I'm running a model in Proc Logistic, modeling the probability of a negative culture (Y/N) with the dichotomous predictors drug (Y/N) and disease severity (Y/N). I also need to include study site (34 of these and many are sparsely populated) as it's a confounder. However, when I do, the model falls apart ("Quasi-complete separation of data points detected...WARNING: The maximum likelihood estimate may not exist....WARNING: The validity of the model fit is questionable."), I guess because there are so many sites. How do I approach this problem? Should I group the sites into several chunks? I don't often run multivariate models. Thank you.
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