Center for Problem-Oriented Policing

Appendix A: About the Data Presented in Table 1

The numbers reported in Table 1 are from an analysis conducted by Guerette and Bowers (2009) of more than 200 evaluations of situational crime prevention measures. Each evaluation is accessible from the Center for Problem-Oriented Policing website (http://www.popcenter.org/library/scp/). Of the more than 200 evaluations, 102 provided some inspection of displacement or diffusion effects, and, within these, there were 574 different inspections. Each of these were reviewed and recorded by two independent reviewers whether or not there was evidence "consistent with" the possibility of displacement or diffusion effects. The numbers presented should be used as a general guide as to the probability of the various forms of displacement and diffusion occurring and should not be considered as absolute. This is because many studies lacked sufficient research designs to make confident determinations and a few did not provide data to evidence their reporting of the presence of displacement or diffusion. Additionally, most did not allow for the comparison of displacement and diffusion effects relative to the gains achieved by the intervention.

Those studies included in the evaluation met the following conditions:

  1. It was written in English.
  2. It was published as a journal article, government report, organizational report, or book (including book chapters).
  3. It reported an evaluation of a crime prevention effort that was predominantly or exclusively a situational intervention. In some cases the crime prevention effort also included dispositional interventions (i.e., directed to modifying criminal motivation) but the situational intervention(s) was predominant. The situational techniques employed in the intervention were classifiable under Cornish and Clarke's (2003) listing of 25 situational crime prevention techniques.
  4. Studies that involved targeted police tactics were not included (even though they could have been classified as "strengthening formal surveillance"). Studies that used predominantly situational measures but which also involved some targeted police efforts were included.
  5. The evaluation used some quantitative measure of crime.
  6. The article reported original research findings. Systematic reviews or other meta-analyses of prevention projects themselves were not included.
  7. In cases where the same project was reported in two different publications (e.g., in a government report and in a journal article), only the manuscript with the most detailed information was included.