In the context of group sequential methods(GSM) for a clinical trial with two treatment arms, it is usually assumed that data follows normal distribution $N(\mu,\sigma^2)$. Then GSM such as Pocock's test and O'Brien and Fleming test can be applied to the problem.
My question is that if data does not follow normality assumption, what kind of test should one use ? Since in reality many data are from skewed distributions,e.g., exponential, chi-square or lognormal, violation of normality should often be seen. Then in this case what method should we use to do group sequential test? Any reference or material would be appreciated.