Police body cams worn by 2,600 officers in the nation's capital did not affect citizen complaints or the use of force by the Metropolitan Police Department (MPD), according to a new study.

"We found essentially that we could not detect any statistically significant effect of the body-worn cameras," according to Anita Ravishankar, an MPD researcher at a city government group named Lab @ DC.

To conduct the study, researchers identified officers across the seven metro police districts that fit a specific criteria: the officer had to have active, full duty administrative status without a scheduled leave of absence during the study; the officer had to hold a rank of sergeant or below; and the officer had to be assigned to patrol duties in a patrol district or to a non-administrative role at a police station. From there, officers were split into control (no body cams) and treatment groups. "Our sample consisted of 2,224 MPD members, with 1,035 members assigned to the control group, and 1,189 members assigned to the treatment group," the study notes. The study (PDF) then measured four outcome factors: reported uses of force, civilian complaints, policing activities (which includes tickets, warnings, arrests, etc.), and judicial outcomes, specifically whether MPD arrest charges led to prosecutions.

DC Police Chief Peter Newsham told NPR that everybody was expecting a different conclusion about the agency's $5.1 million program. "I think we're surprised by the result. I think a lot of people were suggesting that the body-worn cameras would change behavior. There was no indication that the cameras changed behavior at all."

Police body-worn cameras (BWCs) are becoming a common staple in American policing in the wake of many high-profile shootings. The Department of Justice doled out $23 million in grant funding for them in 2015, and 95 percent of the nation's largest police departments have adopted them or intend to deploy them.

The stated goal for body cam usage is to promote public accountability and transparency at a time when seemingly everybody is filming one another with their mobile phones. Body cams have become such a focus in policing that we now have documented instances of police fabricating footage. As a general rule, body cam footage across the US is currently not a public record.

But the all-important barometer of citizen complaints or use of force has not changed by the use of body cams, according to the new study (PDF):

We estimated very small average treatment effects on all measured outcomes, none of which rose to statistical significance. These results suggest that we should recalibrate our expectations of BWCs' ability to induce large-scale behavioral changes in policing, particularly in contexts similar to Washington, DC.

Michael Tobin, the director of the DC Office of Police Complaints, told The Washington Post that a review of body cam footage shows that, in tense situations, "people react the same whether they are on camera or not." However, he added that, "in routine encounters, when people know the camera is active, I believe we see people acting differently—more professionally, more formally."

Despite its overall finding, the report notes that body cam footage has other value—like its use in courts and for purposes of training and transparency.