Sample Size Calculation
We used the DWELL index as our primary outcome measure for the calculation. As this is a novel intervention, and with the absence of other relevant data from the literature, we based our calculations on initial data from the pilot study and from the index validation study. We analyzed matched observations from pre- and postintervention questionnaires and used WinPepi to perform the calculations. The mean prepilot DWELL index was 47.5 (SD 18.07), and the mean postpilot DWELL index was 63.57 (SD 11.98). The Pearson correlation coefficient was For a difference of this magnitude to be detected, we used three different scenarios: (1) a 15-point increase in DWELL score found in the pilot; (2) a 10-point difference for more conservative results; and (3) a 5-point difference assuming that the control group could be contaminated by trial participation, with a smaller effect compared with the intervention group. We assumed that the participants in the RCT might be less engaged with the group than the participants in the pilot study, leading to a reduced expected effect of the program.We calculated the required sample for power at 80% and 90% and the 2-sided significance level at 5% and 1%. Using 80% power, a 5% 2-sided significance level, and 5 as the difference to be detected, we found that 146 participants (73 pairs) were required as our minimum number to recruit. As online interventions suffer from a high attrition rate [