Practical Regression: Fixed Effects Models
This is the fifth in a series of lecture notes which, if tied together into a textbook, might be entitled "Practical Regression." The purpose of the notes is to supplement the theoretical content of most statistics texts with practical advice based on nearly three decades of experience of the author, combined with over one hundred years of experience of colleagues who have offered guidance. As the title "Practical Regression" suggests, these notes are a guide to performing regression in practice. This technical note discusses fixed effects models. Though a unified example, the note shows how omitted variable bias can plague estimates in cross-section regressions and how focusing attention on intragroup ("within") variation over time can allow for identification of regression coefficients that are free of potential bias. The note demontrates the mathematical principles behind fixed effects modeling and also explains why, in some cases, it may be preferable not to include fixed effects.