Questionnaires have become the primary method for measuring physical activity in epidemiologic studies.

- As epidemiologic studies typically involve hundreds to thousands of subjects, questionnaires are extremely practical and cost-efficient. They are easy for subjects to complete, often in self-administered form.

- Questionnaires are non-reactive meaning that they tend not to interfere with daily activities.

- Questionnaires can be tailored to the characteristics of a particular population (i.e., age, gender) or limited to a particular period of time (i.e., past year, or lifetime).

- The disadvantages of questionnaires involve their precision in measuring absolute energy expenditure. This issue along with the actual performance of such questionnaires in terms of reproducibility and validity will be considered in this lecture.