Double Machine Learning for Static Panel Models with Fixed Effects

Recent advances in causal inference have seen the development of methods which make use of the predictive power of machine learning algorithms. In this paper, we use double machine learning (DML) (Chernozhukov et al., 2018) to approximate high-dimensional and non-linear nuisance functions of the confounders to make inferences about the effects of policy interventions from panel data. We propose new estimators by adapting correlated random effects, within-group and first-difference estimation for linear models to an extension of Robinson (1988)'s partially linear regression model to static panel data models with individual fixed effects and unspecified non-linear confounder effects. Using Monte Carlo simulations, we compare the relative performance of different machine learning algorithms and find that conventional least squares estimators performs well when the data generating process is mildly non-linear and smooth, but there are substantial performance gains with DML in terms of bias reduction when the true effect of the regressors is non-linear and discontinuous. However, inference based on individual learners can lead to badly biased inference. Finally, we provide an illustrative example of DML for observational panel data showing the impact of the introduction of the minimum wage on voting behavior in the UK.