Power calculation for non-normal distribution I'm trying to do a power analysis for a research study aimed at characterizing between group differences in depression for 2 different populations. As I understand it, depression scores are a non-normal distribution in the general population.
For a P value <0.05, specified power of 0.8 and a difference of PHQ-9 scores between populations of 7 (scale is 0-27), what is the best/proper power analysis equation?
Are there any other variables I need given that PHQ-9 scores are a non-normal distribution (right skewed)?
Thank you so much!!

If you have a representative sample of the scores you can compute the power or sample size for a Wilcoxon test.  These are functions of the entire frequency distribution of the scores, and the effect size is stated in terms of an odds ratio, which can be converted to a difference in means for interpretation.  Detailed examples with R code are in Chapter 7 of BBR.  Note that the Wilcoxon test is a special case of the proportional odds model, and this model can handle arbitrarily strong clumping at zero.
