METHODS	The models included : i ) the Mantel-Haenszel test , ii ) logistic regression with fixed center effect and fixed treatment effect , iii ) center-size weighted and iv ) un-weighted logistic regression with fixed center effect and fixed treatment-by-center interaction , iv ) logistic regression with random center effect and fixed treatment effect , v ) logistic regression with random center effect and random treatment-by-center interaction , and vi ) generalized estimating equations .\