problem: Dependence, since the observations are, very likely, not independent. • Modeling the potential dependence creates different models. Panel Data Sets ...
Lecture 15 Panel Data Models
Panel Data Sets • A panel data set, or longitudinal data set, is one where there are repeated observations on the same units. • The units may be individuals, households, enterprises, countries, or any set of entities that remain stable through time. • Repeated observations create a potentially very large panel data sets. With N units and T time periods => Number of observations: NT. • Repeated observations can be great for estimation: Large sample! We use the repeated observations to our advantage. • But, repeated observations from the same individual introduce a problem: Dependence, since the observations are, very likely, not independent. • Modeling the potential dependence creates different models.
Panel Data Sets • The National Longitudinal Survey (NLS) of Youth is an example. The same respondents were interviewed every year from 1979 to 1994. Since 1994 they have been interviewed every two years. • Panel data allows us a researcher to study cross section effects –i.e., along N, variation across the firms- & time series effects –i.e., along T, variation across time. Cross section y11 y 12 Time series y1t y1T
y 21 y 22
y 2t y 2T
yi1 yi 2
y N1 y N 2 y Nt y NT
Panel Data Sets • Notation: x11 yi1 y11 x y y 12 12 i2 y1 ;....;yi X1 x1t yit y1t y1T yiT x1T1
w11 w21 x21 ... xk1 w x22 ... xk2 12 w22 ... ;....;Xi x2t ... xk2 w1t w2t x2T1 ... xkT1 w1Tj w2Tj
... wk1 ... xk2 ... ... wk2 ... wkTj
• A standard panel data set model stacks the yi’s and the xi’s: y = X + c + X is a ΣiTixk matrix is a kx1 matrix c is ΣiTix1 matrix, associated with unobservable variables. y and are ΣiTix1 matrices
Panel Data Sets • Financial data – COMPUSTAT provides financial data by firm (N =99,000) and by quarter (T = 1962:I, 1962:II, ..., ) – CRSP daily and monthly stock and index returns from 1962 on. – Datastream provides economic and financial data for countries. It also covers bonds and stock markets around the world. – Intra-daily data: Olsen (exchange rates) and TAQ (stock market transaction prices). Essentially infinite T, large N. – OptionMetrics, also known as “Ivy DB”, is a database of historical prices, implied volatility for listed stocks and option markets. – International Financial Statistics (IFS from the IMF) covers economic and financial data for almost all countries post-WWII.
Balanced and Unbalanced Panels • Notation: yi,t, i = 1,…,N; t = 1,…,Ti • Mathematical and notational convenience: - Balanced: NT (that is, every unit is surveyed in every time period.) N - Unbalanced: i=1 Ti Q: Is the fixed Ti assumption ever necessary? SUR models. • The NLS of Youth is unbalanced because some individuals have not been interviewed in some years. Some could not be located, some refused, and a few have died. CRSP is also unbalanced, some firms are listed from 1962, others started to be listed later.
Panel Data Models • With panel data we can study different issues: - Cross sectional variation (unobservable in time series data) vs. time series variation (unobservable in cross sectional data) - Heterogeneity (observable and unobservable individual heterogeneity) - Hierarchical structures (say, zip code, city and state effects) - Dynamics in economic behavior - Individual/Group effects (individual effects) - Time effects
Panel Data Models: Example 1 - SUR • In Zellner’s SUR formulation (no linear dependence on yit) we have: (A1) yit = xit’i + it (A2) E[i|X] = 0, (A3’) Var[i|X] = i2IT --groupwise heteroscedasticity. E[it jt |X] = ij --contemporaneous correlation E[it jt’ |X] = 0 when t≠t’ (A4) Rank(X) = full rank • In (A1)- (A4), we have the a GR model with heteroscedasticity. OLS in each equation is OK, but not efficient. GLS is efficient. • We are not taking advantage of pooling –i.e., using NT observations! • Use LR or F tests to check if pooling (aggregation) can be done.
Panel Data Models: Example 2 - Pooling • Assumptions (A1) yit = xit’ + zi’ γ + it i = 1, 2, ...., N t = 1, 2, ...., Ti (A2) E[i|X,z] = 0, (A3) Var[i|X,z] = 2I. (A4) Rank(X) = full rank
- the DGP - we think of i as individuals or groups. - usually, N >> T. - X and z: exogenous - Heteroscedasticity can be allowed.
• We think of X as a vector of observed characteristics. For example, firm size, Market-to-book, Z-score, R&D expenditures, etc. • We think of z as a vector of unobserved characteristics (individual effects). For example, quality of management, growth opportunities, etc.
Panel Data Models: Basic Model • The DGP (A1): k
y it 1 j X j2
Z pi t it
- The index i refers to individuals –i.e., the unit of observation–, t refers to the time period, and j and p are used to differentiate between different observed and unobserved explanatory variables. - A time trend t allows for a shift of the intercept over time, capturing time effects –technological change, regulations, taxes, etc. - If the implicit assumption of a constant rate of change is strong, the trend can be replaced by a set of dummy variables, one for each time period except the reference period. - The Xj variables are usually the variables of interest, while the Zp variables are responsible for unobserved heterogeneity (& dependence on the yi’s) and as such constitute a nuisance component of the model.25
Panel Data Models: Basic Model • Since the Zp variables are unobserved, there is no means of obtaining information about the pZp component of the model. Usually, it is convenient to define a term c known as the unobserved effect, representing the joint impact of the Zp variables on yi. s
• We can rewrite the regression model as: k
y it 1
c i t it
The characterization of the ci component is crucial. Different ways of thinking about zp create different panel data models. 30
Panel Data Models: Basic Model k
y it 1
c i t it
• Note that if the Xj’s are so comprehensive that they capture all the relevant characteristics of the individual, there will be no relevant unobserved characteristics. => Now, c can be dropped. Pooled OLS may be used, treating all the observations for all of the time periods as a single sample. • In general, dropping c leads to missing variables problem: bias! • We usually think of c as contemporaneously exogenous to the conditional error. That is, E[it|c ] = 0, t =1,..., T 30
Panel Data Models: Basic Model • A stronger assumption can solve the estimation problem. Strict exogeneity can also be imposed. In this case, E[it|xi1, xi2,...,xiT,c ] = 0,
t =1,..., T
Then, E [ y it | x it , c i ] 1
=> The βj’s are partial effects holding c constant. • Violations of strict exogeneity are not rare. If xit contains lagged dependent variables or if changes in it affect xit+1 (a “feedback” effect). • But to estimate β we still need to say something about the relation between xit and c. Different assumptions will give rise to different models.
Panel Data Models: Types • The basic DGP:
yit = xit’ + zi’ γ + it
& (A2)-(A4) apply.
Depending on how we model the heterogeneity in the panel, we have different models. • Four Popular Models: (1) Pooled (Constant Effect) Model zi’γ is a constant. zi = α. Dependence on the yit may enter through the variance. That is, repeated observations on individual i are linearly independent. In this case, yit = xit’ + α + it ⟹ OLS estimates α and consistently. We estimate k+1 parameters.
Panel Data Models: Types (2) Fixed Effects Model (FEM) The zi’s are correlated with Xi Fixed Effects: E[zi|Xi] = g(Xi) = α*i; effects are correlated with included variables –i.e., pooled OLS will be inconsistent. Assume zi’ γ = αi (constant; it does not vary with t). Then, yit = xit’ + αi + it ⟹ the regression line is raised/lowered by a fixed amount for each indvidual i (the dependence created by the repeated observations!). In econometrics terms, this is the source of the fixed-effects. ⟹ We have a lot of parameters: k+N. We have N individual effects! OLS can be used to estimates α and consistently.
Panel Data Models: Types (3) Random Effects Model (REM) The differences between individuals are random, drawn from a given distribution with constant parameters. We assume the zi’s are uncorrelated with the Xi. That is, (if Xi contains a constant term, μ=0 WLOG). E[zi|Xi ] = μ Add and subtract E[zi’ γ] = μ* from (*): yit = xit’ + E[zi’ γ] +( zi’ γ) - E[zi’ γ] + it = xit’ + μ* + ui + it We have a compound (“composed”) error –i.e., ui + it= wit. This error wit introduces contemporaneous cross-correlations across the i group. ⟹ OLS estimates μ and consistently, but GLS will be efficient.
Panel Data Models: Types (4) Random Parameters /Coefficients Model We introduce heterogeneity through i . But, this may introduce additional N parameters. A solution is to model i. For example, yit = xit’( + hi) + αi + it hi is a random vector that induces parameter variation, where hi ~D(0, σ2hi). That is, we introduce heteroscedasticity. Now, the coefficients are different for each individual. It is possible to complicate the model by making them different through time: it = ( + hi) + θt where θt ~D(0, σ2t). Estimation: GLS, MLE. Long history: Rao (1965) and Chow (1975) worked on these models.
Compact Notation • Compact Notation: yi = X i + c + i Xi is a Tixk matrix is a kx1 matrix cis a Tix1 matrix yi and i are Tix1 matrices y=X+c+ • Recall we stack the yi’s and Xi’s: X is a ΣiTixk matrix is a kx1 matrix c, y and are ΣiTix1 matrices Or y = X* * + ,
X * = [X ι] * = [ c]’
- ΣiTix(k+1) matrix. - (k+1)x1 matrix
Assumptions for Asymptotics (Greene) • Convergence of moments involving cross section Xi. Usually, we assume N increasing, T or Ti assumed fixed. – “Fixed T asymptotics” (see Greene, p. 196) – Time series characteristics are not relevant (may be nonstationary) – If T is also growing, need to treat as multivariate time series. • Rank of matrices. X must have full column rank. (Xi may not, if Ti < K.) • Strict exogeneity and dynamics. If xit contains yi,t-1 then xit cannot be strictly exogenous. Xit will be correlated with the unobservables in period t-1. Inconsistent OLS estimates! (To be revisited later.)
