# What's the difference between volatility and variance?

How do they differ in what they imply about an underlying's (or any variable's) movement?

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## 6 Answers

Volatility is typically unobservable, and as such estimated --- for example via the (sample) variance of returns, or more frequently, its square root yielding the standard deviation of returns as a volatility estimate.

There are also countless models for volatility, from old applied models like Garman/Klass to exponential decaying and formal models such as GARCH or Stochastic Volatility.

As for forecasts of the movement: well, that is a different topic as movement is the first moment (mean, location) whereas volatility is a second moment (dispersion, variance, volatility). So in a certain sense, volatility estimates do not give you estimates of future direction but of future ranges of movement.

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So what is the difference from your pov ? – nicolas Jan 27 '12 at 10:53
Volatility is not ever the "(sample) variance of returns". Volatility is always expressed as standard deviation and hence squared results in variance. – Freddy Dec 30 '12 at 12:58

By volatility people usually refer to to annualized standard deviation of an asset. For an asset it's usually quoted as a percentage of the asset price (i.e. the return volatility). For a portfolio, it is often quoted in currency units. Variance is the square of the standard deviation. It is usually not quoted directly because it doesn't have an intuitive unit of measure. Instead, it is used in variance decomposition, e.g. the idiosyncratic variance of a portfolio is 6% of the total portfolio variance.

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• The main underlying difference is in their definition. Variance has a fixed mathematical definition, however volatility does not as such. Volatility is said to be the measure of fluctuations of a process.

• Volatility is a subjective term, whereas variance is an objective term i.e. given the data you can definitely find the variance, while you can't find volatility just having the data. Volatility is associated with the process, and not with the data.

• In order to know the volatility you need to have an idea of the process i.e you need to have an observation of the dispersion of the process. All the different processes will have different methods to compute volatilities based on the underlying assumptions of the process.

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Suppose X is a random variable representing the returns of an asset having finite mean $\mu$ and variance $\sigma^2>0$.

• Variance $\sigma^2$ represents the expected squared deviation of $X$ from $\mu$. Intuitively, this is a measure of how dispersed returns are about the mean. If returns are measured in $\%$, then the units of variance are $\%^2$. However, for many people $\%^2$ is difficult to interpret.
• Volatility $\sigma$ is the square root of variance, and has units $\%$. This change in units makes volatility more interpretable, furthermore a better tool for analysis. If we further assume $X$ follows a Gaussian distribution, then $\sigma$ provides many more additional insights.

Volatility is a tool commonly used in univariate cases, e.g. when speaking of returns of one stock, one bond, or one portfolio.

In the multivariate setting, variance is used, e.g. a covariance matrix, because taking the square root of a matrix is an unecessary additional layer of complexity.

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Volatility is essentially quadratic variation. It is a property of sample paths, not probability measures. In other words, it can be calculated given a single historical path and doesn't depended upon the probability you assign to that path.

Variance, and standard deviation, are functions of the probability you assign to events.

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I think variance is called quadratic measure, or double moment, not volatility. – S_H Feb 8 '11 at 0:48

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I think you missed the point, Harpreet. If you take e.g. a standard Brownian motion and an Ornstein-Uhlenbeck (aka Vasicek) process, they both have the same (constant) instantaneous volatility. But their variances are different ; in the BM case the variance grows like time, whereas in the OU case the variance converges rapidly to a finite limit (stationary regime). – egoroff Feb 8 '11 at 10:57

the only difference between volatility and variance is the square. everything else is bs, as concept that apply to one applies to the other (historical vs implied, blabla)

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i dont know who down voted, but I stand by this comment. there is no difference except the square. if you disagree, please argue. – nicolas May 28 '11 at 16:34
Variance has properties that standard deviation does not. For example, variance is additive with stable distributions while standard deviation is not. I didn't down vote btw. – strimp099 Dec 25 '11 at 20:47
@strimp099 yes, variance is additive, and is an easier, less error prone way of manipulating additive quantities. but then standard deviation is square root additive. and then standard deviation is multiplicative. bottom line is, the difference is technical, not conceptual. – nicolas Jan 27 '12 at 10:17
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