Pre-print, Working paper: For static panel data models that include endogenous time-invariant variables corre- lated with individual e¤ects, exogenous averages over time of time-varying variables can be internal instruments. To pretest their exogeneity, we .rst estimate a random e¤ects model that includes all averages over time of time-varying variables (Mundlak, 1978; Kr- ishnakumar, 2006). Internal instruments are then selected if their parameter is statistically di¤erent from zero (Mundlak, 1978; Hausman and Taylor, 1981). Finally, we estimate a Hausman-Taylor (1981) model using these internal instruments. We then evaluate the bi- ases of currently used alternative estimators in a Monte-Carlo simulation: repeated between, ordinary least squares, two-stage restricted between, Oaxaca-Geisler estimator, .xed e¤ect vector decomposition, and random e¤ects (restricted generalized least squares).