During the 2014 Ferguson protests, America woke up to a surprising fact: There are no good national numbers on police conduct. While the federal government collects reasonably accurate crime statistics, it doesn't know much about law enforcement patterns such as racial profiling and police use of force. It turned out even the government's most basic statistic — the number of people killed by police — was way off.

The White House says it wants to change that with the Police Data Initiative. It's an outgrowth of the President's Task Force on 21st Century Policing, whose final report called for greater data transparency as a means to build trust between police and communities. The Police Data Initiative encourages departments to anticipate the kind of numbers their communities want to see, and provide them, preferably in database format. As an example, the White House cites the online data portal on police shootings set up by the Dallas Police Department.

But there's a caveat, here: This is all voluntary.

The White House says 53 jurisdictions so far have pledged to share this kind of data, a list that includes some big agencies such as the NYPD. But an additional 17,000 or so law enforcement agencies have not yet signed on, and they account for about 85 percent of the country's population.

The White House simply doesn't have the authority to require police departments to share statistics. And administration officials say they prefer the voluntary approach.

"This is how any great thing that has occurred in our industry ... has started, with a handful of agencies that had the progressiveness and the courage to do it," said Ron Davis, director of the Justice Department's Office of Community Oriented Policing Services. At a White House-sponsored meeting of participating police departments Friday, Davis predicted the effort would catch on.

"We know that if you start collecting the data," Davis said, "that 50 today will be 100 in two months, and 200 by next year, and the question three years from now is, 'Why would you not be having open data?' "

The White House is also trying to sell police chiefs on the idea that data transparency can defuse community tensions. Administration officials say accurate data can help a police department avoid being "defined by rumors."

"Providing open data is telling your community that ... I serve you," says Davis.

Dallas Police Chief David Brown agrees, citing his city's release of years' worth of detailed data on police shootings.

"This built quite a bit of trust," he says. "In 2015, our department's excessive-force complaints were reduced by 67 percent, and our deadly force incidents have been reduced by 45 percent."

Still, this openness to providing data seems to be most prevalent in police departments that are already in cooperative relationships with the federal government. Many of them receive federal grants, observes David L. Carter, a professor in the School of Criminal Justice at Michigan State University. Other participating agencies are already in the "progressive" camp.

"In many cases, progressive police executives feel it's 'the right thing to do,' and will volunteer," says Carter in an email. But he thinks others may take a pass, especially if they think data transparency will cost them money. "You will have agencies that have no inherent opposition to the PDI, but it does not really provide them a return on investment."

The result? There may be good stats on places like LA and Dallas, while thousands of smaller communities — places the size of Ferguson — will continue to be white spaces on the statistical map.

Finally, there's the question of standardization. The White House has no plans to set rules for how data should be collected.

"Some agencies appear to have very good data, and others don't," says Robert Kaminski, who studies police use of force at the University of South Carolina. "It varies tremendously."

Even basic definitions can vary. Kaminski gives the example of handcuffing: Some departments classify that as a use of force; others don't. Differences like that make comparisons between data sets extremely difficult.

The White House is even leaving it up to departments to decide which categories of data to release.

Carter says he understands why the White House is telling departments to "report what you feel comfortable reporting," as he puts it.

"One, it [helps] sell it to agencies to participate," he says. He thinks the White House is eager to "get the data flowing" and that things will become more structured once the Police Digital Initiative is institutionalized.

And despite the lack of a mandate, researchers say they have detected a new attitude among police departments. Kaminski has been requesting data from police for two decades, and he says just in the past few years, departments seem more willing to hand over numbers — even those that might make them look bad.

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