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But the collateral effects of the violence created ongoing problems for big-city mayors.

Mr. Ludwig cited research by colleague, Steve Levitt, author of the book Freakonomics, on the decimating effect.

“Every homicide reduces a city population by 70,” he said, referring to residents’ desires to move to locales they consider safe. Comparatively, for every other type of crime recorded in a city, the population shrinks by one person, Mr. Ludwig said.

“Getting the homicide problem under control is particularly important to mayors who want to grow their city’s tax base,” he said.

In some cases, setting goals and choosing numbers hasn’t worked out as intended.

As mayor of Baltimore from 1999 to 2007, Maryland Gov. Martin O’Malley pledged to reduce homicides to fewer than 175 a year — a goal that the city has yet to achieve. The city dropped below 200 homicides last year for the first time since 1977, only to again surge past the benchmark figure this year with 215 killings as of Friday.

When first elected mayor of Philadelphia in 2007, Michael Nutter also made promises to reduce homicides in the City of Brotherly Love by 30 percent to 50 percent. The reduction goals were echoed by Commissioner Charles H. Ramsey, the former chief in the District, who made note of aiming to cut homicides to less than 300. But a violent 2012 sent the number of killings soaring, opening public officials to criticism when the homicide total raced past 300 in November to 329 as of Monday.

In Chicago, the number 500 took on significance for the wrong reason. The city recorded 512 homicides in 2008 before three years of declines. A violent 2012 was capped Friday when Superintendent Garry F. McCarthy acknowledged with a public statement that the city had logged its 500th killing, calling the milestone a “tragic number.”

Perhaps wary of the political liability of making such promises after New Orleans failed to meet Police Superintendent Ronal Serpas’ goal of even a 5 percent homicide decrease in 2011, officials left any numerical benchmark out of a comprehensive plan developed for last year to reduce the city’s homicide rate.

“It is very common to see this kind of goal-setting. They don’t always reach their goal and that runs the risk of them being called a failure,” Mr. Fox said.

It’s also a risk that makes Chief Lanier’s pledge all the more notable.

“You wouldn’t have heard a police chief saying that 20 years ago,” Mr. LaFree said.

An improbable goal

By openly voicing a goal of recording fewer than 100 homicides in a year, Chief Lanier acknowledges she took a risk.

Her first two years in office, the homicide total increased, from 169 in 2006 when she was appointed to 181 in 2007 and 186 in 2008.

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