On Saturday night, the emergency room staff knows all too well what's coming — people showing up with a broken jaw, a knife wound or a bashed-in face, often after too many hours in a pub. Doctors at the emergency department in Cardiff, Wales, realized that many of the people who were injured in fights never reported it to the police. That realization led to a simple program that has radically reduced the toll of violence.

The hospital already had information on where and how people in the emergency room had been injured — "which bar, which club, which street, which park, which school," says Dr. Jonathan Shepherd, a professor of oral and maxillofacial surgery at Cardiff University and an author of the study.

Many people who are injured in a fight never reported it to police, it turned out. That was particularly true with fights happening inside pubs and clubs. "They don't know who the perpetrator was, so what's the point of going to the police?" Shepherd tells Shots. "And they're afraid of having their own conduct scrutinized. If it's a fist fight or gang related or drug related, nobody's going to want to go to the police."

So the hospital started sharing its information with the police, after removing names and other identifying information. With that anonymous information, authorities found they could do a much better job of targeting violence hot spots.

That and other measures reduced the social and economic costs of violence in Cardiff by about $11 million in 2007, according to the study, which was published in the journal Injury Prevention.

That includes lower costs of health care and criminal justice. The program itself costs the city about $338,000 a year, including data-management costs and prevention efforts.

Some of those prevention efforts were novel indeed. After it became clear that a lot of people were getting injured in fights inside pubs and bars, the establishments were required to switch from glass to plastic cups. The number of people injured with a broken bottle or glass dropped by 70 percent.

A section of the Cardiff city center rich with nightlife was blocked off to cars on weekend nights, making it less likely that revelers would run afoul of a motor vehicle. And some bars that proved to be big trouble spots were closed.

An earlier study of the Cardiff experiment found hospital admissions reduced by 42 percent, compared to cities without the reporting plan and preventive efforts, and a 32 percent reduction in wounding recorded by police.

Word on the success of the reporting system spread. By mid-2012, one-third of emergency departments in England had implemented it in full, and almost all were well on their way to doing so, Shepherd says.

"In the U.K. gun violence is very, very rare," Shepherd says. "There are some years when the local police force here doesn't have one firearms offense to deal with. That means the focus shifts from gun violence to knife violence and fistfights." But he says that this same approach could be useful in addressing gun violence as well.

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