The Trial for Activity in Adolescent Girls (TAAG) is a group-randomized trial (GRT) to reduce the usual decline in moderate to vigorous physical activity (MVPA) among middle school girls. We report the school-level intraclass correlation (ICC) for MVPA from the TAAG baseline survey of sixth grade girls and describe the relationship between the schedule of data collection and the ICC.
Each of six sites recruited six schools and randomly selected 60 sixth grade girls from each school; 74.2% participated. Girls were grouped in waves defined by the date measurements began and asked to wear an Actigraph accelerometer for 6 d. Occasional missing data were replaced by imputation, and counts above 1500 per 30 s were treated as MVPA, converted into metabolic equivalents (METs), and summed over 6 a.m.-midnight to provide MET-minutes per 18-h day. Mixed-model regression was used to estimate ICC.
The school-level ICC were higher when estimated from a single wave compared with three waves (e.g., 0.057 vs 0.022) and across weekdays compared with weekend days (e.g., 0.024 vs 0.012). Power in a new trial would be greater with some schedules (e.g., 88% given three waves and 6 d) than with others (e.g., 23% given one wave and Tuesday only).
The schedule of data collection can have a dramatic effect on the ICC for MVPA. In turn, this can have a dramatic effect on the standard error for an intervention effect and on power. Investigators will need to consider the expected magnitude of the ICC and the validity of the MVPA estimates associated with their data collection schedule in planning a new study.
1Division of Epidemiology, School of Public Health, The Ohio State University, Columbus, OH; 2Departments of Nutrition and Epidemiology, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, NC; 3Division of Epidemiology and Community Health, School of Public Health, University of Minnesota, Minneapolis, MN;4Department of Biostatistics, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, NC; 5Department of Exercise Science, University of South Carolina, Columbia, SC; 6Graduate School of Public Health, San Diego State University, San Diego, CA; 7Department of Biostatistics, School of Public Health and Tropical Medicine, Tulane University, New Orleans, LA; and 8Division of Epidemiology and Clinical Applications, National Heart Lung and Blood Institute, Bethesda, MD
Address for correspondence: David M. Murray, Ph.D., Chair, Division of Epidemiology, School of Public Health, The Ohio State University, B222 Starling Loving Hall, 320 West 10th Avenue, Columbus, Ohio 43210; E-mail: email@example.com.
Submitted for publication June 2005.
Accepted for publication November 2005.