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Police drug diversion (PDD) – realist impact, process and economic evaluation

Key details

Lead institution
Principal researcher(s)
Professor Alex Stevens
Police region
South East
Collaboration and partnership
  • Cabinet Office (funder)
  • College of Policing
  • Bradford Institute for Health Research
  • Department of Health and Social Care
  • Durham Constabulary
  • National Police Chiefs' Council (NPCC)
  • Loughborough University 
  • Open University
  • Thames Valley Police
  • University of Kent
  • University of Sheffield
  • University of York
  • User Voice
  • West Midlands Police
Level of research
Professional/work based
Project start date
Date due for completion

Research context

Dame Carole Black’s Independent review of drugs (2020) articulated the scale of the drug problem in England and Wales. There are about three million people who use illicit drugs each year, of whom about 300,000 use heroin and/or crack cocaine. The total annual cost of drug misuse is estimated to be £19.3 billion, including:

  • £8.5 billion for drug-related crime
  • £6.3 billion for the cost of drug-related death
  • £1.4 billion in additional costs of health and social care

Drug-related deaths are also at record levels.

The Black Review (2020) and the government's 10-year drugs plan (2022) have recommended expansion and evaluation of schemes that divert people away from the criminal justice system, and into education and treatment. Previous research shows the potential for diversion to increase the proportion of offences that are brought to justice, to cut crime, and reduce costs. Less is known about whether schemes have health benefits, have similar or different effects for different types of people, and are cost-effective.

Research questions

The PDD evaluation aims to fill these gaps in knowledge by answering the following research questions.

  • What effects have PDD schemes had on offending?
  • What effects have PDD schemes had on health outcomes?
  • What other effects have PDD schemes had?
  • Were there inequalities in the use and effects of PDD schemes?
  • What were the cost-consequences of PDD schemes for health, police and other service providers?


The PDD evaluation has received funding from the Cabinet Office's Evaluation Accelerator Fund.

Research methodology

Work package one – development of intervention manuals and a theory of change

This aimed to produce manual of the PDD schemes in three main study forces. Workshops with more than 70 stakeholders were carried out across the force areas and nationally, to develop and refine the detailed descriptions of the schemes. A theory of change on the alternatives to criminalisation for drug possession was also adapted to describe out how PDD might bring about outcomes.

Work package two – process evaluation

This focuses on the implementation of PDD and how closely implementation of schemes follows the manuals for the three study forces. The process evaluation also seeks to understand how contexts and mechanisms combine to effect change. The research involves semi-structured interviews (n=225) and focus groups (n=6) with police officers, service providers, people who were diverted and people who were not diverted despite being eligible, and analysis of aggregated police data.

Work package three – quantitative outcome assessment

This will assess the impact of PDD on offending and health outcomes. It involves complex data linkage across organisations. Analysis will compare outcomes for people who had contact with the police for drug-related offences in forces with PDD schemes, with those for similar people in forces without PDD schemes. Outcomes include reoffending, access to drug or alcohol treatment and relevant hospital visits during the 12-month follow-up. Intention to treat and per protocol analysis is planned.

Work package 4 – cost-consequence analysis

This looks to establish the financial costs and benefits of PDD, and on which organisations they fall.

Work package 5 – equity assessment

This will examine whether the use and impact of PDD varies by ethnicity, sex and location.

Work package 6 – realist synthesis

Lastly, this will bring together all the evaluation findings to detail what works, for whom, when and why.

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