Panel Data Models: (A3’) - No Homoscedasticity • We can relax assumption (A3). The new DGP model: y = X* * + , with X * = [X ι] - ΣiTix(k+1) matrix. * = [ c]’ - (k+1)x1 matrix Now, we assume (A3’) E[ '|X] = Σ ≠ σ2 IΣiTi • Potentially, a lot of different elements in E[ '|X] in a panel: - Individual heteroscedasticity. Usual groupwise heteroscedasticity. - Autocorrelation (Individual/group/firm) effects. Errors have arbitrary correlation across time for a particular individual i: - Temporal correlation (Time) effects. Errors have arbitrary correlation across individuals at a moment in time (SUR-type correlation). - Persistent common shocks: Errors have some correlation between different firms in different time periods (but, these shocks are assumed to die out over time, and may be ignored after L periods).
Panel Data Models: (A3’) - Error Structures • To understand the different elements in Σ, consider the following DGP for the errors, εit’s: εit = θi’ft+ ηit , ft~D(0, σf2) & ηit = ϕ ηit-1 + ςit, ςit~D(0, σςi2) ft : vector of random factors common to all individuals/groups/firms. θi: vector of factor loadings, specific to individual i. ςit: random shocks to individual i, uncorrelated across both i and t. ηit: random shocks to i. This term generates autocorrelation effects in i. • θi’ft generates both contemporaneous (SUR) and time-varying crosscorrelations between i and j . (Autorrelations die out after L periods.) - If ft is uncorrelated across t, =>only contemporaneous (SUR) effects. - If ft is persistent in t, =>both SUR and persistent common effects.
Panel Data Models: (A3’) - Error Structures • Different forms for E[ '|X]: - Individual heteroscedasticity. E[i2|X] = σi2 => standard groupwise heteroscedasticity driven by ςit. - Autocorrelation (Individual) effects: E[itis|X] ≠ 0 (t≠s) => auto-/time-correlation for errors, it driven by ηit. - Temporal correlation effects: E[itjt|X] ≠ 0 (i≠j) => contemporary cross-correlation for errors driven by ft. - Persistent common shocks: E[itjs|X] ≠ 0 (i≠j) and |t − s| < L => time-varying cross-correlation for errors driven by ft. • Remark: Heteroscedasticity points to GLS efficient estimation, but, as before, for consistent inferences we can use OLS with (adjusted for panels) White or NW SE’s.
Panel Data Models: (A3’) – Clustered SE • For consistent inferences, we can use OLS with White or NW SE’s: - White SE’s adjust only for heteroscedasticity: S0 = (1/T) i ei2 xi xi. - NW SE’s adjust for heteroscedasticity and autocorrelation: ST = S0 + (1/T) l wL(l) t=l+1,...,T (xt-let-l etxt+ xtet et-lxt-l) • But, cross-sectional (SUR) or “spatial” dependencies are ignored. If present, the White’s or NW’s HAC need to be adjusted. • Simple intuition: Repeating a dataset 10 times should not increase the precision of parameter estimates. However, the i.i.d. assumption will do this: Now, we divide by NT, not T or N. We cannot ignore the dependence in the data. An obvious solution is to aggregate the repeated data. That is, aggregate in groups!
Panel Data Models: (A3’) – Clustered SE • In general, the observations are not identical, but correlated within a cluster –i.e., a group that share certain characteristic. We assume correlation within a cluster, but independence across clusters. • Simple idea: Aggregate over the clusters. The key is how to cluster. • Canonical example: We want to study the effect of class size on 1st graders' grades, the unobservables of 1st graders belonging to the same classroom will be correlated (say, teachers’ quality, recess routines) while will not be correlated with 1st graders in far away classrooms. Then, we can cluster by school/teacher. • In finance, it is reasonable to expect that shocks to firms in the same industry are not independent. Then, we can cluster by industry.
Panel Data Models: (A3’) – Clustered SE • We remove the dependence by assuming correlation within a cluster, but independence across clusters. That is, we think of the data as y ig 1 x ig' c ig t ig 0 E [ ig jg ' ]
g 1,..., G
if g g ' if g g '
Or stacking the data by cluster: yg = X g + cg + g
g = 1, ..., G. G Ng Ng
• Easy extension of the White SE:
S0 xig x'jg wig, jg g 2 i 1 j 1
where w2ig,jg = E[ig jg|Xg] is the error covariance for the igth and jgth observations. We use S0 to calculate clustered White SE.
Panel Data Models: (A3’) - PCSE • Driscoll and Kraay (1998) provide an easy extension to estimate robust NW SE’s in panels with cross-sectional dependencies: Average the xtet over the clusters we suspect cause dependence. • Within a cluster, we assume that the correlations within a cluster are the same for different observations. • The KxK sandwich matrix is defined as ^
X ' X
ht (b )ht j (b )'
t j 1
with ht ( b )
N (t )
h i 1
• The NW method is applied to the time series of cross-sectional averages of hit(b). We average over the G clusters to get G ht(b). We use the sandwich matrix to estimate clustered NW SE, using w(l) as usual Bartlett or QS weights –other weights are OK.
Panel Data Models: (A3’) - PCSE • The NW method is applied to the time series of cross-sectional averages of hit(b). By using cross-sectional averages, estimated SE are consistent independently of the panel’s cross-sectional dimension N. • Clearly, these clustered NW SE reduce to the usual NW SE if each cluster only has one observation. • If we do not suspect autocorrelation problems –not rare, given that many panel data sets have heavy temporally spaced observations-, we can rely on White SE (S0). • These clustered standard errors are called panel corrected SE (PCSE’s) or clustered SE. The clustered White-style SE are, sometimes, called Rogers SE.
Panel Data Models: (A3’) - PCSE • These PCSE’s are robust to very general forms of cross-sectional (and temporal) dependence. • These PCSE’s using HAR estimators (say, based on KVB SE) are also possible. (See Hansen (2007). • Consistency of the PCSE is discussed by White (1984), Liang and Zegger (1986) for panels with finite number of observations per cluster, as G → ∞. Hansen (2007) shows that PCSE can be used with Ng→ ∞ -i.e., long panels--, in addition to G → ∞.
Pooled Model • General DGP yit = xit’ + c + it
& (A2)-(A4) apply.
• The pooled model assumes that unobservable characteristics are constant, independent of i --no heterogeneity. That is, we have: yit = xit’ + α + it Now, we have a CLM, with k+1 parameters. • Stacking the variables in matrices, we have: y=X+αι+ Dimensions: - y, ι and are ΣiTix1 - X is ΣiTixk - is kx1
Pooled Model • We can re-write the pooled equation model as: y = X* * + , X* = [X ι] - ΣiTix(k+1) matrix: * = [ α]’ - (k+1)x1 matrix In this context, OLS produces BLUE and consistent estimator. In this model, we refer to pooled OLS estimation • Of course, if our assumption regarding the unobservable variables is wrong, we are in the presence of an omitted variable, c. • Then, we have potential bias and inconsistency of pooled OLS. The magnitude of these problems depends on how the true model behaves: ‘fixed’ or ‘random.’
Pooled Regression: Heterogeneity Bias • In the pooled model, there is no model for group/individual i heterogeneity. Thus, pooled regression may result in heterogeneity bias: Pooled regression:
• • • •
True model: Firm 4
True model: Firm 3
True model: Firm 2
True model: Firm 1
Pooled Regression: Within Transformation • We can estimate by centering the observations around their group/individual means. That is, k
x ji ) it i
• This method is called the within-groups estimation because it relies on variations within individuals rather than between individuals. That is, this estimator reflects the time-series or within-individual i information reflected in the changes within individuals across time. • We are estimating using the time-series information in the data.
Pooled Regression: Within Transformation • There is a cost in the simplicity of the within-groups estimation. First, the intercept and any X variable that remains constant for each individual (say, gender or College degree) will drop out of the model. The elimination of the intercept may not matter, but the loss of the unchanging explanatory variables may be frustrating. ⟹ Obviously, if we are interested on the effect of gender on CEO compensation, within transformation will not work. But it will work well if we are interested on the effect of an independent Board of Directors, by looking at the compensation pre-/post-BOD. • Under the usual assumptions, pooled OLS is consistent and unbiased.
Pooled Regression: Between Transformation • There is an additional alternative to estimate , by expressing the model in terms of group/individual means. That is, k
y i 1
j x ji
• It is called the between estimator because it relies on variations between individuals (say, i & j). We are estimating using the cross-sectional information in the data (the time-series individual i variation is gone!). ⟹ Obviously, if we are interested on the effect of a new independent BOD during the tenure of a CEO on that CEO’s compensation, between transformation will not work. But it will work well if we study the effect of gender on CEO’s compensation.
Pooled Regression: Between Transformation • We lose observations (and power!): we have only N data points. • Under the usual assumptions, pooled OLS using the between transformation is consistent and unbiased.
Useful Analysis of Variance Notation (Greene) • The
variance (total variantion) quantifies the idea that each individual i –say, each firm-- differs from the overall average. We can decompose the variance into two parts: a within-group/individual part and a between group/individual part. Decomposition of Total variation: Ti Ti Σ Ni=1 Σ t=1 (z it z) 2 Σ Ni=1 Σ t=1 (z it zi .) 2 Σ Ni=1 Ti zi . z
Total variation = Within groups variation + Between groups variation • Interpretation: - Within group variation: Measures variation of individuals over time. - Between group variation: Measures variation of the means across individuals.
