J Exp Criminol
DOI 10.1007/s11292-016-9261-3

Report: increases in police use of force in the presence
of body-worn cameras are driven by officer discretion:
a protocol-based subgroup analysis of ten
randomized experiments
Barak Ariel 1,2 & Alex Sutherland 3 &
Darren Henstock 4 & Josh Young 5 & Paul Drover 6 &
Jayne Sykes 7 & Simon Megicks 8 & Ryan Henderson 9

# The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract
Objectives Our multisite randomized controlled trial reported that police body-worn
cameras (BWCs) had, on average, no effect on recorded incidents of police use of
force. In some sites, rates of use of force decreased and in others increased. We wanted
to understand these counter-intuitive findings and report pre-specified subgroup analyses related to officers’ discretion on activating the BWCs.
Methods Using pre-established criteria for experimental protocol breakdown in terms
of treatment integrity, ten experimental sites were subgrouped into Bhigh-compliance^
(no officer discretion applied to when and where BWCs should be used; n = 3), Bnocompliance^ (treatment integrity failure in both treatment and control conditions; n = 4),
* Barak Ariel
barak.ariel@mail.huji.ac.il; ba285@cam.ac.uk

1

Institute of Criminology, Faculty of Law, Hebrew University, Mount Scopus,
Jerusalem 91905, Israel

2

Institute of Criminology, University of Cambridge, Sidgwick Avenue, Cambridge CB3 9DA, UK

3

Communities, Safety & Justice RAND Europe, Westbrook Centre, Milton Road, Cambridge CB4
1YG, UK

4

Birmingham South Police, Birmingham, UK

5

Ventura Police Department, Ventura, CA, USA

6

Wolverhampton Police, Wolverhampton, UK

7

West Yorkshire Police, West Yorkshire, UK

8

Cambridgeshire Constabulary, Ely, UK

9

Police Service of Northern Ireland, Belfast, UK

B. Ariel et al.

and tests where officers applied discretion during treatment group but followed protocol
in control conditions only (n = 4).
Results When officers complied with the experimental protocol and did not
use discretion, use of force rates were 37 % lower [SMD = (−.346);
SE = .137; 95 % CI (−.614) – (−.077)]; when officers did not comply with treatment
protocol (i.e., officers chose when to turn cameras on/off), use of force rates were 71 %
higher [SMD = .392; SE = .130; 95 % CI (.136) – (.647)], compared to control
conditions. When full discretion (i.e., overall breakdown of protocol) was applied to
both treatment and control conditions, null effects were registered
[SMD = .009; SE=.070; 95 % CI (−.127) – (.146)], compared to control conditions.
Conclusions BWCs can reduce police use of force when then officers’ discretion to
turn cameras on or off is minimized—in terms of both case types as well as individual
incidents. BWCs ought to be switched on and the recording announced to suspects at
early stages of police–public interactions. Future BWCs tests should pay close attention
to adherence to experimental protocols.
Keywords Body-worn cameras . Use of force . Police . Treatment integrity . Subgroup .
Experimental protocol

Introduction
What would it take to reduce unnecessary police use of force? At present, many police
departments globally are attempting to do that by equipping police officers with bodyworn cameras (BWCs) in order to potentially de-escalate volatile encounters through the
deterrent threat of apprehension for noncompliant behavior. Billions of taxpayers’ dollars
(Friedman 2015), as well as headlining news coverage, are not mirrored by a similar
increase in research evidence on this new technology (Lum et al. 2015; White 2014).
We have previously reported results from a global multisite randomized controlled
trial on the effect of BWCs on various outcomes, including use of force, complaints
against the police, and assaults against officers (Ariel et al. 2016). Averaged over ten
trials, we reported that the use of police BWCs had no overall effect on use of force.
However, our results varied, with force increasing in some trials and reducing in others.
These conflicting results were puzzling and disturbing. Why would officers, knowing
that their actions were being filmed by their own equipment, choose to apply force
more often when cameras were on in some instances? Similarly, why would suspects’
demeanor become more aggressive or noncompliant under these circumstances? This
runs contrary to both common sense and a good deal of research across disciplines on
the effect of deterrence on compliance behavior as well as the law (see Nagin 2013; in
the framework of BWCs, see Ariel 2016). Here we report on planned subgroup
analyses aimed at disentangling both of these effects.

