A leading sociological theory of crime is the "routine activities" approach (Cohen and Felson, 1979). The premise of this ecological theory is that criminal events result from likely offenders, suitable targets, and the absence of capable guardians against crime converging nonrandom4 in time and space. Yet prior research has been unable to employ spatial data, relying instead on individual-and household-level data, to test that basic premise. This analysis supports the premise with spatial data on 323,979 calls to police over all 115,000 addresses and intersections in Minneapolis over 1 year. Relatively few %ot spots" produce most calls to Police (50% of calls in 3 % ofplaces) and calls reportingpredatory crimes (all robberies at 2.2% of places, all rapes at 1.2% of places, and all auto thefs at 2.7% of places), because crime is both rare (only 3.6% of the city could have had a robbery with no repeat addresses) and concentrated, although the magnitude of concentration varies by offense type. These distributions all deviate signijicantly, and with ample magnitude, from the simple Poisson model of chance, which raises basic questions about the criminogenic nature of places, as distinct from neighborhoods or collectivities.Is crime distributed randomly in space? There is much evidence that it is not. Yet there are many who suggest that it is. In a leading treatise on police innovations, for example, Skolnick and Bayley (1986: 1) observe that "we feel trapped in an environment that is like a madhouse of unpredictable violence and Quixotic threat." People victimized by crime near their homes

5 In Milwaukee, Wisconsin, for example, the prosecution rate for misdemeanor domestic battery was about ten percent at the time the Wisconsin State legislature enacted mandatory arrest for probable cause cases of that offense. In the Milwaukee experiment reported in this article, the prosecution rate was under five percent of all arrests.

and 35 Officers of the Milwaukee Police DepartmentPersons arrested for misdemeanor domestic violence are held in custody for wideb varying lengths of time. To test the eflects of this variance, we randomly assigned short (2 = 2.8 hours), full (f = 11.1 hours), and no arrests (warning only) to a sample of 1,200 cases with predominantly unemployed suspects concentrated in black ghetto poverty neighborhoods in Milwaukee. Victim interviews and one official measure showed that short arrest had a substantial initial deterrent effect relative to the warning group. Longer term follow-up and before-a fter analysis, however, found neither arrest group reflected any deterrence. On the most comprehensive official measure, short arrest consistently showed signijicantly higher longterm recidivism than no arrest. Its deterrent effect ended at 30 days, but its criminogenic effect was significant afrer one year. We conclude that short-custody arrests for domestic violence in poverty ghetto areas may pose a dilemma between short-and long-term crime control, but longer custody arrests have no clear long-term effect in either direction.

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With an increasing number of criminal justice scholars conducting randomized field experiments, there are several analytic issues related to such studies that our discipline must begin to address more systematically. For example, treatment dilution and treatment migration are common forms of randomization implementation failure in field experiments, and a review of the criminological literature on experiments reveals a lack of consensus as to how these problems should be handled when evaluating treatment effects. In addition, issues related to statistical power and desired sample size remain unresolved. Given the relatively longer history of dealing with these issues in medicine, literature from that field is reviewed, providing additional insights regarding the dilemmas created by various design failures in randomized field experiments.