WHO Data (Greene)
Note: The variability is driven by between groups variation
Pooled Model: Living with (A3’) • We start with the pooled model: y = X* * + , with X * = [X ι] * = [ α]’ Now, we allow E[i j'|Xi ] = σij Ωij
- ΣiTix(k+1) matrix. - (k+1)x1 matrix
• Potentially a lot of different forms for E[i j'|Xi] in a panel: - Individual heteroscedasticity. E[i2|Xi] = σi2 - Individual/group effects: E[itis |Xi] ≠ 0 (t≠s) - Time (SUR or spatial) effects: E[itjt |Xi] ≠ 0 (i≠j) - Persistent common shocks: E[itjs|Xi] ≠ 0 (i≠j) and |t − s| < L
Pooled Model: Living with (A3’) • Heteroscedasticity points to GLS efficient estimation, but, for consistent inferences we can use OLS with White/NW SE. • Before calculating the NW SE, we cluster the data to remove the dependence caused by the within group correlation of the data. • Driscoll and Kraay (1998): Average the xtet over the clusters. Then, the KxK sandwich matrix is defined as ^
X ' X
N (t )
h (b)h t
t j (b )'
ht (b )
t j 1
it (b )
The NW method is applied to the time series of cross-sectional averages of hit(b). If we do not suspect autocorrelation problems, we can rely on White SE (S0), also referred as Rogers SE.
Pooled Model: PCSE - Clustering • We can cluster the SE by one variable (say, industry) or by several variables (say, year and industry) --“multi-level clustering.” If these several variables are nested (say, industry and state), cluster at highest level. • We assume that the correlations within a cluster (a group of firms, a region, different years for the same firm, different years for the same region) are the same for different observations. • Different clusters can produce very different SE. We want to cluster in groups that produce correlated errors. Usually, we cluster using economic theory (clustering by industry, year, industry and year).
Pooled OLS: Clustered SE – Results (Greene) Ordinary LHS=LWAGE Residuals
least squares regression ............ Mean = 6.67635 Sum of squares = 522.20082 Standard error of e = .35447 Fit R-squared = .41121 Model test F[ 8, 4156] (prob) = 362.8(.0000) Panel Data Analysis of LWAGE [ONE way] Unconditional ANOVA (No regressors) Source Variation Deg. Free. Mean Square Between 646.25374 594. 1.08797 Residual 240.65119 3570. .06741 Total 886.90494 4164. .21299 --------+------------------------------------------------Variable| Coefficient Standard Error b/St.Er. P[|Z|>z] --------+------------------------------------------------EXP| .04085*** .00219 18.693 .0000 EXPSQ| -.00069*** .480428D-04 -14.318 .0000 OCC| -.13830*** .01480 -9.344 .0000 SMSA| .14856*** .01207 12.311 .0000 MS| .06798*** .02075 3.277 .0010 FEM| -.40020*** .02526 -15.843 .0000 UNION| .09410*** .01253 7.509 .0000 ED| .05812*** .00260 22.351 .0000 Constant| 5.40160*** .04839 111.628 .0000
Pooled OLS: Clustered SE – Results (Greene) |Variable | Coefficient | Standard Error |b/St.Er.|P[|Z|>z] | +---------+--------------+----------------+--------+---------+ Constant 5.40159723 .04838934 111.628 .0000 EXP .04084968 .00218534 18.693 .0000 EXPSQ -.00068788 .480428D-04 -14.318 .0000 OCC -.13830480 .01480107 -9.344 .0000 SMSA .14856267 .01206772 12.311 .0000 MS .06798358 .02074599 3.277 .0010 FEM -.40020215 .02526118 -15.843 .0000 UNION .09409925 .01253203 7.509 .0000 ED .05812166 .00260039 22.351 .0000 Clustered SE Constant 5.40159723 .10156038 53.186 .0000 EXP .04084968 .00432272 9.450 .0000 EXPSQ -.00068788 .983981D-04 -6.991 .0000 OCC -.13830480 .02772631 -4.988 .0000 SMSA .14856267 .02423668 6.130 .0000 MS .06798358 .04382220 1.551 .1208 FEM -.40020215 .04961926 -8.065 .0000 UNION .09409925 .02422669 3.884 .0001 ED .05812166 .00555697 10.459 .0000
Note: Clustered SE’s tend to be bigger. The more correlation allowed, the higher the SE.
Pooled Model: PCSE – Clustering: Remarks • Since we allow for correlation between observations, clustered SE will increase CIs. The higher the clustering level, the larger the resulting SE. • Thus, different clusters can produce different SE. Rely when possible on economic theory/intuition to cluster. • It is not a bad idea to try different ways of defining clusters and see how the estimated SE are affected. Be conservative, report largest SE. • Practical rules - If aggregate variables (say, by industry, or zip code) are used in the model, clustering should be done at that level. - When the data correlates in more than one way, we have two cases: - If nested (say, city and state), cluster at highest level of aggregation - If not nested (e.g., time and industry), use “multi-level clustering.”
Pooled Model: PCSE - Remarks • The bigger the cross-sectional correlation, the bigger the SE. That is, NW SE’s tend to be smaller than Driscoll and Kraay SE. • In simulations, it is found (as expected) that the PCSE perform better when there is cross-sectional dependence in the data. But, when there is no dependence in the cross-section, the standard White or NW SE do better. In some cases, these differences can be significant • Testing for cross-sectional dependence may be a good idea, especially when results are not robust to different SE. LM tests can be easily implemented. Pesaran (2004) proposes an easy test.
Pooled Model: PCSE - Remarks • The computational issues are straightforward for balanced data. We need only the vector of residuals, the model matrix (X), and indicators for group (and, usually, time) to form the clusters. • But for unbalanced there are two approaches - Create a balanced subset of the panel to estimate Ω. Advantage: Computationally simple. - Loop over eit ejt pairs to estimate covariances over available overlapping time frames (loop over all pairs can take a long time). Advantage: Information is not thrown out. • More on PCSE later.
Cornwell and Rupert Data (Greene) Cornwell and Rupert Returns to Schooling Data, 595 Individuals, 7 Years Variables in the file are EXP WKS OCC IND SOUTH SMSA MS FEM UNION ED BLK LWAGE
= work experience = weeks worked = occupation, 1 if blue collar, = 1 if manufacturing industry = 1 if resides in south = 1 if resides in a city (SMSA) = 1 if married = 1 if female = 1 if wage set by unioin contract = years of education = 1 if individual is black = log of wage = dependent variable in regressions
These data were analyzed in Cornwell, C. and Rupert, P., "Efficient Estimation with Panel Data: An Empirical Comparison of Instrumental Variable Estimators," Journal of Applied Econometrics, 3, 1988, pp. 149-155. See Baltagi, page 122 for further analysis. The data were downloaded from the website for Baltagi's text.
Application 1: Cornell and Rupert (Greene)
Application 2: Bid-Ask Spread (Hoechle)
Pooled Model with (A3’) - GLS • We start with the pooled model: y = X* * + where X * = [X ι] - ΣiTix(k+1) matrix: * = [ α]’ - (k+1)x1 matrix Now, we allow E[i j'|Xi ] = σij Ωij • We can use OLS with PCSE’s or we can do GLS. Note: Why GLS? Efficiency. • Suppose Ωij = IT Then, we only have cross-equation correlation, not time correlation. We are back in the (aggregation) SUR framework
Pooled Model with SUR - GLS • Suppose Ωij = IT. We are in the (aggregation) SUR framework: ˆ GLS ( X 'V 1 X )1 X 'V 1 y ( X '[ I ]1 X )1 X '[ I ]1 y ( X '[1 I ]X )1 X '[1 I ]y
• For FGLS, use the pooled OLS residuals ei and ej to estimate the covariance σij. Note that
1 T 1 ˆ et et ' E ' E T t 1 T where E is a TxN matrix and et=[e1t e2t ... eNt ]' is Nx1 vector. We need to invert ˆ (NxN matrix). Note: In general, the rank(E) ≤ T. Then, rank(ˆ ) ≤ T < N => singularity, FGLS cannot be computed. This is a problem of the data, not the model.
Pooled Model with Heteroscedasticity - GLS • Now, suppose we have groupwise heteroscedasticity. That is, E[i j'|Xi ] = 0 for i≠j E[i2|Xi ] = σi2 • We do FGLS, as usual, using the pooled OLS residuals ei to estimate the variance σi2 and, thus, to estimate Σ: 12 0 .. 0
0 ... 0 ... .. ... N2 ...
• We can test this model with H0: σ12 =σ22 =...=σN2. We can use: W = Σi (si2 – s2pooled)/Var(si2) → χ2N where si2 is computed using the pooled OLS ei residuals.
Pooled Model with Autocorrelation - GLS • Now, suppose we have individual autocorrelation. That is, E[it js'|Xi ] = 0 for i≠j -for example, it i it 1 u it E[it it-p'|Xi ] ≠ 0 Var[it|Xi ] = σ2 • We do FGLS, as usual, using the pooled OLS residuals ei to estimate the ρi and, thus, to estimate Σi: 1 2 u (i ) i i 1 i2 .. T 1 i
i 1 ..
... ... ... ...
i T 1 0 .. 1
• We can test this model with H0: ρ1 = ρ2 =...= ρN=0. We can use an LM test to test H0.
Pooled OLS with First Differences • From the general DGP: yit = xit’ + c + it
& (A2)-(A4) apply.
It may still be possible to use OLS to estimate , when we have individual heterogeneity. We can use OLS if we eliminate the cause of heterogeneity: c We can do this by taking first differences of the DGP. That is, Δyit = yit – yit-1 = (xit- xit-1)’ + Δ c + Δ it = Δxit’ + uit Note: All time invariant variables, including cdisappear from the model (one “diff”). If the model has a time trend –economic fluctuations- , it also disappear, it become the constant term (the other “diff”). Thus, this method is usually called “diffs in diffs” (DD or DiD).
Pooled OLS with First Differences • With strict exogeneity of (Xi,ci), the OLS regression of Δyit on Δxit is unbiased and consistent, but inefficient. • Why? The error is not longer it, but uit. The Var[u] is given by: i,2 i,1 2 2 2 i,3 i,2 Var 0 i,T i,T 1 0 i i
2 2 2 2
0 2 2
0 (Toeplitz form) 2 2 2
• That is, first differencing produces heteroscedasticity. Efficient estimation method: GLS. • It turns out that GLS is complicated. Use OLS in first differences and use Newey-West SE/PCSE with one lag.
