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Testing – step seven

Measuring the amount of serious violence prevented.

First published
Serious violence hot spots policing guide
3 mins read


  • It is important to assess whether the hot spots policing strategy has had the desired effect. 
  • Randomised controlled trials (RCTs) are the best way to test the approach's effectiveness.
  • Parallel track testing and repeat crossover testing have been used for hot spots policing RCTs.
  • Parallel track testing is the more common approach in hot spots policing studies. It can take a long time and often involves a large sample of hot spots. Control group hot spots are often left without hot spots policing for significant periods of time.
  • Repeat crossover testing can require fewer hot spots where visible patrolling is used. It can be carried out more quickly. All hot spots are policed some of the time.

It is important to test or assess whether:

  • the desired outcomes have been achieved
  • there have been any unintended negative consequences

Testing does not need to be expensive or complicated. It should be proportionate to the nature of the problem or response. Serious violence is a high-harm problem and assessment would usually be appropriate. For example, an outcome for assessment might be a reduction in violent offending.

Outcomes should at the least be compared:

  • before and after implementation of hot spots policing
  • in the area that received hot spots policing and in a similar area that did not

More sophisticated testing can help to prove that any change in violent offending was caused by the hot spots policing strategy. For example, using randomised controlled trials (RCTs).

Forces are likely to need to seek advice from experts to undertake an RCT.

Parallel track RCTs have traditionally been used to test effectiveness. Recent research suggests that repeat crossover designs can also be effective when relying on visible patrolling (Sherman, 2022). Both approaches have strengths and weaknesses.

Parallel track testing

Hot spots policing is often tested using parallel track comparisons (Sherman and others, 1989). This involves assigning hot spots to either:

  • receive the hot spots policing strategy (the treatment group)
  • continue with business-as-usual activity (the control group)

Comparing the two groups' crime rates tests the effectiveness of hot spots policing.

Having two groups means the process can involve many hot spots. This can take a long time to develop and put in place. It also leaves control group hot spots without potentially effective tactics for a significant period of time.

Repeat crossover testing addresses several of these issues, but should only be used in certain conditions. 

Repeat crossover testing

Repeat crossover designs involve treating each hot spot as a control and treatment group (Bland and others, 2021).

In this approach, hot spots receive more patrols on some days but not on others. Crime is compared on days when there are more patrols and when there are not. Every hot spot receives patrols, but none of them are daily. 

This approach is quicker and needs fewer hot spots. It requires careful consideration of the potential deterrent effects of hot spots activity.

Crime may reduce in a control hot spot because of the deterrent effect of previous patrols. As a result, it is important to be mindful of residual deterrence when planning the hot spot strategy (Barnes and others, 2020).

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