Police, Camera, Evidence: London’s Cluster Randomised Controlled Trial of Body Worn Video



Policy areas

Organisation name Metropolitan Police Service

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Contact person: Catherine Owens


In the UK, the police use of Body Worn Video (BWV) has been increasing. The technology is being used in a variety of ways, most often to capture police operational activity first hand, using helmet or vest mounted cameras. The uptake of the technology may be explained by its perceived potential to assist with a range of policing problems. Notably, cameras have been proposed as key to increasing the transparency, efficiency and effectiveness of police conduct (Drover, P., & Ariel, B. (2015). Leading an Experiment in Police Body-Worn Video Cameras. International Criminal Justice Review, 25(1), 80–97). Despite the growing popularity of BWV there is much to be learned regarding its effectiveness, particularly its role in reducing attrition through the Criminal Justice System (CJS), as well as understanding any impact it has on the nature of police–public encounters.

The project tested implementing BWV in UK’s largest police force, the Metropolitan Police Service (MPS) via a cluster randomised controlled trial (RCT). This explored the impact of BWV on complaints against the police, frequency of stop and search, and criminal justice outcomes for violent incidents in ten MPS boroughs over a year. Entire teams of officers were randomly assigned to wear or not wear BWV, which provided a strong basis to draw inferences regarding their effects, establishing cause and effect relationships, and ruling out other explanations.

During the trial, 814 officers in 19 teams were assigned to wear cameras and 1246, in 29 teams, were assigned to not receive cameras. BWV captured 48 281 recordings, totalling 12 212 hours of video, of which 28% were tagged as ‘evidential’ for potential use within the CJS. Across both groups, approximately 11 300 stop and searches, 261 complaints and 64 355 notifiable crime reports, 16 191 classified as violent incidents, were included.

The use of BWV itself, the trial methods and publication strategy were aimed at promoting social inclusion and tapping into what works in the complex area of building public confidence and legitimacy in policing. A large range of data sources, including unusual sources, were used to triangulate the impact. For example, officers were asked to show clips that reflected their experience to researchers, a survey of those stopped and search, and sentiment analysis of tweets. This work has also led to innovations:

  • social media to communicate directly with young people
  • sharing stop and search footage with community groups

The project led to the MPS investing in 22 000 BWV cameras and a wide-scale roll-out across London. The project has also supported evidence-based decision-making in other forces nationally and internationally, as well as the use of the technology in areas of public sector delivery beyond policing.

The trial was published and all instruments used as part of the evaluation have been made freely available, so it can be effectively replicated in other areas nationally and internationally. An academic paper has also been published. In addition, both the research and operational leads have hosted and promoted the learning from the project nationally and internationally, supporting trials and pilots in the Netherlands, Germany, Romania, the United States, South America and Scotland. Officers from the MPS presented the trial design at a CEPOL (College of European Policing) conference, to engage cross-European working. The college-lead researcher has already attended a working group on BWV evaluation and implementation for the Americas and Caribbean, as well as planning a European version. This shows clear transferability of learning, and an appetite for collaboration and replication. As well as transferability with other police agencies internationally, there have been a number of requests from other public service providers for information on BWV. In the trial for the prison service, secure children’s homes, revenue collection agencies, transport workers, hospitals and schools they have all considered or are implementing BWV. It is important in these areas to fully understand the impact that the new technology could have and all of the intended and unintended consequences of a possible introduction.

From a police perspective, the use of BWV presents a significant opportunity to improve public confidence. It increases professionalism and police efficiency, improves sanction detection rates, increases victimless prosecutions for domestic violence cases, decreases complaints and improves the efficiency of other parts of the CJS. These benefits support the London Mayoral priorities in the Police and Crime Plan of boosting public confidence, reducing crime, delivering better value for money and reducing delays in the CJS.

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