Hi! I need help with my dissertation regarding sample size calculation. I will be grateful if anybody can help me out with this...

My study involves two aspects...

1. Comparison of a quantitative variable - between cases and controls

2. The cases are further classified into four groups... followed by Comparison of the quantitative variable between the four groups.

Since the variable follows skewed distribution, I will be using Mann Whitney U test for comparison between cases and controls, Kruskal Walis with post hoc Bonferroni for comparison between the four groups.

What is the formula for calculation of minimum sample size required in this scenario?