Friday, September 28, 2012

60 Steps Revisited: Step 53 - Test For Significance

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.

In this post in my journey through Crime Analysis For Problem Solvers we are going to look at Step 53 - Test For Significance. I should warn you though, there is some egghead statistics involved in this post. But before you give up, let's see if we can make it through.

Have you ever noticed that crime sometimes goes up and down for no apparent reason? Given this seeming unpredictability, how do we measure the effectiveness of our solutions? After all, we want to know if crime went down based on our efforts or if there is some other factor at work. A test for significance will give you an idea of whether it was your solution or random chance at work for the variation in the level of crime.

Rather then try to explain it all here, you really need to read it at the original publication. If all this went right over your head (like it did tended to do with mine) the authors have this advice:

The investigation of randomness can become very complex, as there are many different types of significance tests for many different situations. There are some very useful websites, as well as books, which can help you to choose among them, and there are many statistical software programs that can make the required calculations. But if there is a great deal riding on the outcome of a significance test, or a p-value, and you are not well educated in probability theory or statistics, you should seek expert help from a local university or other organizations that use statistics on a regular basis.

Sounds like good advice for the mathematically challenged. Next time, we'll look at Step 54 - Tell A Clear Story.

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