What works to reduce crime

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We want to invest where it will make the biggest difference, so we need to know what works to reduce crime. Evidence Briefs summarise the evidence about how well an investment reduces crime, how much is spent on it, and whether there is scope to increase the level of investment.

Each Evidence Brief is overseen by a research committee chaired by the Justice Sector Science advisor, Associate Professor Ian Lambie. This ensures that the briefs fairly reflect the evidence.

Evidence Briefs are designed as general reference documents rather than advice to support a particular funding decision. Although the Evidence Briefs focus on crime, many of the investments covered will be designed to do more than reduce crime. There will always be broader issues to consider when taking an investment decision such as cultural values, fairness and equity, and societal benefits.

Correctional Alcohol and Drug Treatment

Evidence brief: Alcohol and Drug Treatment [PDF, 273 KB]

  • There is strong evidence that AOD treatment is effective in reducing crime.
  • AOD treatment is most effective for offenders at high risk of reoffending.
  • Both psychological treatment and treatment using pharmaceuticals can be effective.

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Cognitive Behavioural Therapy

Evidence brief: Cognitive Behavioural Therapy [PDF, 305 KB]

  • CBT is the cornerstone of modern approaches to rehabilitate offenders.
  • There is strong evidence that CBT is effective in reducing crime.
  • In a different context, there is also strong evidence that CBT is effective at treating a wide range of mental health conditions.

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Restorative Justice

Evidence brief: Restorative Justice [PDF, 317 KB]

  • There is strong evidence that RJ is effective at reducing crime.
  • The international evidence shows the RJ also provides benefits for some victims.

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Evidence brief methodology

Each Evidence Brief is designed to advise decision makers on how confident they can be that investment will reduce crime, based on the strength of the evidence. Each Evidence Brief provides an evidence rating based on the same objective criteria. The final rating is built around two separate assessments, one reflecting international evidence, and another – New Zealand evidence.

This approach reflects that effective programmes in one country often, but not always, work in other countries. Therefore, even if international research shows that an investment type can reduce offending, it may not be a strong investment option unless we are able to replicate the results in New Zealand’s crime environment.

Both international and New Zealand evidence is assessed using the Maryland Scale of Scientific Methods, a 5-point scale with randomised controlled trials at the top (level 5) of the scale. At lower levels, there is an increasing risk that findings are subject to selection bias and a wide range of other challenges to validity. The academics who developed the scale consider Level 3 the minimum in order to conclude the intervention reduces crime¹.

The final rating is measured in a six-grade scale varying from “poor” evidence to “very strong” evidence.

  New Zealand studies
    At least one level 4 or 5 study finds a statistically significant negative impact, no conflicting L4+ studies Studies show conflicting results, or no impact, or no level 3+ study exists At least one level 3 study finds a statistically significant positive impact, no conflicting L3+ studies At least one level 4 study finds a statistically significant positive impact, no conflicting L4+ studies At least one level 5 study find a statistically significant positive impact, no conflicting L5 studies
International studies Meta-analysis/systematic review of 5+ studies finds a statistically significant positive impact, no conflicts Fair (promising) Very promising Strong Strong Very strong
MA/SR with fewer than 5 studies finds positive impact, or no MA/SR exists and level 4 or 5 studies find a positive impact Speculative Fair (promising) Fair (promising) Very promising Strong
MA/SR find conflicting results Speculative Speculative Fair (promising) Very promising Strong
MA/SR shows no impact, or no MA/SR exists Poor Speculative Fair (promising) Very promising Strong
MA/SR shows negative impact, no conflicting results Poor Poor Speculative Fair (promising) Strong

There is also a standard interpretation for each evidence rating, as summarised in the following table.

Investment categoryInterpretation
Very strong
  • Very robust international and local evidence that investment tends to reduce crime.
  • Investment likely to generate a return if implemented well.
  • Simple monitoring approach should confirm the investment is providing a positive return.
  • Little additional evaluation required.
Strong
  • Robust international and local evidence that investment tends to reduce crime.
  • Investment likely to generate a return if implemented well.
  • Could benefit from additional evaluation to confirm investment is delivering a positive return and to support fine-tuning of the investment design.
Very promising
  • Robust international or local evidence that investment tends to reduce crime.
  • Investment may well generate a return if implemented well.
  • Further evaluation desirable to confirm investment is delivering a positive return and to support fine-tuning of the investment design.
Fair/promising
  • Some evidence that investment can reduce crime.
  • Uncertain whether investment will generate return even if implemented well.
  • May be unproven in New Zealand or be subject to conflicting research.
  • May benefit from trial approaches with a research and development focus.
  • Robust evaluation needed to confirm investment is delivering a positive return and to aid in detailed service design.
Speculative
  • Little or conflicting evidence that investment can reduce crime.
  • Highly uncertain whether investment will generate return even if implemented well.
  • Primarily suited to trial approaches with a strong research and development focus.
  • Full rollout should be subject to high-quality evaluation to ensure investment is delivering a positive return, and to deliver insights into detailed service design questions.
Poor
  • Robust evidence that investment does not reduce crime or that it increases crime.
  • Should be priority for divestment.

¹ Sherman, L., Farrington, D., Welsh, B., & Mackenzie, D. (Eds). (2002). Evidence-Based Crime Prevention. New York: Routledge

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