To date, epidemiologic research on AEE has relied on very crude measurement tools—primarily questionnaires. These self-reported data capture only a small fraction of the total variance in activity and are potentially confounded when they focus on leisure-time PA. By contrast, direct measurement with doubly labeled water (DLW) provides a precise, unbiased estimate of all forms of non-resting energy expenditure. DLW has also been used to validate the new generation of accelerometers which are efficient measurement tools for larger studies.