A systematic review with a meta-analysis by the Campbell Collaboration evaluated the three existing types of focused deterrence: on violent groups, on highly violent individuals, and on open drug markets [1]. The evidence found indicates that focused deterrence in general (containing those three types) is an effective mechanism for reducing crime and violence.
Specifically, the analysis of focused deterrence interventions on violent groups indicates that these programs produce a statistically significant effect on crime and delinquency. This effect is comparatively greater (three times greater) than that observed among interventions focused on violent individuals, and almost eight times greater than the effect of interventions focused on open drug markets.
Another study analyzed a wide range of policing strategies from a large set of systematic reviews [2]. One of the strategies analyzed was deterrence focused on violent groups, which was evaluated through a 2012 Campbell Collaboration review (older, therefore, than the one cited above). The authors of this study concluded that deterrence focused on groups is, of all the strategies evaluated, the most effective to reduce homicides. Although the authors recommend caution when interpreting the results (due to the absence of randomized controlled trials in the systematic review), they point out that 9 out of 10 studies reported substantial reductions in crime and violence, with reductions ranging from 34% to 63% in the specific case of homicides [2].