Monday, July 23, 2012

60 Steps Revisited: Step 27 - Know How To Use Rates and Denominators

Back in 2009, I did a series of posts covering the excellent book Crime Analysis For Problem Solvers. The book is published by the US DOJ's Problem Oriented Policing Center (POP Center). Because of the value I think this book has for crime analysts, and policing in general, I am going to re-post this series on here on the blog.

This post in my series examining Crime Analysis For Problem Solvers is going to cover Step 27 - Know How To Use Rates and Denominators. A question I get often from people is "What is the crime rate?" Most often this question comes from citizens who aren't really looking for rate but instead are looking for something else. In fact, the confusion occurs so often I tried to explain it with this post. But in this step, we're going to look at determining rates on a smaller scale to help you identify problem areas.

If you were to look at a crime map of the city where I work you'd likely see a lot of points concentrated in one area of town. In fact, you might even think this area of town had a serious crime problem. However, if you compare that map to a map of population density, the same area of town with the most crime points is also the most densely populated. This makes sense, where you have more people, you are going to have more crimes. In order to identify problem areas, we need to know where the crime does not correspond with the population, specifically where the number of crimes for a given population (crime rate) is greater than other areas.

But to be even more useful, the authors suggest determining rates based on the target of the crime. For example, instead of determining the number of vehicle burglaries per X number of persons, determine the number per vehicles registered for this area. The authors included a great example of this in the book.

Calculating rates can be very helpful in finding risky facilities (Step 28). Karin Schmerler and her colleagues in the Chula Vista, California, Police Department investigated calls from the city's motels. The 10 national chain and 16 local independent motels generated similar numbers of calls, but the national chains contained more rooms.

When they added up all the calls for the local independents and divided this by the rooms in these motels, Schmerler found that the average call rate for the independent motels was 1.8 per room. Doing the same for the national chains yielded a call rate of 0.5. Clearly, the local independents generate many more calls per room.
Calculating the rates of crime per target may require some outside of the box thinking to calculate an appropriate target value. In fact, you may not be able to determine the number of actual targets and instead have to use a proxy for this. It may not be possible for you to determine the number of vehicles in a shopping center parking lot, but you might be able to use the number of parking spaces as a proxy for this. The authors caution that you make sure your proxy is relevant to the target of your crimes.
Should you put more emphasis on high numbers or high rates? If your objective is to reduce the volume of crime, then focusing on numbers may the best choice. But if your objective is to reduce the chances of harm, then focus on rate.
The effort required to calculate rate may not always be worth it, but it is another tool to put in your toolbox for when the need arises.

Next time, Step 28 - Identify Risky Facilities.

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