Douglas Dion, Evidence and Inference in the Comparative Case Study, Comparative Politics, Vol. 39, 1998, pp. 127-45.

Abstract

Comparative case studies often rely on a practice known as selecting on the dependent variable. This techniques involves choosing some phenomenon of political interest, gathering data on occurrences of the phenomenon, then determining what characteristics the occurrences have in common. This techniques has recently come under rather sharp attack. If observations are selected based on the value of the dependent variables, then estimates obtained by ordinary least squares will be biased. This bias can not be corrected by the introduction of control variables. Gathering more data will not solve the problem.

The paper offers a qualified defense of analyses of a small number of cases selected on the dependent variable. As is well-known, selecting on the dependent variable is perfectly admissible if one is evaluating necessary (as opposed to sufficient) conditions. However, with a few exceptions, this argument has one unnoticed in the debate on case selection. By concentrating on the logic of testing necessary conditions, this paper attempts to show hat the methodological implications of selecting on the dependent variable are much less threatening to the validity of small-n comparative case study work than has often been claimed.