Sample size calculation
RECRUITMENT
TRIUMPH was designed to detect a mean difference of 2.0 points between arms on the IPSS at 12 months post-randomisation with 90% power, as this is the mean decrease in IPSS among men who rate their condition as slightly improved when the baseline scores are <20 points.Early in the study, however, we observed variability between sites in the number of participants recruited, thus necessitating a revision of the sample size calculation. Using recruitment data available at the time, we estimated that the mean number of participants who would need to provide consent at each site would be 26 and that the coefficient of variation of the mean cluster size would be 0.26. Ignoring clustering and loss to follow-up, we determined that 263 participants in total would be required to detect a difference of 2 units in IPSS with 90% power assuming a common standard deviation of 5. Our updated design effect assumed an intraclass correlation coefficient of 0.05; a mean of 26 patients consenting in each site but only 70% providing primary outcome data resulting in a mean cluster size of 18.2; and a 0.26 coefficient of variation in cluster sizes. Under these revised assumptions, the design effect is 1.92, meaning that 506 participants in total would be required to provide primary outcome data. Given our assumed loss to follow-up, 724 patients needed to provide consent to participate in the study, and as each site was expected to obtain consent for 26 patients, this translated to 28 sites in total. Allowing for some practices not to perform as expected, 30 were ultimately recruited in agreement with the trial management group, steering committee, and funder.