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“Police departments are using data in a more timely manner and more wisely,” he said.

Commissioner Ramsey, a former D.C. police chief, said Philadelphia created patrol beats and checkpoints at the 125 most violent intersections in the city and subsequently experienced a 22 percent reduction in violent crime in those areas. He said he also focused police resources on the most violent police districts at times when violent crimes were most likely to occur.

In another indication of a declining crime rate in 2009, the deaths of police officers in the line of duty was at its lowest number in the past 50 years.

According to a study by the National Law Enforcement Officers Memorial Fund, 124 officers were killed while on duty, the lowest number since 1959, when 108 were killed. The study found a 7 percent decline in officer deaths when comparing 2009 with 2008.

But the National Law Enforcement Officers Memorial Fund also found reason for concern in 2009, as the number of firearm-related cop killings increased 23 percent from 39 in 2008 to 48 in 2009. The overall decrease was largely driven by a decrease in traffic-related fatalities.

FBI spokesman Bill Carter said the bureau does not generally comment on trends in crime statistics because of the complexity of factors that contribute to them.

“The amount and type of crime varies from place to place depending on a number of factors that affect crime in a particular community. We don’t give a single reason why,” he said.

Some analysts expected crimes to increase in 2009 because of tough economic times. But James Alan Fox, a professor of criminal justice at Northeastern University in Boston, said the link between the economy and crime is largely a myth.

“All the research on economic indicators and violent crime in particular show there really isn’t a link,” Mr. Fox said. “People don’t essentially become criminals because they are out of a job; the decision to pursue a life of crime is made independently of the economy.”

However, Mr. Fox said an indirect connection between the economy and crime can be made in some cases. When the economy lags, he said, fewer resources are available for police budgets and other crime-prevention programs, which can have an impact on crime rates, particularly property offenses.

c Sarah Abruzzese contributed to this report.