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 arm | Total sample size for RCT |
|---|---|---|---|
| 0.50 | 0.50 | 526 | 1170 |
| 0.45 | 0.45 | 521 | 1158 |
| 0.40 | 0.40 | 505 | 1124 |
| 0.35 | 0.35 | 479 | 1066 |
| 0.30 | 0.30 | 442 | 984 |
*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.