driving (Nagin, 1998). The next section describes serious difficulties that are involved in relating enforcement actions to changes in drug activity. A principal problem here is the lack of any direct measures of the number of drug dealers or of the level of drug dealing in a place.

Many studies use number or rate of arrests as a proxy for the level of drug dealing activity. See, for example, Braga et al. (1999), Green (1996), and Worden et al. (1993). Arrests, however, are still an inaccurate measure of drug dealing activity (Weisburd and Green, 1994, Worden et al. 1993). Arrests reflect both the level of drug-dealing activity and the skill or intensity of police activity in arresting drug dealers. Thus, an absolute increase in the level of arrests could reflect some mixture of an increase in the level of drug dealing, a change in the mode of drug dealing that increases the probability that dealing will lead to arrest (e.g., moving an indoor market into the street), or a change in police policy encouraging arrest when a drug deal is detected. Thus, arrest levels depend on the extent and effectiveness of police surveillance as well as the skill of dealers at concealing their operations (e.g., by moving them indoors or by operating delivery services). If, however, arrests occur primarily when drug dealing is highly visible and disruptive to the community, then measures of arrests could well be useful as indicators of the visibility of drug markets and the disruption that they cause. Also, shifts in the level of arrests without some indication of changes in arrest policy, or shifts in arrest of one subgroup without shifts in others, could serve as an indicator of changing participation in the market. In summary, because of the uncertainty about what factors cause any particular change in arrests, a reliable measure of the effectiveness of local law enforcement in deterring individuals from engaging in drug dealing cannot be generated.

Refraining from illegal acts is only one possible response of drug dealers or potential dealers to pressure from law enforcement authorities. There are many ways in which drug dealers can adapt to law enforcement in order to continue their sales operations with minimal disruption. One is by forming delivery or beeper services, eliminating the need for meetings between dealers and customers and thereby greatly reducing the visibility of a drug sales operation to the police. Similarly, the use of brokers or other intermediaries makes it harder for law enforcement officials to identify and locate dealers. Other forms of adaptation include changing the locations of drug-dealing operations or the hours of the day that they operate. The police cannot be active everywhere all the time. Thus, a crackdown at one location may lead to increased activities at another. Similarly, dealers may know the patrolling routines of police and arrange not to be visible during hours of police presence.

There is little evidence on the extent to which these adaptations take place and their consequences for the effectiveness of law enforcement. It

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