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American Journal of Epidemiology Vol. 153, No. 10 : 987-995
Copyright © 2001 by The Johns Hopkins University School of Hygiene and Public Health

ORIGINAL CONTRIBUTIONS

Sources of Variance in Daily Physical Activity Levels in the Seasonal Variation of Blood Cholesterol Study

Charles E. Matthews1, James R. Hebert1, Patty S. Freedson2, Edward J. Stanek III2, Philip A. Merriam3, Cara B. Ebbeling4 and Ira S. Ockene3

1 Department of Epidemiology and Biostatistics, School of Public Health, University of South Carolina, Columbia, SC.
2 Departments of Exercise Science and Biostatistics and Epidemiology, School of Public Health and Health Sciences, University of Massachusetts, Amherst, MA.
3 Divisions of Cardiovascular Medicine and Preventive and Behavioral Medicine, University of Massachusetts Medical School, Worcester, MA.
4 Department of Endocrinology, Children's Hospital, Boston, MA.

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 activity (metabolic equivalent-hours/day) were obtained over 12 months. Random effects models were employed to estimate variance components for subject, season, day of the week, and residual error, from which the number of days of 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 seasonal 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 activity, 21–28 days of assessment were required. This study is among the first to have examined the sources of variance in daily physical activity levels in a large population of adults using 24-hour physical activity recall. These findings provide insight for understanding the strengths and limitations of short term and long term physical activity assessments employed in epidemiologic studies.

epidemiologic measurements; exercise; reproducibility of results; seasons

Abbreviations: MET(s), metabolic equivalent(s); SD, standard deviation; SEASON, Seasonal Variation of Blood Cholesterol


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