RESUMO
BACKGROUND: Objective methods can improve accuracy of physical activity measurement in field studies but uncertainties remain about their use. METHODS: Children age 11 years from the Avon Longitudinal Study of Parents and Children (ALSPAC), were asked to wear a uni-axial accelerometer (MTI Actigraph) for 7 days. RESULTS: Of 7159 children who attended for assessment, 5595 (78%) provided valid measures. The reliability coefficient for 3 days of recording was .7 and the power to detect a difference of 0.07 SDs (P