Monday, February 3, 2014

How to calculate crime rate

One question I get all the time is “What’s the crime rate?” However, most of the time when people ask that question they don’t really understand the question they are asking. When citizens call my office and ask “What’s the crime rate?” they are usually asking it in relation to a particular neighborhood. The community where I work is right next to a large military base so we have people moving in and out of the community on a regular basis. What they usually mean is “I want to know if the neighborhood where I am considering purchasing a house is safe?” The answer to that one is complex. They ask about crime rate because they hear the term used in the media and think it will tell them something about the community.

The strict definition of mathematical rate is: “is a ratio between two measurements with different units”. Calculating Rate is a way of normalizing between different units. Crime rate is the number of crimes that occur in a given population. Let’s look at it this way.

• Community A has a population of 50,000. Last year they had 5 murders.
• Community B has a population of 5,000. Last year they had 2 murders.

Which community had a bigger problem with murders last year? Calculating the crime rate can help us normalize the populations between these two very different sized towns.

Crime rate is normally expressed as the number of crimes per 100,000. In order to calculate this the formula would look like this:

(Number of Crimes / Population) x 100,000 = Crime Rate Per 100,000

Remember your order of operations. You need to do the part of the equation enclosed in parenthesis first. Now let’s look at our two communities and see what their murder rate is.

Community A

(5 / 50,000) x 100,000 = 10 murders per 100,000 population

Community B

(2 / 5,000) x 100,000 = 40 murders per 100,000 population

This is quite the difference. Community B’s murder rate is four times greater than Community A even though Community A had three more murders than Community B. Of course this brings up a whole other discussion on whether it’s fair to directly compare crime statistics in two different communities. The FBI has a warning on their Uniform Crime Reports site that goes into detail about why this isn’t a good practice. However, rates can also be used to compare a city against itself at different times to account for a population growth or decline.

For instance, the community where I work is consistently listed as one of the 10 fastest growing areas of the United States. When I started my law enforcement career in 1991 the population of Killeen, TX was around 57,000. Now, 23 years later the population is over 134,000. Given than kind of population growth it’s a valid question to ask if we are doing better or worse in our crime suppression efforts now than we did back then. But you don’t have to limit calculating rate to just crimes. You can use this to normalize traffic accidents, citations, arrests, calls for service or any other metric.

What metric could you understand better by calculating it’s rate?

1. Hi Scott,
I am preparing a project on studying crime data and finding the safety index/crime index of several states of India. I was here to understand more about crime index and your article helped me a lot. But my another question is that if we are considering multiple types of crime for each state then what should be the weight given to each type of crime. So that we are not biased while calculating the crime index for different state.
I hope I am clear with my question to you. Please revert if there is any confusion.

2. Aditya, You certainly could give different weights to different types of crime. However, here in the US we don't usually weight them but instead break them into two broad classifications; violent crime and non-violent crime. I think it would be more important to be consistent with how it's normally measured in your country. Unfortunately, I have no experience with how crime statistics are gathered in India.

Best of luck.

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