- Questionnaires have become the primary method for measuring physical activity in epidemiologic studies. Because epidemiologic questionnaires are based upon self-reported physical activity data, documenting the reproducibility and validity of any new questionnaire is critical.

- Because we are interested in the association between physical activity and disease, the goal is most often to assess typical or usual physical activity (energy expenditure).

- Both subjective and objective comparison measures are available to validate questionnaires, each with their own strengths and weaknesses.

- The study design should be based upon the time frame of the questionnaire, variation in physical activity, and consider subject burden. Ideally, comparison measures should be administered for a sufficient number of days to represent average activity.

- Finally, because there is no perfect measure of typical or long-term energy expenditure, interpretation of results of validity/reproducibility studies should be informed by an understanding of the sources of error associated with both the questionnaire and the comparison method.