OLS with First Diffs: Treatment Application • Suppose there is random assignment to treatment and control groups, like in a typical medical experiment. • We compare the change in outcomes across the treatment and control groups to estimate the treatment effect. (We used this method –“natural experiment”– in Lecture 8 to deal with endogeneity.) • With two periods –i.e., before and after– and strict exogeneity: yit = yi2 – yi1 = δ0 + δ1 Treatmenti + (xi2'– xi1' ) uit (This is a CLM. OLS is consistent and BLUE). Then, E[yit |Treatmenti = 1] = δ0 + δ1 + E[(xi2'– xi1' )|Treatmenti = 1] E[yit |Treatmenti = 0] = δ0 + E[(xi2'– xi1' )|Treatmenti = 0]
OLS with First Diffs: Treatment Application • Assuming that controls are orthogonal to Treatment: δ1 = E[yit |Treatmenti = 1] – E[yit |Treatmenti = 0] δ1 is the difference in average change in the two periods –i.e., before and after– between the treated and control groups. This is the diffs in diffs (DD, DiD) estimator. • Typical problem: Exogeneity (randomness) of treatment. That is, (A2) E[uit |Treatmenti ] = 0. • In medical experiments, diffs in diffs estimation is routinely used to evaluate the effectiveness of a new treatment and/or medication. Usual H0: δ1 = 0. It can be tested with a t-test (using HAR/PCSEs).
OLS with First Diffs: Treatment Application • Same result can be derived by looking at levels DGP (yit, xit) including two dummies: One for Treatmentit and one for after Treatmentit (Posti ): yit = xit’ + c + γ0 + γ1 Treatmenti + γ2 Posti + δ1 Treatmenti x Posti uit Now, it is easy to separate cross-sectional differences from time-series differences. - Cross-sectional difference E[yit |Treatmenti=1, Posti=1] – E[yit |Treatmenti= 0, Posti=1] = γ1 + δ1 Note: Unbiased if γ1=0 ⇒ No permanent difference between the treatment and control groups.
OLS with First Diffs: Treatment Application - Time-sectional difference E[yit |Treatmenti=1, Posti=1] – E[yit |Treatmenti= 1, Posti=0] = γ2 + δ1 Note: Unbiased if γ2=0 => No common trend over the pre- and posttreatment times. • In Lecture 8 we emphasized that we need to make sure that the treatment is the only difference between the two groups. That is, in the absence of treatment, the average change in yit would have been the same for both groups. • This is a key assumption behind the DD estimator, usually “tested” by looking at a graph of the behavior of both groups before treatment – recall example in Lecture 8 from Redding & Sturm (2008).
OLS with First Diffs: Natural Experiment • In Finance & Economics, especially in Corporate Finance, we apply the DD method when we use natural experiments (a change in a law, a policy or a regulation) to study the effect of Xt on yt. (Recall Lecture 8.) • We have two periods: Before and after the natural experiment (the treatment). • If we also have a well-defined control group, where the treatment was not administered –i.e., the natural experiment never occurred–, then, we can use DD estimation. • The number of groups, S, (treated & not treated) under consideration is usually small –typically 2. N is usually very large.
Diffs in Diffs: Natural Experiment - 1 Example 1: We are interested in the effect of labor shocks on wages and employments. Natural experiment: The 1980 Mariel boatlifs, a temporary lifting of emigration restrictions in Cuba. Most of the marielitos (the 1980 Cuban immigrants) settled in Miami. • Two periods: Before and after the 1980 Mariel boatlifs. • Control group: Low skilled workers in Houston, LA and Atlanta. • Calculate unemployment and wages of low skilled workers in both periods. Then, regress yit against a set of control variables (industry, education, age, etc.) and a treatment dummy: yit = yi2 – yi1 = δ0 + δ1 Treatmenti + (xi2'– xi1' ) uit • H0: δ1=0. Card (1990) found no effect of massive immigration.
Diffs in Diffs: Natural Experiment - 2 Example 2: Suppose we are interested in the effect of accounting transparency on analysts forecast errors. We can use the July 30, 2002 Sarbanes-Oxley (SOX) law as a natural experiment. • Two periods: Before and after SOX. • Control group (firms under no SOX law): It may be difficult to find a good control group. Maybe, we can use analysts in Canada or in Germany as a control group. • Calculate the dispersion of the analysts’ forecasts, yi, in both periods & regress yit against a set of control variables (experience of analyst, education, age, etc.) and a treatment dummy: yit = yi2 – yi1 = δ0 + δ1 Treatmenti + (xi2'– xi1' ) uit • H0: δ1=0.
Diffs in Diffs: Remarks • We express the DGP in terms of i (indiviuals), s (groups), and t (time): yist = δs + δt + δ1 Treatmentst + xist' ist • Usually, we have small S and T; but large N. Since, in general, we have within group correlation (treated individuals show similar errors), the asymptotics of the t-test are driven by S*T. • Donald and Lang (2004): Under the usual (generous) assumptions, it converges to a normal distribution (a tST-K may work better). • Intuition: Suppose that within s,t groups the errors are perfectly correlated. Then, we only have S*T independent observations! • Given the potential (time-varying) correlations in the errors, OLS SE can be terrible. PCSE tend to do better.
Dealing with Attrition • Attrition problem: If an unbalanced panel is a result of some selection process related to εit, then endogeneity is present and need to be dealt with using some correction methods. Otherwise, we have attrition bias. • Example: In the "Quality of Life for cancer patients" study discussed in Greene, appearance for the second interview was low for people with initial low QOL (death or depression) or with initial high QOL (don’t need the treatment). • Solutions to the attrition problem – Heckman selection model (used in the study) • Prob[Present at exit|covariates] = Φ(z’θ) (Probit model) • Additional variable added to difference model i = ϕ(zi’θ)/Φ(zi’θ) – The FDA solution: fill with zeros. (!)
Pooled Model: ML Estimation • In the pooled model, y = X + , we assume t ~N(0, Σ), where t = [1t, 2t,..., Nt ]' and Σ is an NxN matrix. • We can write the log likelihood function as: L = log L(, Σ|X) = -NT/2 ln(2π) – T/2 ln|Σ| - ½ Σt t'Σ-1t • The ML estimator is equal to the iterated FGLS estimator. • Testing is straightforward with likelihood ratio test. Example: H0: No cross correlation across equations: The off-diagonal elements of Σ are zero. LR = T (ln|ˆR| - ln|ˆ U|) = T (Σi ln si2 - ln|ˆ|), which follows a χ2 with N(N-1)/2 d.f.
Main Models: FEM and REM • Two main approaches to fitting models using panel data: (1) Fixed effects regressions. (2) Random effects regressions. • The key difference between these two approaches is how the unobservable characteristics –the individual effects- are modeled. • Terminology from experimental design (say, psychology or medicine), where the emphasis was on the kind of sample at hand and inferences: - FE: The individuals are fixed. The differences between them are not of interest, only is interesting. No intent on generalizing the results. - RE: The individuals come from a random sample drawn from a larger population, and the variance between them is interesting and can be informative about the larger population.
The Fixed Effects Model (FEM) • The fixed effects (FE) model (A1) yit = xit’ + ci + it -observation for individual i at time t. for all t and s. - Xi and ci strict exogenous. (A2) E[εit|Xis,cis]=0, • The unobserved component, ci, is arbitrarily correlated with xit: E[ci|Xi ] = g(Xi) = constanti ⇒ Cov[xit,ci] ≠0 ⇒ Under the FE assumption, since we omit ci, pooled OLS is biased and inconsistent. • We summarize (“control for”) these unobservable effects with αi, a constant. All time invariant characteristic of individual i (location, gender, nationality, etc.) are swept away under this formulation. Note: In a FEM, individuals serve as their own controls.
Estimation with Fixed Effects • Whatever effects the omitted variables have on the individual i at one time, they will also have the same effect at a later time, thus, their effects will be constant, or “fixed.” • For this, we need the omitted variables to have time-invariant values with time-invariant effects. Typical example, a CEO’s IQ/managerial skills. We expect this variable to have the same effect at t=1 or t=10. • As we will see, FEM are estimated using the within transformation. Thus, if individuals do not change much (or at all) across time, a FEM may not work very well. We need within-individuals variability in the variables if we are to use individuals as their own controls.
Estimation with Fixed Effects • Matrix notation - In matrix notation for individual i: - ci is a Tix1 vector. (Each individual yi = xi’ + ci + i has Ti observations.) - In matrix notation for all individuals –i.e., stacking: y=X+c+ - we have ΣiTi observations. Now, c, y, and are ΣiTix1 vectors. • Dummy variable representation N
y it x it '
j d jit
1 (i j )
Assumptions for the FE Model (FEM) • The individual unobservable characteristics (effects) are correlated with the included variables: E[ci|Xi ] = g(Xi) ⇒ Cov[xit,ci] ≠ 0 • The FE model assumes ci = αi (constant; it does not vary with t): yi = Xi + diαi + εi, for each individual i. • Stacking
y1 X1 y2 X2 y X N N
d1 0 0
0 d2 0
0 0 0
0 0 β ε α dN
β = [X, D] ε α = Zδ ε
FEM: Estimation • The FEM is the CLM, but with many independent variables: k+N. • OLS is unbiased, consistent, efficient, but impractical if N is large. • The OLS estimates of β and α are given by: 1
X D X y b X X a D X D D D y U s in g th e F r is c h - W a u g h t h e o r e m b
= [ X M D X ] 1 X M D y
Note (Greene): LS is an estimator, not a model. Given the formulation with a lot of dummy variables, this particular LS estimator is called Least Squares Dummy Variable (LSDV) estimator.