Background
Police are, to mix two phrases spanning the Atlantic, the Bthin blue line^ that Bprotects
and serves^. However, as the legitimate use of force rests with police officers, citizens

Report: increases in police use of force in the presence of...

should be—and are in fact—concerned that force is used proportionately and fairly.
Time and again we learn that some police officers use excessive force in a manner
wholly unnecessary for a situation (The Guardian 2015), or are unable to de-escalate
tense engagements with members of the public (see Piquero et al. 2006; Sherman
1980). Similarly, we know that some citizens’ demeanor promulgates use of force,
often through verbal or physical assault of officers (Reisig et al. 2004; Terrill and
Mastrofski 2002; but cf. Engel et al. 2000); these instances are likely to result in
aggressive arrests and use of more police force. Thus, reducing police use of force is a
laudable aim for law enforcement agencies as this can improve public perceptions of
police, whilst at the same time making policing more effective (Mazerolle et al. 2013;
The White House 2014).
BForce^ can be used to achieve a lawful objective, such as making a lawful arrest,
subduing a resisting individual, acting in self-defense, or protecting others (College of
Policing 2015). However, measuring what Bpolice force^ is, at which point it becomes
excessive, unnecessary, or disproportionate (Harris 2010)—or even who instigates the
use of force beyond what is required—is far from clear (Ariel et al. 2014). There is no
tracking system of police force that is completely reliable or even valid—since the
amount of Bforce^ necessary in a given situation is subjective, primarily self-reported,
and heavily underreported (Hickman et al. 2008). Yet at its core, police use of some force
is an essential requirement against certain offenders, under specific circumstances. The
issue is not whether or not force needs to be applied, but how it can be minimized.
It is at this juncture that BWCs enter: the basic motivation for police BWCs is, among
many other things, to reduce use of force (Miller et al. 2014). The theoretical basis for the
use of cameras—that being monitored changes behavior—is deterrence theory (see
review in Ariel et al. 2014:4–6). Police BWCs—at least as implemented in our trials—
fulfill the causal mechanisms of deterrence to regulate police–citizen encounters. BWCs
increase the perceived certainty of apprehension for rule violations. In this sense, the
underlying assumption is that officers will use excessive or unnecessary force less
frequently than during control conditions. From the citizen/suspect side of things,
assaulting the officer, resisting arrest or committing detectable offenses is also bound to
lead to further sanctions, which (rational) actors will tend to avoid (see Cornish and Clarke
2014)—a point we return to in the conclusion. When BWCs are actually turned on and
appropriately activated, they can efficiently detect rule violations and law breaking by
officers and/or citizens—and this process can send a credible deterrence threat (see Jervis
et al. 1989:3). In this sense, BWCs are unlike CCTVs (Welsh and Farrington 2009),
dashboard cameras, or bystanders’ mobile phone cameras. For a more elaborate discussion on this theoretical mechanism behind BWCs, see Ariel (2016:105–109).
A recent review of the available evidence conducted by Lum et al. (2015) has shown
that there are, currently, 12 existing empirical studies of BWCs and about 30 ongoing
research projects (see also review by White 2014). While there were attempts to
implement BWCs in policing nearly a decade ago (Goodall 2007; Harris 2010),
evidence on their effectiveness has only surfaced in the last couple of years (see Lum
2015). Four of the studies employed randomized controlled trials (Ariel et al. 2014;
Grossmith et al. 2015; Jennings et al. 2015; Owens et al. 2014), and others have used
less robust designs (e.g., Ariel 2016). The most recent published studies employed
different units of analysis to measure various consequences of the use of BWCs in
police routine operations: in Orlando, Florida, the unit of analysis was the individual

