“Waiting for Life to Arrive”: A history of the regression-discontinuity design in Psychology, Statistics and Economics

Abstract

This paper reviews the history of the regression discontinuity design in three academic disciplines. It describes the design's birth and subsequent demise in Psychology even though most problems with it had been solved there. It further describes the scant interest shown in the design by scholars formally trained in Statistics, and the design's poor reception in Economics from 1972 until about 1995, when its profile and acceptance changed. Reasons are given for this checkered history that is characterized as waiting for life to arrive.

Introduction

The regression discontinuity design (RDD) occurs when assignment to treatment depends deterministically on a quantified score on some continuous assignment variable. This score is then used as a covariate in a regression of outcome. When RDD is perfectly implemented, the selection process is fully observed and so can be modeled to produce an unbiased causal inference.

This paper is about the history of RDD. Although I am not a trained historian, I know enough to respect the primacy historians place on documenting events and trends. I also know that interpreting these events and trends has to be conditioned by independent knowledge of temporal sequence, by archived specifics that are relevant to the explanations offered, and by recourse to substantiated theories of individual and institutional behavior. Fortunately, most of this paper is about such events, trends and interpretations. But a few parts are not, and historians may well become nervous when I try to interpret events that might have happened but did not. Although some historians are developing a taste for counterfactual or virtual history (Ferguson, 1997), it is deservedly a minority taste. As an amateur historian, I will almost certainly fall into other traps professionals learn to avoid. Of those I can recognize, one is the teleological trap of inferring inevitable-seeming links between past events when, with more secure footing in the original time and place, these events might seem more contingent and many other futures possible. Another problem is that I am not an independent commentator on RDD. I have been peripherally involved in its history, albeit as a disseminator and not a theorist or practitioner. I was also marginally involved in the Northwestern University theory group that developed the design in the early 1970s after its discovery earlier (Thistlethwaite and Campbell, 1960). Doubtless I know more about RDD's history in that context than about other attempts to develop, disseminate or evaluate the design. While I have read many of the original sources reported on here, I have probably relied on secondary sources more than a real historian would. The relatively recent history of RDD has helped me, though, since many of the method's pioneers are still alive and have offered commentary on earlier drafts of this paper as it touched on their work. I have tried to incorporate their memories and sensitivities into this final version, sometimes even citing their notes to me about their work. Nonetheless, the following account is mine and not theirs; and while I respect the simplest norms of writing history, I cannot hope to dip deeply into the historian's bag of analytic tools. So, caveat lector.

This is not the first historical account of RDD. Donald Campbell, the design's originator, wrote his own version of the design's early history (Campbell, 1984), and various scholars have given snapshots of its history since then (Trochim, 1984, Trochim, 2001). However, the present account is more current, detailed and interdisciplinary than its predecessors. Indeed, it is organized around academic disciplines, tracing the history of the design in Psychology and Education, then in Statistics and Biostatistics, and then in Economics. The account speaks to many themes, including the repeated re-invention of the design across these disciplines. This was often done invoking different names for the design, the upshot being that RDD has not attained consistent “brand” status across the various behavioral, social and health sciences. Another theme speaks to the design's differential waxing and waning by discipline, trying to describe and explain what happened. RDD was invented and initially developed in Psychology and Education, but interest in it waned there after about 1990. It has never had much visible growth in Statistics, though its was acknowledged there. And in Economics RDD had a serendipitous birth, a long period of neglect, and then a renaissance after about 1995. This special journal number is part of that revival. Since its invention in 1960, RDD has been, in Samuel Beckett's words: “waiting for life to happen”. Will this revival breathe life into the design in Economics and, who knows, even beyond?