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The theory: police have gotten better at detecting and preventing crime

The case for: CompStat, the crime tracking system that gave rise to “data-driven policing,” was popularized in the 1990s — and many cities that started using it saw serious results. CompStat is usually associated with high-tech, detailed data collection on crime trends. But most of the time, the data collection is paired with a management system, where information and trends are discussed in meetings, divisions quickly try new tactics to address crime as they come up, and police are held accountable for keeping crime down. CompStat is still used in many, if not most, police departments, and it’s been widely praised by police executives. (It’s also been associated with stats-juking scandals.)

compstat Brennan Center for Justice

Some research also suggests that certain police tactics do decrease crime. Hot-spot policing — which directs police not to focus on individual people, but instead to be present and visible in the places where crime is most likely to occur — has solid empirical support. And studies suggest that in some contexts, “community policing” — focusing on partnerships with community members first, and enforcing the laws against people second — can have positive results.

George Tita, a criminologist at the University of California Irvine, argues that police’s ability to better target certain neighborhoods and offenders has built a deterrent to violence. “I think because police are more effective in what they do,” Tita says, “the probability of seeing some action taken after your group is involved in violence is much higher than it was 20 years ago.” And criminologists who study how people can be deterred from committing crimes to begin with argue that if someone knows he’ll be apprehended right after committing a crime, he’s less likely to commit it.

The case against: In the early 1990s, before crime started to decline, many criminologists had given up on the idea that what police did had any impact on crime whatsoever — at least as far as preventing it was concerned. Police tactics have changed between then and now, but that doesn’t mean they caused the crime decline. It’s fully possible that what look like improvements in policing from this perspective could have just been phenomena that happened alongside a decline in crime for other reasons. Add to that the same problems you have in analyzing broken-windows policing — that police tactics are rarely implemented the same way in two different places — and it becomes a hard case to make.

”I hate to say it, but I don’t think it matters,” John Roman of the Urban Institute said, arguing that different cities used very different policing tactics even as all of them saw drops in crime. “There’s a lot of correlation, but not much causation.”

The bottom line: CompStat helped; with other tactics, it’s less clear. The Brennan Center for Justice’s February 2015 report examines crime rates in 50 American cities before and after they started using CompStat. In all, it says CompStat played a small part in the crime decline since the early 1990s. That’s a huge difference from the assumption that police have no impact on crime at all.

It’s harder to analyze other changes in policing tactics, like hot-spot policing and community policing, for the same reasons it’s hard to analyze broken-windows policing. Hot-spot policing, in particular, has a relatively large body of research supporting it (for criminological research on police tactics, at least) — but it’s not enough to assess its impact nationally.