Using Terror Alert Levels to Estimate the Effect of Police on Crime
利用美国国土安全部恐怖警戒级别的变化作为警察出勤的外生冲击，基于华盛顿特区每日犯罪数据，发现高警戒期间犯罪率显著下降，尤其在国家广场区域，为警察对犯罪的因果效应提供了有力证据。
Changes in the terror alert level set by the Department of Homeland Security provide a shock to police presence in Washington, D.C. Using daily crime data during the period the terror alert system has been in place, we show that the level of crime decreases significantly, both statistically and economically, in Washington, D.C., during high-alert periods. The decrease in the level of crime is especially large in the National Mall. This provides strong evidence of the causal effect of police on the level of crime and suggests a research strategy that can be used in other cities. I.