Previous authors have reported an incidence of acute ankle sprains in athletes that ranged from 11% to 20%. On the basis of these data, we projected a sprain rate for this study of 15% in the control group and 7.5% in the intervention group. Using these rates, we calculated the sample size to determine statistical significance by using cluster randomization as well as a 2-tailed test with α = .05 and 1-β = .80. The estimated sample size for this study thus obtained was 680 (340 in both the intervention and control groups).