B. Ariel et al.

officer (Jennings et al. 2015); Ready and Young (2015) compared volunteers versus
non-volunteers, while others have used a version of cluster random assignment
(Grossmith et al. 2015). Lesser designs, without random allocation of units into
treatment and control conditions (e.g., Ariel 2016; Ellis et al. 2014), have produced
mixed results about the effectiveness of BWCs, ranging from supportive evidence, to
null, and even backfiring effects.
The Rialto experiment (Ariel et al. 2014) was the first randomized controlled trial
that looked at the effectiveness of BWCs, and specifically focused on use of force and
complaints. Rialto Police Department, a jurisdiction in California with just over 50
frontline officers, compared nearly 1,000 police shifts during which all police–public
encounters were equally assigned to either treatment or control conditions. During
treatment shifts, Rialto officers were asked to record (audio/video) all their encounters
with members of the public and to store evidence on a secured cloud. In the control
shifts, the officers were instructed to never carry the devices. Outcomes were then
measured, in terms of officially recorded use of force incidents and complaints lodged
against Rialto police officers. Following this 12 months experiment, Ariel et al. (2014)
reported a relative reduction of roughly 50 % in the total number of incidents of use of
force compared to control conditions, and about a 90 % reduction in citizens’ complaints, compared to the 12 months prior to the experiment.
Ready and Young (2015) conducted an experiment with the Mesa, Arizona Police
Department. The study analyzed nearly 3,700 field reports completed by 100 sworn
patrol officers. Random assignment of the officers into treatment and control groups
has resulted in several important findings: first, officers who did not wear body worn
cameras were more likely to conduct stop and search, and were also more likely to
make an arrest. This means that wearing BWCs may cause officers to be more cautious
and risk-averse than officers in control conditions. At the same time, treatment officers
were more likely to give citations and initiate encounters. This suggests that BWCs
may cause officers to be more proactive with this technology, however without
increasing their use of invasive strategies that Bmay threaten the legitimacy of the
organization^ (Ready and Young 2015:445).
Finally, Jennings et al. (2015) have also observed the effect of BWCs on policing,
but focused particularly on response-to-resistance incidents. In their controlled experiment, they randomly assigned 46 (of 89) officers to wear BWCs, with the remaining
43 officers assigned to a no-BWC condition. The study has shown that BWCs reduced
these types of incidents and serious external complaints. The prevalence of response-toresistance incidents and the prevalence and frequency of Bserious^ external complaints
were significantly less for officers randomly assigned to wear BWCs (p. 480).
These studies suggest that the credible deterrent effect of BWCs may rest on four
critical points: that the camera is (i) actually worn by the officer, (ii) turned on, and (iii)
used during the police–public encounter. It also requires that (iv) suspects are fully
cognizant of the BWC, for instance through officers announcing to the suspect that the
interaction is being officially recorded. If the officer applies discretion and does not
record an encounter—for whatever reason—then the deterrent effect of the camera will
be nil. To use a medical analogy, one must take a pill in order for it to be effective.
It is precisely the issue of discretion where we believe that the effect of BWCs can
vary, and the continued debate around police officers’ discretion is at the core of our
present analysis. BPolice work is complex [,] police use enormous discretion [, and]

Report: increases in police use of force in the presence of...