FEM: Estimation M1D 0 MD 0
0 0 (The dummy variables are orthogonal) MND
0 M2D 0
M I Ti d i ( di d i ) 1 d = I Ti (1/Ti ) d i d i D
X M D X = Ni=1 X iMDi X i , X M D y = Ni=1 X iM Di y i ,
X M X X M y i
i t=1 (x it,k -x i.,k )(x it,l -x i.,l )
i t=1 (x it,k -x i.,k )(y it -y i. )
• That is, we subtract the group mean from each individual observation. Then, the individual effects disappear. Now, OLS can easily be used to estimate the k β parameters, using the demeaned data. • We know this method: The within-groups estimation.
FEM: Within Transformation Removes Effects • The within-groups method estimates the parameters using demeaned data. That is, k
Yit 1 j X jit i t it j 2 k
Yi 1 j X ji i t i j 2
Yit Yi j ( X jit X ji ) ( t t ) it i j 2
Recall: It is called within-groups/individuals method because the model is explaining the variations about the mean of the dependent variable in terms of the variations about the means of the explanatory variables for the group of observations relating to a given individual.
FEM: Within Transformation Removes Effects k
y it y i
j ( x jit
x ji ) (t t ) it i
• There are costs in the simplicity of the within-groups estimation. • First, all time-invariant variables (including constant) for each individual i drop out of the model. This eliminates all betweenindividuals variability (which may be contaminated by omitted variable bias) and leaves only the within-subject variability to analyze. The loss of the unchanging explanatory variables may matter. • For example, if we are studying CEO compensation, the within group transformation will lose individual schooling, years of experience, previous job network, etc. That is, schooling effects on CEO compensation cannot be study in this context.
FEM: Within Transformation Removes Effects k
y it y i
j ( x jit
x ji ) (t t ) it i
• Second, the dependent variables are likely to have much smaller variances than in the original specification. Now, they are measured as deviations from the individual mean, rather than as absolute amounts. This is likely to adversely affect the precision of the estimates of the coefficients. It is also likely to aggravate measurement error bias if the explanatory variables are subject to measurement error. • Third, the manipulation involves the loss of N degrees of freedom (we are estimating N means!).
FEM: LS Dummy Variable (LSDV) Estimator • b is obtained by within-groups least squares (group mean deviations). • Then, we use the normal equations to estimate a: D’Xb + D’Da=D’y a = (D’D)-1D’(y – Xb) a i = ( 1 / T i ) Σ tT=i 1 ( y it - x it b ) = e i
Note: - This is simple algebra – the estimator is just OLS - Again, LS is an estimator, not a model. This particular OLS estimator is called LSDV estimator. - Note what ai is when Ti=1. Follow this with yit-ai-xit’b=0 if Ti=1.
FEM: LSDV Estimator • Recall the dummy variable trap: If a constant is present in the model, the number of dummy variable should be N-1. The omitted individual or group becomes the reference category. • However, the choice of reference category is often arbitrary and, thus, the interpretation of the i will not be particularly interesting. • Alternatively, we can drop the 1 intercept and define dummy variables for all of the individuals. This is the more common approach. The i now become the intercepts for each of the i’s. • If E[εit|Xis,cis]≠0, then LSDV cannot be used. It is inconsistent. In this case, we need to use IVs. Or a good natural experiment.
FEM: First-Difference (FD) Method • We can also eliminate the individual FE using the first-difference method. • The unobserved effect is eliminated by subtracting the observation for the previous time period from the observation for the current time period, for all time periods. k
Yit Yit 1
( X j
X jit 1 ) (t (t 1)) it it 1
it it 1
• The error term is now (it – it–1). As before, differencing induces a moving average autocorrelation if it satisfies the CLM assumptions. Note: If it is subject to AR(1) autocorrelation and is close to 1, taking first differences may approximately solve the problem.
FEM: Estimation – FE or FD? • Fixed-effects (or Within) Estimator – Each variable is demeaned -i.e., subtracted by its average. – Dummy Variable Regression -i.e., put in a dummy variable for each cross-sectional unit, along with other explanatory variables. This may cause estimation difficulty when N is large. • FD Estimator – Each variable is differenced once over time, so we are effectively estimating the relationship between changes of variables.
FEM: Estimation – FE or FD? • Theoretically, when N is large and T is small but greater than 2 (for T=2, FE=FD), FE is more efficient when εit are serially uncorrelated while FD is more efficient when εit follows a random walk (ρ=1). • When T is large and N is small – FD has advantage for processes with large positive autocorrelation. (If is near 1, FD solves the nonstationary problem!) – FE is more sensitive to nonnormality, heteroskedasticity, and serial correlation in εit. – On the other hand, FE is less sensitive to violation of the strict exogeneity assumption. Then, FE is preferred when the processes are weakly dependent over time
FEM: Calculation of Var[b|X] •
Assume strict exogeneity: Cov[εit,(xjs,cj)]=0. Every disturbance in every period for each person is uncorrelated with variables and effects for every person and across periods. Now, we have OLS in a CLM.
Asy.Var[b|X] = ( 2 / Ni=1 Ti )plim[( 2 / Ni=1 Ti ) Ni=1 X iMDi X i ]1 which is the usual estimator for OLS 2 ˆ
Ti Ni=1 t=1 (y it -ai -x itb)2
Ti - N - K
(Note the degrees of freedom correction)
FEM: PCSE • All previous comments and remarks apply to the FEM. • If dependence structure is suspected, we cluster the SE. We can clustered them by one variable (say, industry, region) or by several variables (say, year and industry) --“multi-level clustering.” • We build the SE according to the type of data we have: - If we do not suspect autocorrelated errors –not a strange situation-, we can rely on clustered White SE’s (S0). - If we suspect autocorrelated errors, then the Driscoll and Kraay SE should be used.
FEM: Testing for Fixed Effects • Under H0 (No FE): αi = α for all i. => That is, we test whether to pool or not to pool the data. • Different tests: – F-test based on the LSDV dummy variable model: constant or zero coefficients for D. Test follows an F(N-1,NT-N-K) distribution. – F-test based on FEM (the unrestricted model) vs. pooled model (the restricted model). Test follows an F(N-1,NT-N-K) distribution. – A LR can also be done –usually, assuming normality. Test follows a χ2N-1 distribution.
FEM: Hypothesis Testing • Based on estimated residuals of the fixed effects model. (1) Estimate FEM: yit = xit’β + αi + it => Keep residuals eFE,it (2) Tests as usual: – Heteroscedasticity • Breusch and Pagan (1980) – Autocorrelation: AR(1) • Breusch and Godfrey (1981)
NT2 eFE ' eFE1 d LM 12 T 1 eFE ' eFE
Application: Cornwell and Rupert Data (Greene) Cornwell and Rupert Returns to Schooling Data, 595 Individuals, 7 Years Variables in the file are: (Not used in regressions) EXP = work experience, EXPSQ = EXP2 WKS = weeks worked OCC = occupation, 1 if blue collar, (IND = 1 if manufacturing industry) (SOUTH = 1 if resides in south) SMSA = 1 if resides in a city (SMSA) MS = 1 if married FEM = 1 if female UNION = 1 if wage set by unioin contract ED = years of education (BLK = 1 if individual is black) LWAGE = log of wage = dependent variable in regressions (Y) These data were analyzed in Cornwell, C. and Rupert, P., "Efficient Estimation with Panel Data: An Empirical Comparison of Instrumental Variable Estimators," Journal of Applied Econometrics, 3, 1988, pp. 149-155.
Application: Cornwell and Rupert (Greene) (1) Returns to Schooling - Pooled OLS Results
RSS & R2 X only
Application: Cornwell and Rupert (Greene) (2) Returns to Schooling - LSDV Results
RSS & R2 X and group effects
FEM: Testing for FE (and other formulations)
• Calculations: F-test594,3566 = [(651.78-83.89)/594]/[83.89/3566] = 40.64
The Random Effects Model (REM) • Recall the general DGP: yit = xit’ + zi’ γ + it
-observation for individual i at time t.
When the observed characteristics are constant for each individual, a FEM is not an effective tool because such variables cannot be included. • An alternative approach, known as a random effects (REM) model that, subject to two conditions, provides a solution to this problem. • Conditions: (1)It is possible to treat each of the unobserved Zp variables as being drawn randomly from a given distribution. (2) The Zp variables are distributed independently of all of the Xj variables.
The Random Effects Model (REM) • Conditions: (1) The unobserved Zp variables are drawn randomly from a given distribution. Thus, the ci may be treated as RV (thus, the name of this approach) drawn from a given distribution. Let’s call it ui. Then, k
Yit 1 j X jit ui t it j 2 k
1 j X jit t wit j 2
wit ui it
• We deal with the unobserved effect by subsuming it into a compound disturbance term, wit. We will assume that the Zp is drawn from a distribution with zero mean and constant variance. Then, E ( wit ) E (ui it ) E (ui ) E ( it ) 0
The Random Effects Model (REM) • The zero mean assumption –E[ui] = 0-- is not crucial, any nonzero component is being absorbed by the intercept, 1. (2) The Zp variables are distributed independently of all of the Xj variables. Otherwise, ui and the compounded error, wit, will not be uncorrelated with Xj. The RE estimation will be biased and inconsistent. Note: We would have to use the FEM, even if the first condition seems to be satisfied. • If (1) and (2) are satisfied, we can use the REM, but there is a complication. Now, wit will be heterosccedastic.
REM: Error Components Model • REM Assumptions: yit = xit’ + zi’ γ + it = xit’ + ui + it = xit’ + wit E[it |Xi]=0 E[it2|Xi]= σε2 E[ui |Xi]=0 E[ui2|Xi]= σu2 E[uiit|Xi] = E[uijt |Xi ] =0 - u and are independent. E[uiuj |Xi] = 0 (i≠j) -no cross-correlation of RE. E[itjt |Xi]= 0 (i≠j) -no cross-correlation for the errors, it. E[itjs |Xi]= 0 (t≠s) -there is no autocorrelation for it.
