Identification of self-selection biases in field experiments using stated preference experiments

If left unidentified and uncorrected, self-selection biases may greatly compromise the external validity of the outcomes of field experiments. We show that self-selection biases in terms of observed und unobserved characteristics can be well identified and corrected by means of a complementary stated preference (SP) experiment conducted among the participants and non-participants of a field experiment. In the SP experiment, respondents are confronted with hypothetical choice situations that closely resemble the choice situations present in the field experiment. The SP experiment does not only allow us to compare participants and non-participants with respect to their behavior and implied preferences in the hypothetical choice situations, but also renders it possible to infer how non-participants would have behaved if they had decided to participate, using an innovative modeling approach to elicit the corresponding preference structures. We apply this approach in the context of a large-scale field experiment in which train commuters received monetary rewards for traveling outside peak hours. We find strong self-selection biases, especially with respect to the marginal utility of income, which is significantly higher among participants of the field experiment.