Focused deterrence: A protocol for a realist multisite randomised controlled trial for evaluating a violence prevention intervention in the UK

Introduction
Focused deterrence (FD) is a frequently cited intervention for preventing violence, particularly against violent urban gangs. The Youth Endowment Fund (YEF) believes it could be effective in the UK, based primarily on research conducted in the US. However, we contend that these studies have inadequate methodological designs, lack of rigorous testing, and small sample sizes. Therefore, the evidence supporting focused deterrence as an effective method, particularly outside the US, is inconclusive. The aim of the protocol is to better understand the potential effects of FD in the context of the UK, using a multisite evaluation experimental design to more closely investigate the evidence of its likely impact.

Methods
We planned a realist randomised controlled trial. The design is focused on a multisite trial consisting of two-arm randomised experiments in five locations. Each trial location will test their implementation of a core programme specified by the funder. The multisite nature will allow us to understand differential impacts between locations, improving the external validity of the results. Participants will be randomly selected from a wider pool of eligible individuals for the intervention. We estimate a sample size of approximately N = 1,700 individuals is required. Based on this pooled sample size, a relative reduction of 26% would be detectable in 80% of trials. The trial is coupled with a formative process evaluation of delivery and fidelity. The formative evaluation will use a mixed methods design. The qualitative aspect will include semi-structured cross-sectional and longitudinal interviews with programme leads, programme delivery team, and programme participants, as well as observations of the meetings between the programme delivery team (i.e., community navigators/mentors) and programme participants. The quantitative data for the formative evaluation will be gathered by the sites themselves and consist of routine outcome performance monitoring using administrative data. Sampling for interviews and observations will vary, with the researchers aiming for a higher number of individuals included in the first round of cross-sectional interviews and retaining as many as possible for repeat interviews and observations.

Discussion
This protocol outlines the process and impact evaluation methodology for the most extensive multisite evaluation of focused deterrence to date in the UK. Spanning five distinct sites with seven trials, the evaluation includes a cohort of 2,000 individuals, marking it as the only multisite trial of focused deterrence. Employing an integrated realist evaluation framework, the study uses qualitative and quantitative research methods. The anticipated findings will offer pivotal insights for formulating future violence prevention policies in the UK. They are also expected to contribute significantly to the corpus of literature on violence prevention and intervention evaluation.