It is unclear whether it is possible to accurately estimate physical activity energy expenditure (PAEE) by self-report in youth. Objective: We assessed the validity and reliability of 4 self-reports to assess PAEE and time spent at moderate and vigorous intensity physical activity (MVPA) over the previous week in British young people between 4 and 17 y of age. Design: PAEE and MVPA were derived from the Children's Physical Activity Questionnaire, Youth Physical Activity Questionnaire, and

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... h Adolescent Physical Activity Questionnaire; a lifestyle score indicative of habitual activity was derived from the Child Heart and Health Study in England Questionnaire. These data were compared with criterion methods, PAEE, and MVPA derived from simultaneous measurements by doubly labeled water and accelerometry in 3 age groups: 4-5 y (n ¼ 27), 12-13 y (n ¼ 25), and 16-17 y (n ¼ 24). Validity was assessed by using Spearman correlations and the Bland-Altman method, and reliability was assessed by using intraclass correlation coefficients. Results: The strength of association between questionnaire and criterion methods varied (r ¼ 0.09 to r ¼ 0.46). Some questionnaires were able to accurately assess group-level PAEE and MVPA for some age groups, but the error was large for individual-level estimates throughout. Reliability of the Youth Physical Activity Questionnaire and Child Heart and Health Study in England Questionnaire was good (intraclass correlation coefficient: 0.64-0.92). Conclusions: Absolute PAEE and MVPA estimated from these selfreports were not valid on an individual level in young people, although some questionnaires appeared to rank individuals accurately. Age (the outcome of interest) and whether individual or group-level estimates are necessary will influence the best choice of self-report method when assessing physical activity in youth. Am J Clin Nutr 2009;89:862-70.