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Randomized Controlled Trial of the Metropolitan Police Department Body-Worn Camera Program

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Two years ago, we launched a police body-worn camera (BWC) program here in Washington, DC. Like many cities across the United States, we adopted this technology with the hope that, alongside other Metropolitan Police Department (MPD) initiatives, it would benefit the District by improving police services, increasing accountability for individual interactions, and strengthening police-community relations. The cameras might encourage positive behavioral change and the video footage might be useful as evidence.

But what are, in fact, the impacts of body-worn cameras in the District? We designed a rigorous field experiment to begin answering this question.

The Approach

Chief Newsham and MPD staff worked with The Lab @ DC to design and implement a randomized controlled trial (RCT). With this design, individual officers were randomly assigned—imagine flipping a coin—to either wear a body camera (treatment) or not (control). We then compared these two groups. With over 2,200 officers involved, this is one of the largest and most rigorous studies on this issue to date.

We used administrative data to measure effects. The primary outcomes of interest were documented uses of force and civilian complaints, although we also measure a variety of additional policing activities and judicial outcomes.

From the beginning, we prioritized scientific rigor and transparency. We created a detailed write-up of the planned methodology and statistical analyses we would use—what scientists call a pre-analysis plan. We shared this document publicly, including on the Open Science Framework, before we analyzed the data. This means that we could not, intentionally or unintentionally, change our analytic approach after the fact to support a particular viewpoint.

The Results

What did we find when we compared the treatment and control groups? We found that body-worn cameras had no statistically significant effects on any of the measured outcomes.

What does this mean?

We learned that BWCs do not have detectable average effects on documented uses of force or civilian complaints. That said, we cannot rule out that BWCs cause small decreases or small increases on these two outcomes.

For example, we find that a group of 1,000 officers with BWCs is estimated to report 74 more uses of force in a year than officers without BWCs. This is our best estimate. However, the data are also consistent with the real effect of BWCs being anywhere from a decrease of 97 uses of force to an increase of 244 uses of force per 1000 officers, per year.

Because this range spans negative, zero, and positive values, the result is considered “statistically insignificant,” or “null.” More plainly, we interpret this to mean that BWCs had no detectable, meaningful effect on documented uses of force.

To explore descriptions and an interactive graphic explaining results on the other outcomes, click below.

Conclusions

Our experiment suggests that we should recalibrate our expectations of BWCs. Law enforcement agencies (particularly in contexts similar to Washington, DC) that are considering adopting BWCs should not expect dramatic reductions in documented uses of force or complaints, or other large-scale shifts in police behavior, solely from the deployment of this technology.

We would also temper expectations about (and suggest further research into) the evidentiary value of BWCs. The administrative court data we had access to has certain limitations, but preliminary analyses do not uncover any clear benefits.

That said, no single scientific study can provide all the answers to every question we might have about a program or policy. There may be effects our study was not designed to measure. We do not rule out that BWCs may affect a small number of incidents, too few to detect in an average estimate.

Looking Forward

This study is part of our broader commitment to evidence-based policy-making and governance. As we design and adopt new policies and programs, it’s critical to measure outcomes in order to understand what works—and what doesn’t—so that we can continuously identify ways to improve how District government is serving its residents.

To this end, we should remember that the BWC program is only one of many initiatives to improve policing in the District. It is rarely realistic to depend on a single policy or program to achieve desired outcomes. Policing is complicated, and while technology can support this complex work, it alone cannot achieve our public safety goals.

As we move forward, there are a number of questions we’re working to answer, which will help inform how to further optimize performance of the BWC program. For example, how does the BWC program impact perceptions of police and judicial legitimacy, or how might the camera footage be used to best train police cadets and officers?