Glossary Entry

Fixed Effect

An ordinary regression coefficient treated as an unknown constant shared across all observations, in contrast to a random effect drawn from a distribution per group.

Models Statistics

Also called: fixed effects

Seed source: UCLA OARC, Introduction to Linear Mixed Models

In a mixed effects model, the fixed effects are the coefficients estimated as unknown constants that apply to everyone: the population-level intercept and slopes a stakeholder would ask about. They are what remains of ordinary regression inside the mixed model.

The word carries a second, related meaning in econometrics, where a “fixed effects model” estimates a separate dummy coefficient per group instead of drawing group effects from a distribution. That estimator makes no assumption that group effects are uncorrelated with the predictors, which is why it is often preferred for causal readings even at the cost of noisier per-group estimates.