School-Level Intraclass Correlation for Physical Activity in Adolescent Girls

David M. Murray, Diane J. Catellier, Peter J. Hannan, Margarita S. Treuth, June Stevens, Kathryn H. Schmitz, Janet C. Rice, Terry L. Conway

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Abstract

Purpose: The Trial for Activity in Adolescent Girls (TAAG) is a multi-center group-randomized trial to reduce the usual decline in moderate to vigorous physical activity (MVPA) among middle-school girls. In group-randomized trials, the group-level intraclass correlation (ICC) has a strong inverse relationship to power and a good estimate of ICC is needed to determine sample size. As a result, we conducted a substudy to estimate the school-level ICC for intensity-weighted minutes of MVPA measured using an accelerometer. Methods: To estimate the ICC, each of six sites recruited two schools and randomly selected 45 eighth grade girls from each school; 80.7% participated. Each girl wore an Actigraph accelerometer for 7 d. Readings above 1500 counts per half minute were counted as MVPA. These counts were converted into metabolic equivalents (MET) and summed over 6 a.m. to midnight to provide MET-minutes per 18-h day of MVPA. Minutes of MVPA per 18-h day also were calculated ignoring the MET value. Results: The unadjusted school-level ICC for minutes of MVPA was 0.0205 (95%CI: -0.0079, 0.1727) and for MET-minutes of MVPA was 0.0045 (95% CI: -0.0147, 0.1145). Adjustment for age and BMI had no measurable effect, whereas adjustment for ethnicity reduced both ICC; adjusted values were 0.0175 (95% CI: -0.0092, 0.1622) for minutes of MVPA and 0.0000 (95% CI: -0.0166, 0.0968) for MET-minutes of MVPA. This information was used to calculate the number of schools and girls needed for TAAG to have 90% power to detect a 50% reduction in the decline of MET-minutes of MVPA between sixth and eighth grade. Conclusions: The results called for 36 schools in TAAG, with 120 girls invited for measurements at each school, and a minimum participation rate of 80%.

Original languageEnglish (US)
Pages (from-to)876-882
Number of pages7
JournalMedicine and science in sports and exercise
Volume36
Issue number5
DOIs
StatePublished - May 2004

All Science Journal Classification (ASJC) codes

  • Orthopedics and Sports Medicine
  • Physical Therapy, Sports Therapy and Rehabilitation

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    Murray, D. M., Catellier, D. J., Hannan, P. J., Treuth, M. S., Stevens, J., Schmitz, K. H., Rice, J. C., & Conway, T. L. (2004). School-Level Intraclass Correlation for Physical Activity in Adolescent Girls. Medicine and science in sports and exercise, 36(5), 876-882. https://doi.org/10.1249/01.MSS.0000126806.72453.1C