Our 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.

中文翻译：

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测试热点警​​察巡逻以防不治之症：伦敦地铁实验中的时间和空间威慑作用

在测试相似治疗的整个实验中，对照组中基线治疗剂量水平的变化挑战了我们对随机现场实验中因果关系和效应大小的理解。最清晰的设计是将在基线时没有任何先前治疗的样本中，将治疗病例与未治疗对照进行比较。我们将该策略应用于在以前从未巡逻的伦敦地铁（LU）上的热点警察巡逻的随机测试中。在前测，后测，对照组的设计中，我们随机分配了LU的115个最高犯罪平台中的57个，以15分钟的剂量每天进行4次，每天15次，每天8次，每周4天。 6个月。效果23 在26周内，有272名警察到达了治疗热点，主要是在没有警察的情况下（在实效中占97％），相对于对照，在经过处理的平台上，公共服务电话减少了21％。该效果是领先的系统评价中发现的平均标准化效果大小的六倍。这一发现为在热点地区不进行巡逻的基线反事实提供了一个基准，有强有力的残余威慑力证据，也没有局部位移的证据。