Monday, January 6, 2014

How to calculate an increase or decrease

Probably the biggest promise that crime analysis holds for a police agency is that it can make an agency more efficient at its mission of making the community you serve safer. Efficiency should be the new buzzword for police. If our methods of crime fighting aren’t efficient, then we are wasting the resources we have been entrusted with. I don’t know about your agency, but in the sleepy little burg where I work, we have precious few resources to waste.

But how do you know if your agency is being effective?

The beginning of the process to determine effectiveness is to measure certain crime or Call for Service metrics and ask yourself: Has this metric increased or decreased? Before I became a crime analyst I was a police officer. I signed up to catch bad guys and not “do math” (I hated math in school). That being said, calculating increase or decrease is pretty easy. It’s even easier if you setup a spreadsheet with the formula or use a tool like Wolfram Alpha.

Usually when police agencies are concerned with calculating an increase or decrease it’s comparing one time period to another. For instance an agency wants to know whether vehicle thefts this year have increased or decreased over last year. For an example we’re going to use a real life example of Uniform Crime Reports data for Vehicle Thefts from the agency where I work, Killeen, Texas. We’ll compare the number of reported vehicle thefts from 2011 to those in 2012.

2011 - 187
2012 - 192

The formula for calculating this looks like this:

(New Value - Old Value) / Old Value = Change

Don’t forget when you see a parentheses in a math equation to solve the operation inside the parentheses first. Once you’ve solved the whole equation, then multiply the result (Change) x 100  to turn that number into a Percentage of Increase or Decrease.

Plugging our Vehicle Theft numbers in that formula we get:

(192-187) / 187 = 0.0267

We then multiply 0.0267 x 100 to convert that number to a percentage and get an increase of 2.67%. If the Change result had been a negative number it would indicate a decrease from the previous year.

It gets even easier to calculate this with Wolfram Alpha. Go to the Wolfram Alpha website, and input this into the input box:

187, 192

Keep in mind that the order in which you input these numbers is VERY important. If you are wanting to compare the numbers for two time periods, you must put the Old Value first. Wolfram Alpha will give a whole list of calculations using these two numbers and one of them will be the result of our desired increase / decrease calculation.

It’s also pretty easy to do these calculations in a spreadsheet like Microsoft Excel. By the way, as a crime analyst, learning to use Excel and learning to use it proficiently is a must. It can make your life so much easier and is almost always my go-to tool. Here’s an example of a simple way to lay out a spreadsheet to make these calculations:

A caution is in order here. You must be very careful with comparing values that are zero. If you had 1 murder in 2011 and 0 murders in 2012 you had a -100% decrease. However, if you had 0 murders in 2011 and 1 in 2012 you did not have a 100% increase. Why is this?

Can you divide by zero? The answer is that you can’t. If you don’t believe me, plug the numbers into the formula and see for yourself. The proper way to represent this would be to indicate that the value is Not Calculable. If you input this into Excel it will give you a Divide by Zero error ( #DIV/0! ) in the results cell.

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