Angrist (06 JEC)

Angrist (06 JEC) - Journal of Experimental Criminology...

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Instrumental variables methods in experimental criminological research: what, why and how JOSHUA D. ANGRIST* MIT Department of Economics, 50 Memorial Drive, Cambridge, MA 02142-1347, USA E-mail: angrist@mit.edu Abstract. Quantitative criminology focuses on straightforward causal questions that are ideally addressed with randomized experiments. In practice, however, traditional randomized trials are difFcult to implement in the untidy world of criminal justice. Even when randomized trials are implemented, not everyone is treated as intended and some control subjects may obtain experimental services. Treatments may also be more complicated than a simple yes/no coding can capture. This paper argues that the instrumental variables methods (IV) used by economists to solve omitted variables bias problems in observational studies also solve the major statistical problems that arise in imperfect criminological experiments. In general, IV methods estimate causal effects on subjects who comply with a randomly assigned treatment. The use of IV in criminology is illustrated through a re-analysis of the Minneapolis domestic violence experiment. The results point to substantial selection bias in estimates using treatment delivered as the causal variable, and IV estimation generates deterrent effects of arrest that are about one-third larger than the corresponding intention-to-treat effects. Key words: causal effects, domestic violence, local average treatment effects, non-compliance, two- stage least squares Background I’m not a criminologist, but I ` ve long admired criminology from afar. As an applied economist who puts the task of convincingly answering causal questions at the top of my agenda, I’ve been impressed with the no-nonsense outcome-oriented approach taken by many quantitative criminologists. Does capital punishment deter? Do drug courts reduce recidivism? Does arrest for domestic assault reduce the likelihood of a repeat offense? These are the sort of straightforward and practical causal questions that I can imagine studying myself. I also appreciate the focus on credible research designs re±ected in much of the criminological research agenda. Especially noteworthy is the fact that, in marked contrast with an unfortunate trend in education research, criminologists do not appear to have been af±icted with what social scientist Tom Cook (2001) calls F sciencephobia. _ This is a tendency to eschew rigorous quantitative research de- signs in favor of a softer approach that emphasizes process over outcomes. In fact, of the disciplines tracked in a survey of social science research methods by Boruch et al. (2002), Criminology is the only one to show a marked increase in the use of randomized trials since the mid-sixties.
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This note was uploaded on 07/17/2008 for the course ECON 642 taught by Professor De jong during the Spring '08 term at Ohio State.

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Angrist (06 JEC) - Journal of Experimental Criminology...

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