Thursday, January 14, 2010

New York City Murders - 2009

The Jan 11, 2010 issue of TIME, in its "Numbers" section, reports that NYC had 461 murders in 2009 as of Dec 27. It then says that this is the lowest number of murders there "since the city began keeping records in 1962." Is that good? We can't really tell without additional information that is not presented.

Remember, when you see raw numbers presented like this, that they are almost meaningless alone. We need to calculate proportions, for starters, and also consider contextual changes. For example: if the population of NYC decreased, then it is possible that the murder rate per capita actually increased, despite a decrease in the raw number of murders.

Some poking around at the Census Bureau and the NYC Dept. of City Planning shows that NYC's population has been growing recently (from about 8.0 million to about 8.4 million between 2000 and 2008). A look farther back, however, shows that the population dropped significantly in the 1970s, and did not return to the 1960 level until perhaps late in the 1990s. Because of this, it is less interesting that the murder raw number is the lowest since 1962 than it is that it is the lowest raw number since around 1980, whenever the population was lowest. These comparisons are still inferior to knowing the true murder rates per capita.

Why was NYC singled out? No information is presented that makes NYC seem special for its decrease in murders. In fact, the Department of Justice Crime Victimization Survey (I love it) shows that the 2004 murder rate of the country as a whole is down to where it was in the mid-1960s.

It is interesting to note the sharp decline of the murder rate during the Great Depression of the 1930s, eh? That bit of context helps put the current dip into perspective as well. Perhaps affluence permits a greater demand for illegal drugs, which contributes to homicide among competing sellers. Or affluence permits more carousing, which presents more opportunities for interpersonal conflict than if people only stayed home. Affluence certainly permits a greater ability to purchase guns. Most murders are impulsive acts of anger by armed persons with poor inhibition.

My remaining question concerns the consistency of data collection. Over time, have there been changes in classification of murders versus manslaughter, for example? Did the numbers (not the DOJ numbers) come from individual police precincts, and did they all report, or the CDC, or somewhere else?

Hopefully this helped put Time's poorly presented number in a context that provides it with more meaning and utility.

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