Marginal modeling of multilevel binary data with time-varying covariates

We propose and compare two approaches for regression analysis of multilevel binary data when clusters are not necessarily nested: a GEE method that relies on a working independence assumption coupled with a three.step method for obtaining empirical standard errors, and a likelihood.based method implemented using Bayesian computational techniques.Implications of time.varying endogenous covariates are addressed.The methods are illustrated using data from the Breast Cancer Surveillance Consortium to estimate mammography accuracy from a repeatedly screened population.