Fixed Effects Regression Methods for Longitudinal Data Using SAS

2.6. A Hybrid Method

As we've seen, both the GEE method and the random effects method may produce estimates that are markedly different from the fixed effects estimates. That's because neither of those methods controls for stable, unmeasured characteristics of the individuals. There's another approach, however, that combines some of the virtues of fixed effects and random effects methods. This method produces coefficient estimates that are identical to those from the fixed effects method, but the standard errors and test statistics might be somewhat different, depending on the details of the estimation method.

The basic idea is to decompose the time-varying predictors into two parts, one representing within-person variation, the other representing ...