2wit u2i it u2i 2it 2ui ,it u2 2
it1 w it 2
( u i it
)( u i it 2 )
REM: Notation (Greene) y1 X1 ε1 u1i1 T1 observations y X 2 2 β ε 2 u2i2 T2 observations yN XN εN uNiN TN observations = Xβ+ε+u Ni=1 Ti observations = Xβ+w In all that follows, except where explicitly noted, X, X i and xit contain a constant term as the first element. To avoid notational clutter, in those cases, x it etc. will simply denote the counterpart without the constant term. Use of the symbol K for the number of variables will thus be context specific but will usually include the constant term.
REM: Notation (Greene) 2 u2 u2 Var[ε i +u ii ] 2 u
2 2 u
= 2 I Ti u2ii Ti Ti = 2 I Ti u2ii = Ωi Ω1 0 Var[ w | X ] 0
(Note these differ only in the dimension Ti ) Ω N
• Note: If E[itjt |Xi ]≠ 0 (i≠j) or E[itjs |Xi ] ≠ 0 (t≠s), we no longer have this nice diagonal-type structure for Var[w|X].
REM: Assumptions - Convergence of Moments X X X X Ni1 fi i i a weighted sum of individual moment matrices N Ti i1 T X Ω X X ΩX Ni1 fi i i i a weighted sum of individual moment matrices N Ti i1 T = 2 Ni1 fi
X i X i u2 Ni1 fi x i x i Ti
Note asymptotics are with respect to N. Each matrix
X i X i is the Ti
moments for the Ti observations. Should be 'well behaved' in micro level data. The average of N such matrices should be likewise. T or Ti is assumed to be fixed (and small).
REM: Pooled OLS Estimation (Greene) • Standard results for the pooled OLS estimator b in the GR model - Consistent and asymptotic normal - Unbiased - Inefficient • We can use pooled OLS, but for inferences we need the true variance –i.e., the sandwich estimator: 1
1 XX XΩX XX Var[b | X] N N i1 Ti i1 Ti Ni1 Ti Ni1 Ti 0 Q-1 Q * Q-1 0 as N with our convergence assumptions
REM: Sandwich Estimator for OLS (Greene) 1
X X X Ω X X X N N i 1 T i i 1 T i Ni 1 T i Ni 1 T i X Ω X X Ω X Ni 1 f i i i i , w h e re = Ω i = E [ w i w i | X i ] Ti Ni 1 T
V a r[ b | X ]
In th e s p irit o f th e W h ite e s tim a to r, u s e ˆ w ˆ X X w X Ω X ˆ i = y i - X ib Ni 1 f i i i i i , w Ti Ni 1 T H y p o th e s is te s ts a re th e n b a s e d o n W a ld s ta tis tic s . T H I S I S T H E 'C L U S T E R ' E S T I M A T O R
• Recall: Clustered standard errors or PCSE There is a grouping, or “cluster,” within which the error term is possibly correlated, but outside of which (across groups) it is not.
REM: Sandwich Estimator – Mechanics (Greene) 1
Est.Var[b | X ] X X
N i 1
ˆ iw ˆ i X i X X X i w
ˆ i = set of Ti OLS residuals for individual i. w X i = Ti xK data on exogenous variable for individual i. ˆ i = K x 1 vector of products X i w ˆ i )( w ˆ i X i ) KxK matrix (rank 1, outer product) ( X i w
X wˆ wˆ X N i 1
= sum of N rank 1 matrices. Rank K.
ˆ X . ˆ iw ˆ i X i = Ni1 X i Ω We could compute this as Ni1 X i w i i Why not do it that way?
REM: GLS • Standard results for GLS in a GR model - Consistent - Unbiased - Efficient (if functional form for Ω correct) ˆ = [ X Ω β = [
N i 1
X ] 1 [ X Ω
X i Ω
y ] N i 1
X i Ω
1 ii I Ti 2 2 2 T i u ( n o t e , d e p e n d s o n i o n ly th r o u g h T i ) Ω
• As usual, the matrix Ω-1/2=P will be used to transform the data.
REM: GLS • The matrix Ω-1/2 = P is used to transform the data. That is, y it y i x it x i it where
Asy .Var [ ˆ GLS ] ( X '
T i u2
X ) 1 2 ( X *' X *)
• We call the transformed data: quasi time-demeaned data. As expected, GLS is just pooled OLS with the transformed data. Note: The RE can be seen as mixture of two estimators: - when θ = 0 (σu=0) => pooled OLS estimator - when θ = 1 (σε=0 or σu→ ∞) => LSDV estimator (ui’s become the FE) Then, the bigger (smaller) the variance of the unobserved effect –i.e., individual heterogeneity is bigger-, the closer it is to FE (pooled OLS). Also, when T is large, it becomes more like FE
REM: FGLS - Estimators for the Variances • To transform the data, we need to estimate σε2 and σu2, consistently. • Usual steps (assume a balanced panel): (1) Start with a consistent estimator of β. For example, pooled OLS, b. -It estimates Σi Σt (σε2 + σu2) (2) Compute Σi Σt (yit - xit’b)2 (3) Divide by a function of NT. For example: NT – K – 1 => We have an estimator of σ2, s2pooled = epooled’epooled/(NT – K – 1) We will use s2pooled to estimate the sum: σε2 + σu2 (4) Use LSDV estimation to get ai and bLSDV. Keep residuals, eFE,it. (5) Compute Σi Σt (yit - ai - xit’ bLSDV)2 -It estimates Σi Σt (σε2) (6) Divide by NT-K-N. => We have an estimator of σε2, s2ε=Σi Σt (eFE,it)2 /(NT-K-N) (7) Estimate σu2 as s2u= s2pooled - s2ε
REM: FGLS - Estimators for the Variances Feasible GLS requires (only) consistent estimators of 2 and u2 . Candidates: 2 From the robust LSDV estimator: ˆ
Ni1 tTi 1 (y it ai x itbLSDV )2 Ni1 Ti K N
Ni1 tTi 1 (y it aOLS x itb OLS )2 Ni1 Ti K 1 N (y a x ib MEANS )2 From the group means regression: 2 / T u2 i1 it N K 1 ˆ w ˆ N Ti 1 Ti w 2 (Wooldridge) Based on E[w it w is | X i ] u2 if t s, ˆ u i1 Nt 1 s t 1 it is i1 Ti K N
From the pooled OLS estimator: 2 u2
There are many others.
Note: A slight chance in notation, x’ does not contain the constant term.
REM: Practical Problems with FGLS All of the preceding regularly produce negative estimates of u2 . Estimation is made very complicated in unbalanced panels. A bulletproof solution (originally used in TSP, now LIMDEP and others). 2 From th e robust LSDV estimator: ˆ
Ni1 tTi 1 (y it ai x it bLSDV ) 2 Ni1 Ti
From the pooled OLS estimator: 2 u2 2 ˆu
Ni1 tTi 1 (y it aOLS x it b OLS ) 2 2 ˆ Ni1 Ti
Ni1 tTi 1 (y it aOLS x it b OLS ) 2 Ni1 tTi 1 (y it ai x it bLSDV ) 2 0 Ni1 Ti
• Bullet proof solution: Do not correct by degrees of freedom. Then, given that the unrestricted RSS (LSDV) will be lower than the restricted (pooled OLS) RSS, σu2 will be positive!
Application: Fixed Effects Estimates (Greene) ---------------------------------------------------------------------Least Squares with Group Dummy Variables.......... LHS=LWAGE Mean = 6.67635 Residuals Sum of squares = 82.34912 Standard error of e = .15205 These 2 variables have no within group variation. FEM ED F.E. estimates are based on a generalized inverse. --------+------------------------------------------------------------Variable| Coefficient Standard Error b/St.Er. P[|Z|>z] Mean of X --------+------------------------------------------------------------EXP| .11346*** .00247 45.982 .0000 19.8538 EXPSQ| -.00042*** .544864D-04 -7.789 .0000 514.405 OCC| -.02106 .01373 -1.534 .1251 .51116 SMSA| -.04209** .01934 -2.177 .0295 .65378 MS| -.02915 .01897 -1.536 .1245 .81441 FEM| .000 ......(Fixed Parameter)....... UNION| .03413** .01491 2.290 .0220 .36399 ED| .000 ......(Fixed Parameter)....... --------+-------------------------------------------------------------
REM: Computing Variance Estimators (Greene) Using full list of variables (FEM and ED are time invariant) OLS sum of squares = 522.2008. 2 +u2 = 522.2008 / (4165 - 9) = 0.12565. Using full list of variables and a generalized inverse (same as dropping FEM and ED), LSDV sum of squares = 82.34912. 2 = 82.34912 / (4165 - 8-595) = 0.023119. u2 0.12565 - 0.023119 = 0.10253 Both estimators are positive. We stop here. If u2 were negative, we would use estimators without DF corrections.
