I am running the gee package to to logit regressions with clustered standard errors. I can one model to run correctly. There are other models that are run on subsets of the the years in the data. When I run it on subsets, it returns the error rank-deficient model matrix. This seems strange that we'd have rank defficiency just by dropping some observations at the beginning/end of our dataset based on the year. Any help/insight would be greatly appreciated!! EXL -- ERIC LIN Technology and Operations Management Harvard Business School Boston, MA 02163 elin at hbs.edu