Sources of variance in daily physical activity levels in the seasonal variation of Blood Cholesterol Study

The authors examined sources of variance in self-reported physical activity in a cohort of healthy adults (n = 580) from Worcester, Massachusetts (the Seasonal Variation of Blood Cholesterol Study, 1994-1998). Fifteen 24-hour physical activity recalls of total, occupational, and nonoccupational acti vity (metabolic equivalent-hours/day) were obtained over 12 months. Random effects models were employed to estimate variance components for subject, s eason, day of the week, and residual error, from which the number of days o f assessment required to achieve 80% reliability was estimated. The largest proportional source of variance in total and nonoccupational activity was within-subject variance (50-60% of the total). Differences between subjects accounted for 20-30% of the overall variance in total activity, and season al and day-of-the-week effects accounted for 6% and 15%, respectively. For total activity, 7-10 days of assessment in men and 14-21 days of assessment in women were required to achieve 80% reliability. For nonoccupational act ivity, 21-28 days of assessment were required. This study is among the firs t to have examined the sources of variance in daily physical activity level s in a large population of adults using 24-hour physical activity recall, T hese findings provide insight for understanding the strengths and limitatio ns of short term and long term physical activity assessments employed in ep idemiologic studies.