Bayesian Sensitivity Analysis for Offline Policy Evaluation Jongbin Jung Ravi Shroff
[email protected] [email protected] Stanford University New York University Avi Feller Sharad Goel UC-Berkeley
[email protected] [email protected] Stanford University ABSTRACT 1 INTRODUCTION On a variety of complex decision-making tasks, from doctors pre- Machine learning algorithms are increasingly used by employ- scribing treatment to judges setting bail, machine learning algo- ers, judges, lenders, and other experts to guide high-stakes de- rithms have been shown to outperform expert human judgments. cisions [2, 3, 5, 6, 22]. These algorithms, for example, can be used to One complication, however, is that it is often difficult to anticipate help determine which job candidates are interviewed, which defen- the effects of algorithmic policies prior to deployment, as onegen- dants are required to pay bail, and which loan applicants are granted erally cannot use historical data to directly observe what would credit. When determining whether to implement a proposed policy, have happened had the actions recommended by the algorithm it is important to anticipate its likely effects using only historical been taken. A common strategy is to model potential outcomes data, as ex-post evaluation may be expensive or otherwise imprac- for alternative decisions assuming that there are no unmeasured tical. This general problem is known as offline policy evaluation. confounders (i.e., to assume ignorability). But if this ignorability One approach to this problem is to first assume that treatment as- assumption is violated, the predicted and actual effects of an al- signment is ignorable given the observed covariates, after which a gorithmic policy can diverge sharply.