ambiguous and inconsistent ambiguous and inconsistent "fixed effect" multiple ways of defining fixed vs random random effects long-standing confusion multiple ways of defining fixed vs random random effects Andrew Gelman ambiguous and inconsistent ambiguous and inconsistent "fixed effect" ambiguous and inconsistent ambiguous and inconsistent "fixed effect" varies widely ambiguous and inconsistent ambiguous and inconsistent Andrew Gelman multiple definitions ambiguous and inconsistent "fixed effect" ambiguous and inconsistent