Police use of force is at the forefront of public awareness in many countries. Body-worn videos (BWVs) have been proposed as a new way of reducing police use of force, as well as assaults against officers. To date, only a handful of peer-reviewed randomised trials have looked at the effectiveness of BWVs, primarily focusing on use of force and complaints. We sought to replicate these studies, adding assaults against police officers as an additional outcome. Using a prospective meta-analysis of multi-site, multi-national randomised controlled trials from 10 discrete tests with a total population of +2 million, and 2.2 million police officer-hours, we assess the effect of BWVs on the rates of (i) police use of force and (ii) assaults against officers. Averaged over 10 trials, BWVs had no effect on police use of force ( d = 0.021; SE = 0.056; 95% CI: –0.089–0.130), but led to an increased rate of assaults against officers wearing cameras (d = 0.176; SE = 0.058; 95% CI: 0.061–0.290). As there is evidence that cameras may increase the risk of assaults against officers, more attention should be paid to how these devices are implemented. Likewise, since other public-facing organisations are considering equipping their staff with BWVs (e.g. firefighters, private security, traffic wardens), the findings on risks associated with BWVs are transferrable to those occupations as well.

Introduction Violence by the police and against the police undermines the rule of law (Kirk, 2011; Tyler, 1990). Recent cases of killings by the police, particularly of unarmed minority civilians, have raised concerns about due process and justice (Kennedy, 2011). These events mirror a developed body of research on police legitimacy, accountability and transparency in social sciences (Tankebe and Bottoms, 2014). More than any other behaviour, use-of-force by police officers has the potential to decrease public trust in the police. While most citizens recognize the occasional need for force, the overall frequency of force used by police and force that is perceived to be excessive are clearly of concern to the public. (Stewart et al., 2013: 1) So what would it take for the police to act with fairness and restraint when interacting with citizens? At present, there is a world-wide uncontrolled social experiment taking place with one potential answer to this question: equipping police officers with body-worn videos (BWVs) in order to improve procedural compliance by officers and ‘take the heat’ out of encounters with citizens. This social experiment – underpinned by feverish public debate and billions of dollars of government expenditure around the world (Friedman, 2015) – is one where robust evidence can, just, keep pace with the adoption of the new technology (Lum et al., 2015; White, 2014). Our contribution here is to create that evidence base with one of the largest multi-site prospective randomised controlled trials (RCT) in the history of criminal justice research. In this note, we report that, averaged over 10 trials, camera use had no average effect on police use of force, while in some sites BWVs appear to increase use of force against suspects compared to control conditions. Also, crucially, BWVs appear to lead to an increased rate of assaults against officers.

Methods Our data come from 10 RCTs from eight police forces in six jurisdictions, covering a total population of more than 2,000,000 citizens. Two forces had had two separate geographic areas included in the trial, hence the greater number of trials than police forces. Information on participating sites is presented in Table 1 below. Jointly, the trials involved 2122 officers in eight police departments, with 2,188,712 officer-hours.4 Each study was a two-arm trial that randomly assigned officer shifts to either experimental (with cameras) or control (no cameras) conditions, on a weekly basis. This resulted in 4915 shifts being assigned (M = 491.50; SD = 276.99 per site), with no differences between treatment and control conditions in terms of the distribution of shifts (Table 2). Beyond large-scale cluster-randomised designs – which would perhaps be a ‘gold standard’ design in this context if properly administered – randomising shifts is the most practical approach to implementing BWVs trials with police, as even small forces can leverage large sample sizes (Ariel et al., 2015). Our pre-published protocol (see Supplementary Materials) states that all officers doing ‘camera on’ shifts had to wear a camera, keep the camera on during their entire shift (typically between 8 and 12 hours) and inform members of the public during any encounter that they were wearing a camera that was recording their interaction (see Supplementary Materials). This means that the intervention consisted of [camera + notification]. To be clear, the trial design meant that officers did not have discretion about when cameras were turned on – cameras were supposed to be kept on throughout their shift. The only exceptions were circumstances when officers responded to specific types of incidents that were pre-agreed with senior staff in each force (e.g. when conversing with informants, serious sexual assaults or major public events). Table 1. Descriptive statistics for the 10 participating sites. View larger version Table 2. Trial measures by treatment allocation within each participating site. View larger version The two outcomes reported here for all studies were whether an officer used force during a shift (if so, how many times) and whether or not officers were assaulted (if so, how many times). These were then standardised as rates per 1000 arrests per shift because forces ranged in size. There are differences in how police forces define ‘force’ (e.g. incidents that do or do not account for compliant handcuffing), as well as how assaults against officers are recorded (e.g. physical and/or verbal) – with both likely to vary by jurisdiction and/or police force. To mitigate differences in how ‘force’ was defined, our analyses focus on any physical restraint on the force continuum (Garner et al., 1995; Terrill, 2001) beyond the use of verbal commands during an arrest (so if an officer uses pepper spray, for example). A consistent benchmark of what level of force is included in the trial helps with comparability between sites, even if they have differing base rates for use of force. (The differing base rates do not affect the within-force results from the trials – the RCTs are testing between-group differences in each trial site – but may be important for understanding variation in outcomes between trials, particularly in light of the different base rates in use of force. We discuss this point below.) Assaults against officers – as noted above – can be more difficult to capture, because it depends on the willingness of the officer to report their assault (see Bierie, 2015). For this study, we had to be pragmatic in how data on assaults against police were captured. Imposing new reporting requirements would have been problematic (particularly in having this agreed across forces and jurisdictions). As such, we relied on the routine reporting requirements already in place. We used the Comprehensive Meta-Analysis Version 2 software (CMA) to synthesise the results from the trials and present the overall results, using standardised difference of means (Cohen’s d; Cohen, 1992) to compare treatment and control conditions across all sites. We then used the standardised mean difference as a summary statistic in a meta-analytic procedure, and the corresponding 95 percent confidence intervals as a measure of reliability of the estimation procedure. As each trial uses the same design and outcomes then it is appropriate to combine and report them in this manner (Lipsey and Wilson, 2001). The data inputted into CMA consisted of: (i) the number of treatment and control shifts; (ii) the rate of incidents of use of force per 1000 arrests; and (iii) the rate of recorded assaults on police officers on patrol per 1000 arrests in each shift.

