Raw crime data masks differences among cities

By Robert Friedmann

Police chiefs are accountable to their mayors and residents for the crime figures in their city.

When their crime rates are lower than in other cities, they often claim credit.

When they are higher, they sometimes explain that their crime rates should not be compared with those elsewhere because their city differs from others in ways they cannot control.

Police officials are correct to question raw crime comparisons based on the number of crimes and city population only. Indeed, the FBI warns against making such comparisons.

Thus when Baltimore is compared, say, to San Diego, not only is its crime rate higher, so is its level of unemployment, poverty and other conditions that are correlated with crime.

And the police have little control over those conditions; they’re stuck dealing with the crime that results.

Several years ago my colleagues Alfred Blumstein of Carnegie Mellon University and Richard Rosenfeld of the University of Missouri-St. Louis and I joined efforts to produce better crime comparisons that take account of differences in such crime-producing conditions.

Since 2002 we have produced yearly rankings of American cities according to their homicide rates, after adjusting for differences across cities in poverty, median income, unemployment and other conditions that are strongly associated with city homicide rates and over which the police exert little control.

Basically, the procedure reranks the cities based on their expected homicide rates assuming they all have the same level of poverty, unemployment, etc.

We recently published the homicide rankings for 63 large cities based on crime data for 2008. Some cities, like Detroit, rank near the top of the list before adjusting the data for differences in crime-producing factors.

After adjustment, Detroit’s rank dropped dramatically. This means that Detroit’s homicide rate in 2008 was quite a bit lower than would be expected based on its dire socioeconomic circumstances.

Other cities, such as Albuquerque, N.M.; Las Vegas, and Los Angeles, rose in rank after adjusting for socioeconomic differences, meaning that their homicide rates were higher than expected based on their relatively benign conditions.

What about Atlanta? Atlanta’s homicide rank has dropped each year since 2002 after adjusting the data for differences in socioeconomic conditions.

For example, in 2008 Atlanta ranked No. 11 in homicide before adjusting for socioeconomic differences, yet it dropped to No. 58 after adjustment.

Even in 2007, when the city experienced a sizable homicide increase, Atlanta compares favorably to other large cities with sizable disadvantaged populations.

Meaningful comparisons of city crime problems, especially if they are used to evaluate police performance, should not stop with the raw crime rates.

They should also tell us something about how those rates are affected by economic and social conditions for which the police are not responsible but that make their job more or less difficult.

Taking account of socioeconomic differences when comparing city crime figures does not change the crime risk residents face, nor should it send a message that a city’s crime rate is “acceptable.”

But the results are much more informative about a city’s crime challenges than are the raw crime rates alone.

Robert Friedmann is Distinguished Chair of Public Safety Partnerships at Georgia State University. Go online for a PDF of the complete data on cities.