Sample size and power analysis
For this intervention development pilot project, assessment of feasibility and acceptability of the intervention and research procedures was the primary goal. Nonetheless, pilot data can be used to demonstrate whether the effects of treatment look promising across a set of outcome variables, and to suggest, in concert with results from larger scale clinical trials in related fields, the range of effect sizes that would be reasonable to expect in a future trial. As a result, the goal was to obtain estimates of the differences between treatment conditions with 95% confidence intervals and effect size estimates. The actual enrollment was 50 participants with an 88% retention rate (for an evaluable sample of n = 44).Given the sample size, the unadjusted effect size Cohen’s d = 0.86 was detectable in between-group comparisons with power of .80 at .05 level of significance in two-sided tests. In longitudinal analysis of two repeated measures adjusted for baseline, with correlations between pairs of repeated measures ranging from 0.1 to 0.6, the detectable adjusted effect sizes ranged from 0.57 to 0.66. If the observed effect size was smaller, statistical significance in between-group comparisons would not be reached, however statistical significance was not the goal of this pilot study. Sample size guidelines for treatment development projects [