REM: Application (Greene) ----------------------------------------------------------------------
Random Effects Model: v(i,t) = e(i,t) + u(i) Estimates: Var[e] = .023119 Var[u] = .102531 Corr[v(i,t),v(i,s)] = .816006 Lagrange Multiplier Test vs. Model (3) =3713.07 ( 1 degrees of freedom, prob. value = .000000) (High values of LM favor FEM/REM over CR model) Fixed vs. Random Effects (Hausman) = .00 (Cannot be computed) ( 8 degrees of freedom, prob. value = 1.000000) (High (low) values of H favor F.E.(R.E.) model) Sum of Squares 1411.241136 R-squared -.591198 +---------+--------------+----------------+--------+---------+----------+ |Variable | Coefficient | Standard Error |b/St.Er.|P[|Z|>z] | Mean of X| +---------+--------------+----------------+--------+---------+----------+ EXP .08819204 .00224823 39.227 .0000 19.8537815 EXPSQ -.00076604 .496074D-04 -15.442 .0000 514.405042 OCC -.04243576 .01298466 -3.268 .0011 .51116447 SMSA -.03404260 .01620508 -2.101 .0357 .65378151 MS -.06708159 .01794516 -3.738 .0002 .81440576 FEM -.34346104 .04536453 -7.571 .0000 .11260504 UNION .05752770 .01350031 4.261 .0000 .36398559 ED .11028379 .00510008 21.624 .0000 12.8453782 Constant 4.01913257 .07724830 52.029 .0000
Testing for Random Effects: LM Test • We want to test for RE. That is, H0: σu2 =0. • We can use the Breusch-Pagan (1980) Test for RE effects. Similar to the LM-BP test for autocorrelation, it is based on the pooled OLS residuals, ei. It is easy to compute – distributed as 12: B r e u s c h a n d P a g a n L a g r a n g e M u lt ip lie r s t a t is t ic A s s u m in g n o r m a lit y ( a n d f o r c o n v e n ie n c e n o w , a b a la n c e d p a n e l) 2
Ni 1 ( T e i2 ) Ni 1 [ ( T e i2 ) e i e i ] NT NT 1 N N 2 Ni 1 e i e i 2 ( T - 1 ) i 1 i 1 e it 2 (T -1 ) C o n v e r g e s t o c h i- s q u a r e d [ 1 ] u n d e r t h e n u ll h y p o t h e s is o f n o c o m m o n e f f e c t s . ( F o r u n b a la n c e d p a n e ls , t h e
s c a le in f r o n t b e c o m e s ( Ni 1 T i ) 2 / [ 2 Ni 1 T i ( T i 1 ) ] . )
REM: LM Test Application - Cornwell-Rupert
Note: Check the different standard errors from both models.
FE vs. RE: Understanding Differences • Suppose, we want to study the effect of an MBA on stock trading, controlling for other factors such as income and experience. We have a panel, with individuals measured annually over 10 years. We expect some year-to-year correlation with a given individual i (with unobservable individual-level effects accounting for part of the correlation between yearly trading within the same i). • To understand the difference between FE & RE, we ask: - Does a regression coefficient for an MBA represent a comparison of two i’s, one with an MBA and one without one? => Between Effect (RE). - Or does it compare two yearly trading records from the same i who happened to receive an MBA in the interim? => Within Effect (FEM).
FE vs. RE: Understanding Differences • Between Effect (REM): GLS
⟹ consistent and efficient (under H0)
- Between-individual effects and within-individual effects are identical. - Very efficient, no data is thrown away. • Within Effect (FEM): OLS
⟹ consistent estimates.
- No confounding due to unmeasured i-level characteristics. - Cost: All the between-individual comparisons in the data are thrown away. • Q: Are within and between MBA effects the same?
FE vs. RE • Q: RE estimation or FE estimation? • Case for RE: - Under no omitted variables – or if the omitted variables are uncorrelated with the Xit in the model – then a REM is probably best: It produces unbiased estimates of the coefficients, is efficient, uses all the data available, and produces the smallest standard errors. - RE can deal with observed characteristics that remain constant for each individual. In FE, they have to be dropped from model. - In contrast with FE, RE estimates a small number of parameters - We do not lose N degrees of freedom. - Philosophically speaking, a REM is more attractive: Why should we assume one set of unobservables fixed and the other random?
FE vs. RE • Case against RE: - If either of the conditions for using RE is violated, we should use FE. • Condition (1): The unobserved effects are drawn randomly from a given population. This is a reasonable assumption in many cases: Many of the panels are designed to be a random sample (for example, NLSY). But, it would not be a reasonable assumption if the units of observation in the panel data set were data from the S&P 500 firms. • Condition (2): The unobserved effect be distributed independently of the Xj variables. A violation of condition (2) causes inconsistency in the RE estimation
FE vs. RE • FE estimation is always consistent. On the other hand, a violation of condition (2) causes inconsistency in the RE estimation. That is, if there are omitted variables, which are correlated with the Xit in the model, then the FEM provides a way for controlling for omitted variable bias. In a FEM, individuals serve as their own controls. • Q: How can we tell if condition (2) is violated? A: A DHW test can help.
DHW (Hausman) Specification Test: FE vs. RE Estimator
Random Effects E[ci|Xi] = 0
Fixed Effects E[ci|Xi] ≠ 0
FGLS (Random Effects)
Consistent and Efficient
LSDV (Fixed Effects)
Consistent Possibly Efficient
• Under an H0 (RE is true), we have one estimator that is efficient (RE) and one inefficient (LSDV). We can use a Durbin-Hausman-Wu test. As in its other applications, the DHW test determines whether the estimates of the coefficients, taken as a group, are significantly different in the two regressions.
DHW (Hausman) Specification Test: FE vs. RE ˆ -β ˆ Basis for the test, β FE RE ˆ -β ˆ ;W =q ˆ =β ˆ [Var( q ˆ )]-1 q ˆ Wald Criterion: q FE RE A lemma (Hausman (1978)): Under the null hypothesis (RE) d ˆ - β ] nT [β N[0 ,VRE ] (efficient) RE d ˆ - β ] N[0 ,VFE ] (inefficient) nT [β FE
ˆ - β )-( β ˆ β ). The lemma states that in the ˆ = (β Note: q FE RE joint limiting distribution of
ˆ - β ] and nT [ β RE
ˆ , the nT q
limiting covariance, C Q,RE is 0 . But, C Q,RE = C FE,RE - VRE . Then, Var[ q] = VFE + VRE - C FE,RE - C FE,RE . Using the lemma, C FE,RE = VRE . It follows that Var[ q]= VFE - VRE . Based on the preceding ˆ -β ˆ ) [Est.Var( β ˆ ) - Est.Var( β ˆ )]-1 ( β ˆ -β ˆ ) H=( β FE RE FE RE FE RE
Note: β does not contain the constant term.
Computing the DHW Statistic N 1 2 ˆ ] Est.Var[β ˆ i1 X i I ii X i FE T i
2 N Ti ˆ i ˆu 2 ˆ ] Est.Var[β I , 0 = 1 X ii X ˆ ˆ i RE i1 i i 2 2 Ti ˆ Ti ˆu 2 2 ˆ ] Est.Var[β ˆ ] As long as ˆ and ˆ u are consistent, as N , Est.Var[β RE FE
will be nonnegative definite. In a finite sample, to ensure this, both must 2 be computed using the same estimate of ˆ . The one based on LSDV will
generally be the better choice. ˆ ] if there are time Note that columns of zeros will appear in Est.Var[β FE invariant variables in X.
Note: Pooled OLS is consistent, but inefficient under H0. Then, the RE estimation is GLS.
DHW Specification Test: Application (Hoechle) • Bid-Ask Spread Panel estimation.
• Rejection at the 5% level, like in this case, indicates that βFE ≠ βRE. - Usually, this result is taken as an indication of a FEM.
DHW Specification Test: Application (Greene) +--------------------------------------------------+ | Random Effects Model: v(i,t) = e(i,t) + u(i) | | Estimates: Var[e] = .235236D-01 | | Var[u] = .133156D+00 | | Corr[v(i,t),v(i,s)] = .849862 | | Lagrange Multiplier Test vs. Model (3) = 4061.11 | | ( 1 df, prob value = .000000) | | (High values of LM favor FEM/REM over CR model.) | | Fixed vs. Random Effects (Hausman) = 2632.34 | | ( 4 df, prob value = .000000) | | (High (low) values of H favor FEM (REM).) | +--------------------------------------------------+
• The DHW statistic is used to tests the difference in coefficients between an RE and FE models - A rejection, like in this case, indicates that βFE ≠ βRE - But, rejecting H0 does not imply necessarily H1 is “accepted.” - Either the model is misspecified or ui and xit are correlated - Q: Is the model misspecified (any variable missing)?
Wu (Variable Addition) Test • Under the FE assumptions, the common unobserved effect is correlated with the group means. Add the group means to the RE model. If statistically significant, this suggests that the RE model is inappropriate. • In a panel context, tests based on a regression can be more computationally more stable, since no problems with non-positive definiteness are encountered. • Since the errors and the unobserved effect may not be i.i.d. white noise, Wooldridge (2009) suggests using PCSE.
Mundlak (Augmented) Regression (Greene) +--------+--------------+----------------+--------+--------+----------+ |Variable| Coefficient | Standard Error |b/St.Er.|P[|Z|>z]| Mean of X| +--------+--------------+----------------+--------+--------+----------+ |EXPBAR | -.08769*** .00162096 -54.099 .0000 19.853782| |OCCBAR | -.14806*** .03623348 -4.086 .0000 .5111645| |SMSABAR | .21707*** .03209640 6.763 .0000 .6537815| |MSBAR | .14855*** .05087686 2.920 .0035 .8144058| |UNYNBAR | .07831** .03257465 2.404 .0162 .3639856| |WKSBAR | .00857** .00362039 2.367 .0179 46.811525| |INDBAR | .03998 .02966215 1.348 .1777 .3954382| |SOUTHBAR| -.05487 .04293224 -1.278 .2012 .2902761| |EXP | .11448*** .00225862 50.684 .0000 19.853782| |EXPSQ | -.00045*** .483957D-04 -9.304 .0000 514.40504| |OCC | -.02122 .01380348 -1.537 .1243 .5111645| |SMSA | -.04237** .01945829 -2.178 .0294 .6537815| |MS | -.02969 .01901293 -1.561 .1184 .8144058| |FEM | -.31359*** .05419945 -5.786 .0000 .1126050| |UNION | .03268** .01494574 2.187 .0288 .3639856| |ED | .05150*** .00550816 9.349 .0000 12.845378| |BLK | -.15768*** .04463738 -3.533 .0004 .0722689| |WKS | .00081 .00060031 1.354 .1759 46.811525| |IND | .01909 .01546993 1.234 .2171 .3954382| |SOUTH | -.00176 .03435229 -.051 .9592 .2902761| |Constant| 5.15038*** .20122987 25.595 .0000 | +--------+------------------------------------------------------------+
Wu Test: Application 1 (Greene) --> matr;bm=b(1:8);vm=varb(1:8,1:8)$ --> matr;list;wutest=bm'bm$ Matrix WUTEST has 1 rows and 1 columns. 1 +-------------1| 3006.13788 --> calc;list;ctb(.95,8)$ +------------------------------------+ | Listed Calculator Results | +------------------------------------+ Result = 15.507313
Wu Test: Application 2 (Hoechle) • Bid-Ask Spread Wu test estimation with PCSE’s. Stata code:
Wu Test: Application 2 (Hoechle) • Bid-Ask Spread Wu test estimation with Driscoll and Kraay SE’s. Stata code for auxiliary regression:
• Now, you cannot reject the REM at the 5% level. Here you can say, “after accounting for cross-sectional and temporal dependence, the Hausman test indicates that the coefficient estimates from pooled OLS estimation are consistent.” • Different PCSE’s can give different results.
