jats:titleAbstract</jats:title>jats:pOur understanding of causality and effect size in randomized field experiments is challenged by variations in levels of baseline treatment dosage in control groups across experiments testing similar treatments. The clearest design is to compare treated cases with no‐treatment controls in a sample that lacks any prior treatment at baseline. We applied that strategy in a randomized test of hot‐spots police patrols on the previously never‐patrolled, track‐level platforms of the London Underground (LU). In a pretest–posttest, control‐group design, we randomly assigned 57 of the LU's 115 highest crime platforms to receive foot patrol by officers in 15‐minute doses, 4 times per day, during 8‐hour shifts on 4 days a week for 6 months. The effect of 23,272 police arrivals at the treatment hot spots over 26 weeks was to reduce public calls for service by 21 percent on treated platforms relative to controls, primarily when police were absent (97 percent of the measured effect). This effect was six times larger than the mean standardized effect size found in the leading systematic review. This finding provides a benchmark against the baseline counterfactual of no patrol in hot spots, with strong evidence of residual deterrence and no evidence of local displacement.</jats:p>