A sample size was calculated to have 80% power to detect differences in the proportion of participants adhering to PrEP among the three study arms (Aim 2) using the chi-square test for independence with two degrees of freedom, with an alpha of 0.05, and assuming an attrition rate of 10% for those who (1) withdraw voluntarily from the study, (2) initiate on PrEP but withdraw due to adverse events (e.g. hepatic or renal abnormalities), or (3) become ineligible for PrEP (e.g. seroconvert). The effect size estimate was informed by prior experience from the field, where an effect size of 0.30 has been found between ART clients receiving some adherence support and those receiving none. However, we use a more conservative effect size of 0.15, to account for the fact that adherence support may not lead to the same gains in adherence among HIV-negative PrEP clients as it does with HIV-positive ART clients. The sample size was calculated to be 160 per arm (480 total).

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