10 - Quasi-Experimentation When Random Assignment Is Not Possible

Summary

INTRODUCTION

Although tradition and experience often provide the only guidance for criminal justice practitioners, there is a growing consensus among scholars, practitioners, and policy makers that crime prevention practices and policies should be rooted as much as possible in scientific evidence about “what works” (Cullen and Gendreau 2000; MacKenzie 2000; Sherman 1998; Sherman et al. 2006). “Evidence-based crime prevention” is part of a larger and increasingly expanding movement in social policy to use scientific research evidence to guide program development and implementation. In general terms, this movement is dedicated to the improvement of society through the utilization of the highest quality scientific evidence on what works best (see, e.g., Sherman et al. 2006).

In an evidence-based model, the source of scientific evidence is empirical research in the form of evaluations of programs, practices, and policies. Common evaluation designs include randomized experiments, quasi-experiments, and nonexperimental (sometimes called observational) research designs. Not all evaluation designs are considered equal, however. Some evaluation designs, namely randomized controlled experiments, are considered more scientifically valid than others (Campbell and Stanley 1966; Cook and Campbell 1979; Weisburd 2003). The findings of stronger evaluation designs are privileged over the findings of weaker research designs in determining “what works” in crime and justice interventions. For instance, in their report to the United States Congress on what works in preventing crime, University of Maryland researchers developed the Maryland Scientific Methods Scale to indicate to scholars, practitioners, and policymakers that studies evaluating criminological interventions may differ in terms of methodological quality of evaluation techniques (Sherman et al. 1997).