In this paper I show that the conditional fixed effects negative binomial model for count panel data does not control for individual fixed effects unless a very specific set of assumptions are met. I also propose a score test to verify whether these assumptions are met.

This paper estimates the employer-size wage effect on returns to unobservable skills and measured human capital variables using a novel methodology that allows us to estimate a high number of interactions between unobserved effects and firm size. Our results show that in large firms, returns to...