Background: Researchers have begun to investigate the issue of epoch length on children's physical activity using small sample sizes, and to date, no studies have been conducted in adolescents.
Purpose: The purpose of this study was to investigate the effect of different epoch lengths (5, 15, 30, and 60 s) on derived levels of physical activity in both a child and an adolescent sample.
Methods: Three hundred and eleven children age 7-11 yr and 234 adolescents age 12-16 yr were asked to wear an accelerometer during waking hours for 7 d. The epoch was set at 5 s, and when data were downloaded, the activity counts were then reintegrated into 15-, 30-, and 60-s epochs.
Results: A significant epoch effect was seen for time spent in vigorous physical activity, light physical activity, and rest in the child and adolescent samples and moderate-to-vigorous and moderate physical activities in the child sample only. The Bland-Altman analysis showed reasonable agreement on moderate-to-vigorous, moderate, vigorous, and light physical activities and rest between 5- and 15-, 5- and 30-, and 30- and 60-s epochs in the child sample and between 5- and 15-, 5- and 30-, and 15- and 30-s epochs in the adolescent sample.
Conclusions: A short epoch is strongly recommended for child and adolescent samples to obtain a "real" picture of young people's physical activity behavior and to prevent accumulation of counts reflecting the average activity level when longer epochs are used. Activity prevalence studies using epoch lengths of 5 and 60 s in a child or an adolescent sample should not be compared nor should 15- and 60- and 30- and 60-s epochs in an adolescent sample.