discretion is at the core of police functioning^ (Kelling 1999:6). Discretion underpins
policing by consent (Leyland 2012). Yet Bdiscretion^ per se is a vague term that
broadly reflects the decision-making power afforded to officers with the aim of
allowing them to decide whether or not, and to what extent, to follow police procedures
(Hirby 2015). Codified rules and regulations often limit the use of discretionary
powers—for instance in the area of mandatory arrests for domestic violence (see
review in Hirschel et al. 2007), shooting at moving vehicles (e.g., Alexander City
Police Department 2008), holding suspects in a chokehold position (e.g., NYPD Patrol
Guide 2013), or the use of ‘enhanced interrogations’ on suspects under extreme
circumstances. However, in the area of use of Bforce^, the guidelines are suggestive:
the force continuum, which is meant to align the appropriate amount of force to various
scenarios in order to de-escalate the situation, can be overridden based on the officers’
subjective perception at the time of the incident (see Watson et al. 2008). It is only with
the clear vision of hindsight that a Bforceful incident^ might be deemed to have resulted
from an abuse of decision-making powers. The core of the analysis presented below is
to understand what role police discretion plays in the emergent area of police BWCs.

Methods
Experimental procedure The experimental design underpinning this paper has been
reported elsewhere (Ariel et al. 2016) so is only summarized here. Jointly, the trials
involved 2,122 patrol officers in eight police departments, with 2,188,712 officerhours. The unit of analysis across ten trials is the police shift, in keeping with the
Rialto Experiment (Ariel et al. 2014). Each week, shifts were randomly assigned to
‘cameras on’ or ‘cameras off’. This is the most practical approach to implementing
BWC trials with police, because it means that even small forces, such as Rialto with
only 54 officers, are able to leverage sample sizes as high as nearly 1,000 shifts per year
(Ariel et al. 2014). This resulted in 4,915 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 (Ariel et al. 2016).
In practice, our pre-published protocol stripped officers of their discretion to decide
when, where, and under which conditions BWCs would be applied. Following from the
same theoretical mechanism behind surveillance underpinning the Rialto experiment (Ariel
et al. 2014; see also Tankebe and Ariel 2016), we felt that officers should not be able to
make ad-hoc decisions as to whether or not BWCs should be used on a case-by-case basis.
To try and minimize officer discretion, each participating force agreed to the following
treatment conditions, with the emphases in the original experimental protocols (verbatim):
8. TREATMENT AND COMPARISON ELEMENTS
8.1. Experimental or Primary Treatment
8.1.1. What elements must happen, with dosage level (if measured) indicated.
8.1.1.1. Wearable, personal cameras attached to each patrolling officer during experimental shifts, with capability of capturing and recording police interaction with the
public (offenders, witnesses, victims), in both colour video and audio.
8.1.1.2. Cameras must be turned on during every interaction with the public as soon as
officer(s) get out of the police vehicle, until

