Estimates of intraclass correlation for variables related to behavioral HIV/STD prevention in a predominantly African American and Hispanic sample of young women.

Studies designed to evaluate HIV and STD prevention interventions often involve random assignment of groups such as neighborhoods or communities to study conditions (e.g., to intervention or control). Investigators who design group-randomized trials (GRTs) must take the expected intraclass correlation coefficient (ICC) into account in sample size estimation to have adequate power; however, few published ICC estimates exist for outcome variables related to HIV and STD prevention. The Prevention Options for Women Equal Rights (POWER) study was a GRT designed to evaluate a campaign to increase awareness and use of condoms among young African American and Hispanic women. The authors used precampaign and postcampaign data from the POWER study to estimate ICCs (unadjusted and adjusted for covariates) for a variety of sexual behavior and other variables. To illustrate the impact of ICCs on power, the authors present sample-size calculations and demonstrate how ICCs of differing magnitude will affect estimates of required sample size.