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Volunteers should resist the urge to intervene, Tutko said, even if they happen to see a crime in progress, because they lack training and may become victims themselves. He tells trainees that “you do what you can, when you can, as much as you can, but if you cross the line, everybody loses.”

Scholars say that while watch groups primarily act as deterrents and feed information to the police, they may provide more intangible benefits, too, like improving neighborhood cohesion and giving residents a sense of security.

The authors of a 2008 Justice Department review concluded there was “some evidence that Neighborhood Watch can be effective in reducing crime,” but said that while some programs work as intended, others work less well or not at all.

Often started as a response to persistent crime, they can be a challenge to keep alive once the initial threat fades — either the bad guy is caught or goes elsewhere — and residents turn their attention away.

“Most neighborhood watches don’t last very long. They usually galvanize themselves around an incident, or a series of similar incidents, and then the momentum dies out relatively quickly. That’s why it’s not really an effective crime prevention strategy on a wide scale,” Novak said.

Allentown, an eastern Pennsylvania city of about 100,000, has managed to keep its neighborhood watch system going since the mid-1970s, with more than 20 individual groups and hundreds of volunteers.

They are not armed, and there has never been an incident, said Assistant Police Chief Joe Hanna.

“We tell them that we are the police, that if you see a crime in progress, get on the phone and call 911. We’ll be there promptly, and let us handle the dangerous side of it,” Hanna said. “The last thing we want is for them to put themselves in harm’s way” or hurt someone else.

Associated Press writers Kathy Matheson and JoAnn Loviglio in Philadelphia and news researcher Barbara Sambriski in New York contributed to this report.