B. Ariel et al.

8.1.1.2.1. Situation stabilises and/or;
8.1.1.2.2. Offender is brought into custody.
8.1.1.3. Members of the encounter must be notified through a script (i.e., Byou are
being recorded on tape^)
8.1.1.4. Cameras must be worn on the uniform and visible, during experimental shifts.
8.1.2. What elements must not happen, with dosage level (if measured) indicated.
Element A: cross-over
The overall result from our previous analyses (Ariel et al. 2016) was that there were
no significant differences between treatment and control arms [standardized-meandifference (SMD) = .021; SE = .056; 95 % CI −.089 to .130)]. Taken at face value,
the nonsignificant overall results provide weak support for the deployment of BWCs in
policing, as nonsignificant results are difficult to interpret (on methodological challenges associated with nonsignificant experimental results, see discussion in Ariel
2012: 55–58). However, as Ariel and Farrington (2010: 449–450) concluded, subgroup
analyses are a natural step that comes after testing for main effects, particularly when
the findings are heterogeneous. This was the case in this multisite study, where
significant variability existed in the sites included in our prospective meta-analysis
(Q = 17.902; p < .05; I2 = 49.7 %). Therefore, we sought to conduct further analyses
with the data, as they may provide valuable information about the benefits and hazards
of the intervention in subsets of participants. To be clear, we present results from preplanned subgroup analyses on the efficacy of the treatment for particular groups of
interest, in a pre-specified manner. We did not perform these analyses when presenting
the preliminary main effects in Ariel et al. (2016), as the data we were interested in—
police officers’ discretion—were not available at the time.
Defining use of force Police use of force was measured by official self-reported logs
by police officers in the line of duty. For the purpose of this study, Buse of force^ was
defined as any application of physical restraint on the force continuum beyond
handcuffing, in order to gain control of a suspect or situation. Police regulations often
dictate that whenever officers use force during a shift they are obliged to report this—
although practice and form vary from place to place. As one participating force
dictated, BThe decision to record use [of force] is a matter for the commander and
must be entered in the commander’s policy log and the need for recording stated in the
operational order or briefing.^ Force incidents were calculated as a rate per the number
of arrests—as a way of standardizing this measure and accounting for different sized
police forces involved in the trials. We are cognizant that not all incidents of use of
force are recorded by officers, for various reasons discussed elsewhere (Sommers
2013); however, we assumed that the error rate in recording violations would be
stochastic in treatment and control conditions, given our experimental design.
Measuring compliance/integrity We defined Btreatment integrity failure^ in two
ways: first, when the police department explicitly stated that its officers were
granted discretion to wear and use BWCs, and record incidents despite the
experimental protocol. This noncompliance could be either during control shifts,
such that officers used the devices when they were not supposed to, or if they
were able to decide when to turn devices on during individual encounters (the

Report: increases in police use of force in the presence of...

protocol stated that Bevery^ encounter was to be recorded as soon as the officer
arrived at the scene).
Our second measure of treatment integrity failure was more detailed. Meta-data
automatically collected by camera manufacturers created an objective measure: Any
footage recorded during control shifts was considered a violation of the protocol—that
is, a sign of noncompliance. A control shift with at least 1 min of recorded footage was
considered a breach of protocol, as the devices should not have been worn at all during
control shifts. Conversely, when officer cameras did not record any amount of data during
a shift, this was an indication that the camera had either been left back at the station, or
switched off during the shift; we nominally defined a treatment shift without any
downloaded footage (0 min or 0 megabytes downloaded) as a protocol violation, as it
is unreasonable that any officer on duty would have nil interaction with members of the
public in middle or large police departments. There were some pre-agreed types of cases
where officers were allowed to switch off cameras—such as serious sexual violence cases
or dealing with informants—but otherwise officers were required to keep cameras on
throughout their shifts and record every interaction with members of the public.
Table 1 reports on compliance of each site. As shown, three Bcompliance subgroups^
emerged: sites in which compliance was high (n = 3), sites in which compliance broke
down completely (n = 4), and sites in which the police maintained compliance during
control conditions, but failed to follow experimental protocol during treatment shifts
only (n = 3). Put differently, four sites gave officers complete discretion on when and
where BWCs should be used. Three sites stripped officers of their discretion completely.
Three more sites adhered to the protocol for control conditions, but allowed officers
discretion as to when BWCs should be used in treatment cases, including at what point
during the interaction BWCs were turned on in these treatment cases.

Results
As noted, there was an overall nonsignificant main effect (thus far) of cameras on
police use of force based on analyses synthesizing results from the ten trials with data.
However, this average masks instances where use of force decreased and others where
it increased. Figure 1 shows the results subgrouped by the level of treatment integrity—
remembering that the protocol-dictated Bcompliance^ means officers did not have
discretion about when cameras were turned on.
When officers followed the experimental protocol (Bhigh compliance^), use of force
decreased in line with expectations (SMD = −.346; SE = .137; 95 % CI −.614 to
−.077). However, in trials where officers were able to use full discretion in both
treatment and control conditions (Bno compliance^), the overall effect was nil
(SMD = .009; SE = .070; 95 % CI −.127 to .146). This outcome can be expected, as
there were no differences in the application of the treatment in either experimental or
control conditions. Thirdly, when officers applied discretion during treatment conditions only but followed protocol during control conditions, use of force increased
(SMD = .392; SE = .130; 95 % CI .136 to .647). These patterns are presented in Fig. 1
below. In terms of percentage change, high-compliance trials witnessed rates of use of
force decreasing by 37 %. In trials with no compliance in treatment conditions, rates of
use of force increased by 71 %.

