HIGH-SPEED car chases, shoot-outs, dealing with politicians: life for a Los Angeles police officer can be trying. Yet for sheer stress little can compete with the ordeal of the Compstat meeting. Every seven weeks bureau commanders are grilled by a senior panel, often including the police chief himself, on the whys and wherefores of crime in their jurisdictions. They are expected to have an on-the-spot grasp of the statistics: if there has been a spike in burglaries from vehicles, the captain’s interrogators will want to know what is being done about it. There is no hiding from the numbers: data-laden documents are distributed before the meeting, and overhead map projections pinpoint the sites of individual incidents in pitiless detail. The pressure has reduced officers to tears.

Although some think Compstat introduces incentives for police to fiddle the figures (or “juke the stats”), most analysts agree that it has improved the effectiveness and efficiency of the police in Los Angeles, and other cities with similar systems. (Compstat was brought to Los Angeles from New York by Bill Bratton, a tough-talking police chief who oversaw declines in crime in both cities.) It helps in two ways. First, by mandating the collection and management of detailed crime data it makes it easier to allocate police resources. Second, it introduces accountability of the strictest sort. If you are not reducing crime in your bureau, it doesn’t matter if your children play baseball with the mayor’s: you can forget about that promotion.

Crime in America has plummeted since its numerical peak in 1992; the violent sort by 38%, according to FBI statistics. To what extent can innovations in policing like Compstat explain the decline? Naturally, police departments and politicians take much of the credit. In July Charlie Beck, chief of the LAPD, said that new crime figures showed Los Angeles to be the “safest big city” in the country (though only three other cities matched his definition of “big”: New York, Chicago and Houston). The mayor, Antonio Villaraigosa, boasts that the city is safer than at any time since 1952. Both agree that the police have been doing a fine job.

Driving one evening through Watts, a poor part of South Los Angeles with a troubled past, Sergeant Steve Lurie explains that his colleagues find they are called on to be social workers, child psychologists and marriage-guidance counsellors as often as they are police officers. He knows the projects, the gangs and the problems. He describes the challenges of “hood days”: on dates that correspond to numbered streets (eg, October 3rd for 103rd Street), the gang within whose territory the street falls holds a big party. Often this means nothing more than food, drink and music. But sometimes the gangbangers take the opportunity to go on a murderous shooting spree on neighbouring turf.

At around 10pm Mr Lurie is called to the site of a suspected burglary. There is a hint of danger in the air when police eject an entire family from their home after they think they spot the suspect hiding inside. Neighbours, many of them small children, spill on to the streets to see what is going on. But Mr Lurie and his team delicately reduce the temperature, separating the aggressive men from the rest of the crowd. These are hardly the experiences most parents would want for their children on a Monday evening. Yet residents seem by and large to see the police as arbiters and protectors, and Watts bears little resemblance to the crime-ridden hellhole of 20 years ago. Mr Lurie says he wouldn’t be surprised if this year there were as many murders in all of Los Angeles as there were in his division in 1992. Yet good policing is not the only factor behind the decline in crime. The fall in violent crime in Los Angeles began in 1992, a decade before the introduction of Compstat, and a time when the LAPD was hated by many residents, particularly blacks and Latinos. The truth is that no one predicted America’s great crime decline, and no one has a definitive explanation for it. Particularly confounding has been an acceleration in the drop since 2008; many observers thought a poorer country would be a less law-abiding one. Everything from the removal of lead from petrol to the increased prescription of psychiatric drugs has been credited with the decline. A controversial theory proposed in 2001 by two academics, Steven Levitt (of “Freakonomics” fame) and John Donohue, which attributed half the previous decade’s drop in crime to the legalisation of abortion in the 1970s, still has fans. Today there is growing interest in the role of video games and social-media technologies in providing young men, who are responsible for the lion’s share of violent crimes, with alternative ways to spend their time. Other analysts look to structural or demographic explanations. Jack Levin, a criminology professor at Northeastern University in Boston, acknowledges the success of policing strategies, but notes that an ageing society like the United States should expect to experience less violent crime. Immigration also matters, he says: studies have repeatedly shown that cities with large immigrant populations experience lower rates of violent crime.

Then there is the awkward issue of incarceration. America continues to lock up a scandalously large number of its people: around 1% of the adult population is behind bars at any time. But, says Mr Levin, “the relationship between the incarceration rate and the violent-crime rate is not very strong.” New York has not followed the national mania for imprisonment, and yet the decline in its crime has been among the most impressive. Indeed, in states with a particular fondness for imprisoning citizens, such as California, the policy may have done more harm than good. Jonathan Simon, a law professor at Berkeley, points out that removing young men from their communities for long stretches erodes the social links—an older relative, say—that might otherwise help prod them out of crime. Today, under a controversial policy known as “realignment” forced on California by the Supreme Court, the crowds of inmates in the state’s prisons have at last begun to thin out.

Brusque encounters

An emerging challenge for police in some cities is that tactics that prove effective in the short term may also lose them trust. Their widely used “stop-and frisk” powers in New York City, for example, may have taken thousands of guns off the street, but they have also led to furious allegations of racial profiling. “We’ve figured out that encounters with young people reduce violence, but they also have negative effects,” says Mr Simon. “Let’s see if we can separate the two.” A growing number of police are being killed on duty.

Moreover, the good news hardly extends to every corner of the country. Violent crime remains extremely high in some troubled cities, such as Memphis and Detroit, and in smaller places such as Oakland, California, and Camden, New Jersey. Most striking is an unexpected spike of gang-related violence in Chicago, where murders are up by 28% so far this year. Against a backdrop of a long-term decline in all crime in the city, as well as a 10% decline for the year, the sudden unrest has caused some alarm.

The real challenge will come when crime rates bottom out nationally, as one day they will. Compstat may have given some police officers sleepless nights, but in a broad environment of falling crime there is more credit than blame to go around. When the crime rate starts to rise, Compstat meetings will become that much harsher.