Discussion The overall null use-of-force result may dampen the enthusiasm of those calling for cameras to be used by all police forces. Some may even see this as justification for body-worn videos to be abandoned altogether. We urge caution before coming to any firm conclusions for three reasons. First, there are still more studies being conducted as part of this research and as results come in the synthesised results reported here may change. Second, despite the robust methods used in these studies from around the world, the data used represent a convenience sample of police forces, and experiences in other jurisdictions may vary. Third, as we acknowledge above, different jurisdictions have varying definitions of use of use or how assaults against officers are classified, and this may affect the comparability of results between jurisdictions. Thus, an alternative explanation for the overall null result is that heterogeneity in how use of force was measured between the six jurisdictions accounts for the different results. That is, although the studies operated under the same research design protocol, differences in how the police defined use of force might account for the variation observed between forces. However, this argument is countered by the fact that we implemented a consistent approach to measuring force within all sites, meaning that comparisons within forces are valid. Additional sub-group analyses are needed to unpick these issues; however, such data are not presently available. This is one limitation this study faces, and will require addressing in future research. Furthermore, the variability in our results also tells us that the BWVs worked in some places, some of the time, but did not work in others. Compared to the control conditions, an increase in use of force against suspects as a result of using BWVs is a puzzle. By virtue of deterrence alone, BWVs should increase compliance and, subsequently, less force will be used; yet, BWVs were found to exacerbate force in some instances. We need to understand more about this. One direct explanation might be that BWVs escalate an already inflamed police–public encounter, which results in more rather than less force being used. It might be that when BWVs are introduced into some ongoing police–public interactions, the suspect, officer or both become more aggressive. Our second result concerning the increase in assaults against police – unexpected as it was – also demands attention. Does this mean that officers should be advised to remove BWVs immediately? If the results are accepted uncritically, that is, that BWVs increase the likelihood of assaults against officers, then this might be the conclusion one comes to. However, we cannot rule out alternative explanations at this stage. First, (yet un-evidenced) is the idea that with an ‘objective’ record of events, officers feel more able (or compelled) to report instances when they are assaulted. Second, officers may be less assertive (‘toned down’) because of monitoring, and this could make them more vulnerable to assault. (Officers sometimes deal with people who would very much like to do them – and others – harm.) Third, the strongest results for assaults against police came from the smallest studies – these may be atypical results driven by small sample variation. Fourth, the role of officer discretion is not clear and we will explore this further in a subsequent article. Finally, increased assaults against police may also be a corollary of the decreased use of force in some instances. Whereas police may have taken a tit-for-tat approach – using force when assaulted as a form of ‘natural justice’ – cameras may inhibit the reactive use of force and give officers the impetus and/or confidence to report the incident. The question about the reason for the increased assaults is not something that can be left to debate and must be scrutinised empirically.