For estimating regressions for repeated measures outcome data, a popular choice is the population average models estimated by generalized estimating equations (GEE). We review in this report the derivation of the robust inference (sandwich-type estimator of the standard error). In addition, we present formally how the approximation of a misspecified working population average model relates to the true model and in turn how to interpret the results of such a misspecified model.
Hubbard, Alan E. and van der Laan, Mark J., "Nonparametric population average models: deriving the form of approximate population average models estimated using generalized estimating equations" (June 2009). U.C. Berkeley Division of Biostatistics Working Paper Series. Working Paper 251.