Randomized experiments, quasi-experiments and systematic reviews are critical for developing the evidence for evidence-based policing. In this paper, we explore the role of experiments in contributing to the evidence base in policing. Drawing on the Global Policing Database (GPD) corpus of 3,487 high quality evaluation studies in policing, including 431 randomized controlled trials, we describe the breadth and depth of the evidence base in policing. We find that randomized controlled trials form only about 12 percent of the total evidence based in policing, yet this small number of trials has had enormous policy influence over the last fifty years. We also find that most of the evidence in policing is around frontline policing practices and about half of the RCTs in the world come from the US: a greater proportion of RCTs coming from outside of the US than previously reported. We conclude that the breadth of people and places generating high quality evidence will help generalize policing policies and practices beyond the US and is likely to have a snowball effect in fostering the next generation of experimentalists in policing.

Systematic reviews in policing have become an increasingly common way for researchers to synthesize the state of research on programs, practices, and policies. Reviews utilize comprehensive and transparent search strategies to identify and summarize the evidence base for a particular topic, providing rigorous assessments of the state of scientific knowledge about policing strategies needed for evidence-based policing. This chapter summarizes findings and conclusions from systematic reviews on policing, building on an earlier paper that included 17 policing reviews completed between 2004 and 2015. In the current chapter, we identify updates to five of these reviews, and new reviews on 13 policing topics. Our “review of reviews” on 30 policing topics suggests a growth in primary research in policing, and in particular an increase in reviews on non-crime control topics. But we also suggest existing reviews provide insufficient “how to” guidance for implementing evidence-based strategies. We argue in concluding that scholars have succeeded in providing a “first generation” of studies that tell us whether general policing approaches are effective, but a much larger evidence based is needed for a “second generation” of systematic reviews that would provide specific guidance about choosing and implementing evidence-based practices in the field.