1. A computer-implemented method, comprising: receiving, at data processing apparatus, exposure data and control data, the exposure data describing exposed subjects and the control data describing control subjects, each of the exposed subjects being a subject that was exposed to a treatment, and each of the control subjects being a subject that was not exposed to the treatment, wherein the exposure data includes values for a plurality of features for each exposed subject, the plurality of features for each exposed subject including features describing actions taken by the exposed subject, and wherein the control data includes values for a plurality of features for each control subject, the plurality of features for each control subject including features describing actions taken by the control subject; determining, from the exposure data and the control data, a test statistic for each of a plurality of subject actions including a desired action and a plurality of placebo actions, wherein the desired action is an action that is hypothesized to be possibly affected by the treatment and each placebo action is an action that is hypothesized to be unaffected by the treatment, and wherein the test statistic for an action corresponds to an estimated effect that exposure to the treatment has on a likelihood that a subject will take the action; comparing, in the data processing apparatus, the test statistic for the desired action to the test statistics for the plurality of placebo actions; and determining, in the data processing apparatus, whether the test statistic for the desired action is statistically significant according to the comparison.