207,470.21

2,127,377

Grand
total

236,375.22

249,470

Site K

SD

151,533

Site J

M

26,757

Site I

1,886

188,400

108,817

Site F

Site E

Site H

3,390

285,700

751,500

Site D

267

1,004.64

1,567.70

15,677

1,175

2,193

876

2,591

313

1,097

285,700

203,800

Site C

1,889

Total arrests
during RCT

Site B

161,400

Population
size

Site A

Site

276.99

491.50

4,915

888

675

180

734

784

462

134

462

134

462

No. of
shifts

275.44

212.20

2,122

105

150

60

115

120

870

22

111

23

546

No. of frontline
officers

163,573.99

218,871.20

2,188,712

127,872

135,000

494,640

176,160

235,200

369,600

9,112

410,256

9,112

221,760

Officer hours
during RCT

High compliance

No compliance

No compliance

High compliance

No compliance

High compliance

No compliance

Compliance subgroups

Police announce breakdown in treatment group + empirical Compliance within control
check in treatment group
group only

Police announce breakdown in treatment group + empirical Compliance within gontrol
check in treatment group
group only

Police announce breakdown in treatment group + empirical Compliance within control
check in treatment group
croup only

Empirical

Police announce breakdown + empirical

Police announce breakdown

Empirical/manual read of daily logs

Police announce breakdown

Empirical/manual read of daily logs

Police announce breakdown

Indication of treatment integrity

Table 1 Descriptive statistics in ten experimental sites and breakdown of sites based on treatment integrity failure

B. Ariel et al.

Report: increases in police use of force in the presence of...

Fig. 1 Use of force rates: treatment integrity breakdown

Discussion and conclusions
The background for the trials described above was to assess whether the audial and
visual digital recording of police–citizen interactions could act to deter police from
using force and/or deter suspects from instigating forceful encounters. The results
demonstrate that BWCs are able to achieve this objective, but only in situations where
police relinquish some discretion on activating these devices. In fact, when police used
discretion during treatment shifts, reported use of force increased. The causal mechanism for this increase is not clear, but we speculate that the selective activation of
cameras by police is a corollary to situations that are already escalating in aggression.
Furthermore, we also suggest that activating a camera during a tense situation may
serve to increase the aggression of the citizen/suspect (and thus the officer).
Given our preliminary findings, we think that there is a clear route for amending
these concerns, by improving the implementation of BWCs around the world: cameras
should remain on throughout the entire shift—that is, during each and every interaction
with citizens—and should be prefaced by a verbal reminder that the camera is present.
We argue that the verbal reminder delivered by the officer wearing the camera provides
a mechanism to remind that ‘rules of conduct’ are in play—common courtesy from
officer and citizen for one, and potentially a legal requirement given the weight of
privacy sensitivities in the public domain1. Pushing this further, we argue that the verbal
prompt is a mechanism that pushes mental processing of the situation towards the
rational–deliberative mode of thought (Kroneberg et al. 2010; Thaler and Sunstein

1
Other Bannouncements mechanisms^ could be appropriate as well, such as the wearing of a badge or patch
by the officer indicating the recording of such encounters, in order to strongly convey the message to the
citizen that the interaction is recorded.

B. Ariel et al.

2008), thus enabling the hypothesized deterrent effect of the camera to actually operate
on officer and/or citizen. Future research on body-worn cameras ought to look into
these theoretical as well as practical implications more robustly than presently
available.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International
License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were made.

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