ship between physical activity and mortality. A variety of techniques are available for assessing levels of physical activity, including use of self-reported data from diaries or recall questionnaires and objective measurement using pedometers, accelerometers, or other similar devices. Each method has advantages and drawbacks (Vanhees et al., 2005; Westerterp, 2009). Self-reported data are easier to obtain but are vulnerable to recall error, reporting bias (if respondents systematically tend to exaggerate or downplay their level of physical activity), and cross-national differences in interpretation of questions. The net result of such measurement errors is generally to weaken any effect that is present in the data; thus to the extent that these issues are a problem, the effects of physical activity are likely to be greater than estimated, rather than less.
Cross-national studies also need to take account of cross-cultural differences in customary forms of physical activity (Steptoe and Wikman, 2010). For example, bicycling, ice-skating, playing softball, lawn bowling, and playing boules are all activities whose popularity varies enormously across countries. Differences among countries in the physical demands of occupations, urban design, or the built environment can also generate differences in moderate or light activity that occurs as part of everyday life, which can easily be missed in surveys of leisure activities or purposeful exercise. Objective measurement of physical activity is more expensive but usually preferable as it can eliminate some of the problems associated with obtaining information from self-reports.
A variety of methods for obtaining objective measures are available. The most useful for population studies is the use of accelerometers (Westerterp, 2009). One of the most comprehensive measures of the level of physical activity among Americans was performed by Troiano and colleagues on more than 6,000 participants in the 2002–2003 National Health and Nutritional Examination Survey (NHANES). These subjects, who ranged in age from 6 to older adults, agreed to wear an accelerometer for at least an entire day and, for nearly 5,000 of them, 4 or more days. The accelerometer-derived data indicated that very few Americans engage in the recommended level of physical activity—at least 30 minutes of moderate or vigorous activity at least 5 days per week—and the percentage that do so decreases with age. Teenagers (aged 16–19) performed poorly, with only 7.1 percent of boys and 4.1 percent of girls getting the recommended minimum amount of exercise. But adults fared even worse: among those aged 20–59, just 3.8 percent of men and 3.2 percent of women performed enough exercise, and only 2.5 percent of older men and 2.3 percent of older women (Troiano et al., 2008).1