A METHOD FOR EXPLORING THE EFFECTS OF ATTRITION IN RANDOMIZED EXPERIMENTS WITH DICHOTOMOUS OUTCOMES

Attrition from conditions in randomized experiments is common. Yet it is difficult to assess the possible effects of attrition because the o utcome status of the dropouts is usually unknown. This article develop s methods to assess those effects in studies with dichotomous outcomes , illustrating the methods with randomized experiments in drug abuse t reatment, smoking cessation treatment, and alcoholism treatment. The m ethods include computing the lowest and highest possible effect sizes that could have been observed, enumerating the percent of possible stu dy outcomes below a given threshold, estimating the probability that a n outcome beyond any given threshold would be observed if all particip ants were measured, and constructing attrition analysis plots showing the effects of attrition under varied assumptions. For the kind of stu dy to which they apply, these methods should replace the treatment of missing participants as failures in an ''intent-to-treat'' analysis. A user-friendly personal computer program is available to implement all of these analyses.