INTRODUCTION

In recent years, places (such as small geographic locations, schools, classrooms, and clinics) rather than people have come into focus as the unit of analysis in randomized controlled trials (RCTs). Places can be both the physical location and the organizational unit for human behavior (Boruch, Weisburd, and Berk 2010). As such, many social reform interventions involve larger units of organization, and often policies developed by or for agencies such as police departments and schools are not always hypothesized to work directly at the level of the individual person (Boruch et al. 2004; Campbell 1969). Influential place-based experiments have been conducted in the fields of healthcare (e.g., Donner and Klar 2000; Grimshaw et al. 2005; Sikkema 2005; Imai, King, and Nall 2009), social welfare (Bloom and Riccio 2005), education (Flay and Collins 2005; Parker and Teruel 2005), and criminal justice (e.g., Braga and Bond 2008; Braga et al. 1999; Sherman and Weisburd 1995; Taylor, Koper, and Woods 2011; Weisburd and Green 1995; Weisburd, Morris, and Ready 2008).

Place-based experiments in criminal justice have been particularly influential. Studies of “hot spots policing,” which necessarily focus on small units of geography, have led to encouraging findings regarding the ability of the police to prevent crime (Durlauf and Nagin 2011; Skogan and Frydl 2004; Weisburd and Eck 2004). Indeed, in 2004 the National Research Council of the National Academy of Sciences concluded that “…(S)tudies that focused police resources on crime hot spots provide the strongest collective evidence of police effectiveness that is now available” (Skogan and Frydl 2004: 250). These research findings have had important consequences for police reform in practice (Kochel 2011; Weisburd and Lum 2005). Most larger police agencies in the United States, for example, now use hot spots policing tactics (Weisburd and Braga 2006; Weisburd and Lum 2005; Weisburd et al. 2003).