title: School-level intraclass correlation for physical activity in adolescent girls.
abstract: OBJECTIVE
  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%.
authors.affiliation: Department of Psychology, College of Arts and Sciences, University of Memphis, Memphis, TN 38152-3230, USA. d.murray@mail.psyc.memphis.edu
authors.forename: David M
authors.initials: DM
authors.lastname: Murray
authors.forename: Diane J
authors.initials: DJ
authors.lastname: Catellier
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Hannan
authors.forename: Margarita S
authors.initials: MS
authors.lastname: Treuth
authors.forename: June
authors.initials: J
authors.lastname: Stevens
authors.forename: Kathryn H
authors.initials: KH
authors.lastname: Schmitz
authors.forename: Janet C
authors.initials: JC
authors.lastname: Rice
authors.forename: Terry L
authors.initials: TL
authors.lastname: Conway
mesh_terms: D000054:Acceleration; D000293:Adolescent; D000294:Adolescent Behavior; D000704:Analysis of Variance; D003627:Data Interpretation, Statistical; D015444:Exercise; D005260:Female; D006801:Humans; D010809:Physical Fitness; D012574:Schools; D013997:Time Factors