I have 2 groups with unequal sizes (control 70% and test 30%) and need to find out if there is a significant difference between these groups. I've read that MW test works fine with unequal sized samples, but the statistical power will diminish as the group sizes become more unequal. Hence I was adviced to make groupes of equal sizes (by multiplying by coefficient). I worry that in this case the stat power will diminish as well, however couldn't find published references to prove that.
Could anyone help me with advice what I should do : leave it as it is or try making the groups equally sized.
Appreciate all the help)