Summary: This paper concerns inferring how self-interested subjects, as opposed to altruistic investigators, evaluate treatments in social experiments. The authors argue that the attrition behavior of subjects reveals their evaluation and discuss the usefulness of using such data in performing subject-based evaluation. The authors study the causes of disagreements between investigators and subjects in evaluating treatments and empirically assess the degree to which they disagree. The paper provides an empirical framework for estimating the systematic level of disagreement in the presence of such errors. Using clinical trials, the authors find substantial evidence of overapproval by investigators in about one-third of the trials analyzed.