This approach claims substantial positive results of 40 to 60% reduction, but the evaluations fail in the first requirement of rigorous evaluations, as noted in our evaluations page—randomization—as noted in the conclusion to the meta study mentioned in yesterday’s post.

An excerpt.

The Effects of “Pulling Levers” Focused Deterrence Strategies on Crime

Anthony A. Braga and David L. Weisburd

First published: 02 April 2012

The available scientific evidence on the crime reduction value of focused deterrence strategies has been previously characterized as “promising” but “descriptive rather than evaluative” (Skogan and Frydl, 2004: 241) and as “limited” but “still evolving” (Wellford et al., 2005: 10) by the U.S. National Research Council’s Committee to Review Research on Police Policy and Practices and Committee to Improve Research Information and Data on Firearms, respectively. Our systematic review identified ten evaluations of focused deterrence strategies; nine of these evaluations were completed after the National Research Council reports were published. A better developed base of scientific evidence now exists to assess whether crime prevention impacts are associated with this approach.

The basic findings of our review are very positive. Nine out of ten eligible studies reported strong and statistically significant crime reductions associated with the approach. Nonetheless, we are concerned with the lack of rigorous randomized experimental evaluations of this promising approach. While the biases in quasiexperimental research are not clear (e.g. Campbell and Boruch, 1975; Wilkinson and Task Force on Statistical Inference, 1999), recent reviews in crime and justice suggest that weaker research designs often lead to more positive outcomes (e.g. see Weisburd, Lum, and Petrosino, 2001; Welsh et al., 2011). This does not mean that non-experimental studies cannot be of high quality, but only that there is evidence that non-experimental designs in crime and justice are likely to overstate outcomes as contrasted with randomized experiments. In his review of situational crime prevention evaluations, Guerette (2009) finds that the conclusions of randomized evaluations were generally consistent with the majority conclusion of the nonrandomized evaluations. While our narrative review is consistent with Guerette’s (2009) conclusion, our calculated effect sizes reveal that less rigorous focused deterrence evaluation designs were associated with stronger reported effects. As such, we think that caution should be used in drawing conclusions regarding population effect sizes for the pulling levers intervention.

At the same time, the effects observed in the studies reviewed were often very large, and such effect sizes are evidenced as well in those studies using strong comparison groups (e.g. Papachristos, Meares, and Fagan, 2007). Our review provides strong empirical evidence for the crime prevention effectiveness of focused deterrence strategies. Even if we assume that the effects observed contain some degree of upward bias, it appears that the overall impact of such programs is noteworthy.

These findings are certainly encouraging, and point to the promises of this approach.

We certainly believe that the positive outcomes of the present studies indicate that additional experimental evaluations, however difficult and costly, are warranted. The potential barriers are real, especially in regards to identifying valid treatment and comparison areas. But existing evidence is strong enough to warrant a large investment in multi-site experiments (Weisburd and Taxman, 2000). Such experiments could solve the problem of small numbers of places in single jurisdictions, and would also allow for examination of variation in effectiveness across contexts.

Despite our concerns over the lack of randomized experiments, we believe that the findings of eligible focused deterrence evaluations fit well within existing research suggesting that deterrence-based strategies, if applied correctly, can reduce crime (Apel and Nagin, 2011). The focused deterrence approach seems to have the desirable characteristic of altering offenders’ perceptions of sanction risk. Our findings are also supported by the growing body of scientific evidence that suggests police departments, and their partners, can be effective in controlling specific crime problems when they engage a variety of partners, and tailor an array of tactics to address underlying criminogenic conditions and dynamics (Braga, 2008a; Weisburd and Eck, 2004). Indeed, our study suggests that Durlauf and Nagin (2011) are correct in their conclusion that imprisonment and crime can both be reduced through the noteworthy marginal deterrent effects generated by allocating police officers, and their criminal justice partners, in ways that heighten the perceived risk of apprehension.

While the results of this review are very supportive of deterrence principles, we believe that other complementary crime control mechanisms are at work in the focused deterrence strategies described here that need to be highlighted and better understood (see Weisburd, 2011). In Durlauf and Nagin’s (2011) article, the focus is on the possibilities for increasing perceived risk and deterrence by increasing police presence. Although this conclusion is warranted by the data and represents an important component of the causal mechanisms that have increased the effectiveness of focused deterrence strategies, we believe it misses an important part of the story.   In the focused deterrence approach, the emphasis is not only on increasing the risk of offending, it is also on decreasing opportunity structures for violence, deflecting offenders away from crime, increasing the collective efficacy of communities, and increasing the legitimacy of police actions.   Indeed, we suspect that the large effects we observe come precisely from the multi-faceted ways in which this program influences criminals. (pp 25-26)