DHW Specification Test: Remarks • Issues with Hausman tests –as discussed in Wooldridge (2009): (1) Fail to reject means either: - FE and RE are similar -i.e., this is great! - FE estimates are very imprecise - Large differences from RE are nevertheless insignificant - That can happen if the data are awful/noisy. Be careful. (2) Watch for difference between “statistical significance” and “practical significance.” - With a huge sample, the Hausman test may "reject" even though RE is nearly the same as FE - If differences are tiny, you can feel comfortable using the REM. (3) PCSE’s matter => Q: Which ones to use?
Allison’s Hybrid Approach • Allison (2009) suggests a ‘hybrid’ approach that provides the benefits of FE and RE – Also discussed in Gelman & Hill (2007) textbook – Builds on idea of decomposing X into mean, deviation Steps: – 1. Compute case-specific mean variables – 2. Transform X variables into deviations (within transformation) – 3. Do not transform the dependent variable Y – 4. Include both X deviation & X mean variables – 5. Estimate with a RE model
Allison’s Hybrid Approach • Benefits of hybrid approach: – 1. Effects of “X-deviation” variables (within effects) are equivalent to results from a FEM. • All time-constant factors are controlled – 2. Effects of time-constant X variables (between effects) – 3. You can build a general multilevel model • Random slope coefficients; more than 2 level models… – 4. You can directly test FE vs RE – No Hausman test needed • REM: X-mean and X-deviation coefficients should be equal • Conduct a Wald test for equality of coefficients – Also differing X-mean & X-deviation coefficents are informative.
Measurement Error • It can have a severe effect on panel data models. • It is no longer obvious that a panel data estimator is preferred to a cross-section estimator. • Measurement error often leads to “attenuation” of signal to noise ratio in panels – biases coefficients towards zero.
Heteroskedasticity - Review • Given that there is a cross-section component to panel data, there will always be a potential for heteroskedasticity. • Although there are various tests for heteroskedastcity, as with autocorrelation there is a tendency to automatically use NW’s PCSE, which removes the problem. This is fine for the FEM. But not for the REM: REM tell us the structure of heteroscedasitcity => GLS! • Baltagi (1995) allows ui to be heteroscedastic. There is an efficiency problem, however, since only one observation --the estimated error ui,, repeated Ti per individual-- will be used to estimate s2u(i).
Autocorrelation - Review • Although different to autocorrelation using the usual univariate models, a version of the Breusch-Pagan LM test can be used. • To deal with autocorrelated errors, we can use the usual methods, say pseudo-differencing. In general, we will estimate using the LSDV residuals. • If we allow to vary with i, we lose power (in general, T is small). • We can also reformulate the model, by building a ‘Dynamic Model,’ which basically involves adding a lagged dependent variable. • As usual, OLS plus NW’s PCSE can help you to avoid a complicated FGLS estimation. (The usual problems with HAC SE apply.)
PCSE - Review • Key Assumption – Correlations within a cluster (a group of firms, a region, different years for the same firm, different years for the same region) are the same for different observations. • Procedure – (1) Identify clusters using economic theory (industry, year, etc.) – (2) Calculate clustered standard errors – (3) Try different ways of defining clusters and see how the estimated SE are affected. Be conservative, report largest SE. • Performance – Not a lot of studies –some simulations done for simple DGPs. – PCSE’s coverage rates are not very good (typically below their nominal size). – PCSE using HAR estimators is a good idea.
Dynamic Panel Models • What if we have a dynamic process? • Examples – Cigarette consumption – lots of inertia. – Behavioral finance/momentum models --lagged returns matter. – We might consider a model like:
yit yit 1 xit ci it • Now, yit-1 is included as an explanatory variable. Now, => ci and yit1 are correlated! - Issue: FE, RE estimators are biased. • Time-demeaned (or quasi-demeaned) yit-1 correlated with error. • FE is biased for small T. Gets better as T gets bigger (30+). • RE also biased.
Dynamic Panel Models • One solution: Use FD and instrumental variables – Strategy: If there’s a problem between the error, it, and lag yi, let’s find a way to calculate a new version of lag yi that doesn’t pose a problem • Idea: Further lags of yi are not an issue in a FD model. • Use them as “instrumental variables,” as a proxy for lag yi. – Arellano-Bond (1991): GMM estimator • A FD estimator. • Lag of levels as an instrument for differenced yi. – Arellano-Bover (1995)/Blundell-Bond: “System GMM” • Expand on this by using lags of differences and levels as IVs. • Generalized Method of Moments (GMM) estimation.
Dynamic Panel Models: GMM (Arellano/Bond/Bover, Journal of Econometrics, 1995) y i,t βx i,t y i,t 1 i,t ui Dynamic random effects model for panel data. Can't use least squares to estimate consistently. Can't use FGLS without estimates of parameters. Many moment conditions: What is orthogonal to the period 1 disturbance? E[(i,1 ui )x i,1 ] 0 = K orthogonality conditions, K+1 parameters E[(i,1 ui )x i,2 ] 0 = K more orthogonality conditions, same K+1 parameters ... E[(i,1 ui )x i,1 ] 0 = K orthogonality conditions, same K+1 parameters The same variables are orthogonal to the period 2 disturbance. There are hundreds, sometimes thousands of moment conditions, even for fairly small models.
Dynamic Panel Models: GMM • Key usual assumptions / issues – Serial correlation of differenced errors limited to 1 lag – No overidentifying restrictions (No Hansen - Sargan test) – Q: How many instruments? • Criticisms: - Angrist and Pichke (2009): Assumptions are not always plausible. - Allison (2009) - Bollen and Brand (2010): Hard to compare models.
Dynamic Panel Models: Remarks I • General remarks: - Ignoring dynamics –i.e., lags– not a good idea: omitted variables problem. - It is important to think carefully about dynamic processes: • How long does it take things to unfold? • What lags does it make sense to include? • With huge datasets, we can just throw lots in – With smaller datasets, it is important to think things through.
Dynamic Panel Models: IV Framework • Traditional IV panel estimator:
y it x it Yit c i it • X = exogenous covariates • Y = other endogenous covariates (may be related to εit) • ci = unobserved unit-specific characteristic • εit = idiosyncratic error – Treat ci as random, fixed, or use differencing to wipe it out – Use contemporaneous or lagged X and (appropriate) lags of Y as instruments in two-stage estimation of yit. Note: This approach works well if lagged Y is plausibly exogenous.
Time Series Cross Section (TSCS) Data • Time Series Cross Section (TSCS) Data - Panel Data with large T, small N - Example I: economic variables for industrialized countries Often 10-30 countries Often around 30 to 40 years of data - Example II: financial variables Often more than 1,000 firms Often 40-50 years of data for well-established markets (10-30 for emerging markets). – Beck’s (2001) advice: • No specific minimum for T; but be suspicious of T<10 • Large N is not required (though, it does not hurt)
Time Series Cross Section (TSCS) Data • Typical complications of TSCS Data - Heteroskedasticity and autocorrelation - Autocorrelation cannot be ignored. - As N grows, the probability of cross-correlations (contemporaneous and time-varying) also grows. - Correlation at same time point across cases (world factor affecting all markets) - Correlation at different time points across cases (contagion effects over time)
TSCS Data: OLS PCSE • Beck and Katz (2001) – “Old” view: Use FGLS to deal with heteroskedasticity & correlated errors. • Problem: This underestimates standard errors. – New view: Use OLS regression • With FE to deal with unit heterogeneity – To address panel heteroskedasticity ⇒ With PCSE’s. • With FE to deal with unit heterogeneity – To address serial correlation ⇒ With lagged dependent variable in the model
TSCS Data: Dynamics • Beck and Katz (2009) examine dynamic models – OLS PCSE with lagged Y and FE • Still appropriate • Better than some IV estimators – But, did not compare to System GMM. • Plumper, Troeger, and Manow (2005) • FE is not theoretically justified and absorbs theoretically important variance. • Lagged Y absorbs theoretically important temporal variation • Ideally, economic theory must guide model choices.
TSCS Data: Nonstationary Data • Issue: Analysis of longitudinal (time-series) data is going through big changes • Realization that strongly trending data cause problems – Random walk / unit root (ρ=1) / I(1) / non-stationary or near integrated data. – Mixing stationary, I(0) data, with I(1) data. • The “spurious regression” problem. – Strategies: • Tests for unit roots in time series & panel data • Differencing as a solution – A reason to try FD models.
Panel Data: Final Remarks 1. Panel data strategies are taught as “fixes” – How do I “fix” unobserved effects? – How do I “fix” dynamics/serial correlation? • But, the fixes really change what you are modeling • A FE (within) model is a very different look at your data, compared to pooled OLS. • Goal: learn the “fixes.” But, think about interpretation. 2. Lots of disagreements in literature • What is the best “fix”? • Reformulation of model; final model should have “no problems” –LSE approach.
Panel Data: Final Remarks 3. Very important: Try a wide range of models • If your findings are robust, you are doing fine. • If not, differences may help you figure out a better model. • In both cases, you will not get “surprised” when your results go away after following the suggestion of a referee!