statsmodels/statsmodels — pull request #2162 by kshedden (2014-12-31 23:43:05+00:00)
Title: EqClass covariance structure for GEE

This PR adds a new covariance structure to GEE that allows the covariances among pairs of observations to be specified through a collection of equivalence classes. All pairs of observations within an equivalence class have the same covariance.