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Improving the Interpretation of Fixed Effects Regression Results

  • Jonathan Mummolo and Erik Peterson

Fixed effects estimators are frequently used to limit selection bias. For example, it is well known that with panel data, fixed effects models eliminate time-invariant confounding, estimating an independent variable’s effect using only within-unit variation. When researchers interpret the results of fixed effects models, they should therefore consider hypothetical changes in the independent variable (counterfactuals) that could plausibly occur within units to avoid overstating the substantive importance of the variable’s effect. In this article, we replicate several recent studies which used fixed effects estimators to show how descriptions of the substantive significance of results can be improved by precisely characterizing the variation being studied and presenting plausible counterfactuals. We provide a checklist for the interpretation of fixed effects regression results to help avoid these interpretative pitfalls.

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Jonathan Mummolo is an Assistant Professor of Politics and Public Affairs, Princeton University, Robertson Hall, Room 411, Princeton, NJ 08544 ( Erik Peterson is a Post-Doctoral Fellow at Dartmouth College, 026 Silsby Hall, 3 Tuck Mall, Hanover, NH, 03755 ( The authors are grateful to Justin Grimmer, Dorothy Kronick, Jens Hainmueller, Brandon Stewart, Jonathan Wand and anonymous reviewers for helpful feedback on this project. To view supplementary material for this article, please visit

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Political Science Research and Methods
  • ISSN: 2049-8470
  • EISSN: 2049-8489
  • URL: /core/journals/political-science-research-and-methods
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