Table 2

Sample size for 90% power and one-sided 0.025 significance level

Probability of primary outcome in the control group*Probability of primary outcome in the experimental group*N evaluable per armTotal sample size for RCT
  • *On the basis of the pilot study, 9 of 25 participants (95% CI: 18% to 57%) had condomless sex at least once in the 3-week period immediately following the video intervention. According to a similar RCT we conducted in 2014, the loss to follow-up rate was about 10%; adjustment for loss to follow-up required (N evaluable per arm)/(1–0.1) to be enrolled. A non-inferiority limit of 0.1 was used for all calculations.

  • RCT, randomised control trial.