In hierarchies of research evidence, the randomized controlled trial (RCT) usually appears near the top of the pyramid. RCTs are usually considered to be free of bias; systematic reviews of the literature may exclude studies that are not RCTs. Although controlled trials are excellent methods for establishing causation, they do not assure freedom from systematic bias. This chapter explores biases that are common among RCTs. In particular, we report on the practice of systematically excluding study participants who do not meet specific criteria. It is not uncommon for 90 percent of potential volunteers to be turned away. Common grounds for exclusion include comorbidities, even though living with multiple chronic conditions is almost universal among older adults. The selection of the control group can also increase bias. Control groups might be selected specifically because they increase the likely difference between treated and control conditions. The implications of biases in our RCTs are discussed.

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.