Friday, May 29, 2009

Stats And Other Ways To Fib

The Austin American Statesman has an interesting piece on the Texas Department of Public Safety's recent release of preliminary crime stats for Texas.
"When the Department of Public Safety released its latest crime statistics last week, it crowed that the statewide drop in the 2008 crime rate was due to “Texas’ tough approach to law enforcement.”

That’s funny. Across the country, crime rates have been trending down for years, and more steeply than in Texas. More remarkably, the volume of violent crime is falling, too.

What’s the difference between crime rates and crime volume? Rates — the ratio of offenses per 100,000 population — adjust for population growth. Volume — simply counting the number of offenses reported — does not.

Texas is growing, so the DPS prefers to talk about crime rates statewide. That’s why its press release led with the news that the major crime rate in Texas dropped 2.9% from 2007 to 2008. That includes a 3.2% decline in the rate of property crime and a small decrease (less than 1%) in the rate of violent crime.

But crime volume should be part of the conversation too, especially since those numbers are down nationwide. In the first six months of 2008, the total number of violent criminal offenses (murder, rape, robbery and aggravated assault) dropped 3.5% from the same period in 2007. The statistics are from the FBI’s latest semi-annual Uniform Crime Report. (The next report, due later this year, will cover all of 2008. Both have the same Texas statistics that the DPS reports.)

Hold on, there are even more numbers. In three of the four most-populous states, total violent crimes also declined in 2008: California, by 3.3%; Florida by 4.3%; New York by 2%. Those states are growing, too.

What happened in the fourth state, tough-talking Texas? Here, the total number of violent crimes was up 1.2% in 2008. Yes, up. That was due entirely to an increase in aggravated assaults, which occur far more frequently than other violent crimes." Source: Austin American Statesman

The unfortunate part of the article is Ms. Bell's seeming dismissal of crime rate as being less important than crime volume. It is my professional opinion that rate is often times more important than volume.

If your population grows, you can likely expect more crime and disorder. Where people are there is always a certain amount of crime and disorder. Depending on what is happening to your population numbers and your crime volume numbers you can have a number of interesting scenarios. Crime rate is the ratio of crimes to people. Because it is a ratio it is possible to have your volume increase, yet your rate decrease and vice versa.

Because rate determines the number of crimes in a given population it can be used to compare different populations (areas) to determine the difference in those areas. If you knew that the average rate for a particular type of crime was lower in another city than the rate in your city, this could lead you to believe that this type of offense was more problematic in your city. You should then examine the likely factors causing that disparity. Maybe that's a type of crime you need to focus more enforcement effort on. This is a very simple example as demographic, geographic and other factors can weigh heavily on the incidence of crimes in a population. However, it's great tool to determine potential problem areas for further study.

The biggest problem with crime statistics is not methodology but their misuse. Rather than using them as a tool that reasoned heads can use to make their cities safer, some with an axe to grind use them for political reasons, some use them to enlarge their fiefdoms, and some hide their flaws behind statistical chicanery.

We should always keep in mind the old saw, "there are lies, damn lies, and statistics."

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