Percent changes are useful to help people understand changes in a value over time. Again, figuring this one requires nothing more than fourth-grade math.

Simply *subtract* the old value from the new value, then
*divide* by the old value.

*Multiply* the result by 100 and slap a % sign on it. That's
your percent change.

Let's say Springfield had 50 murders last year, as did Capital City. So there's no difference in crime between these cities, right? Maybe, maybe not. Let's go back and look at the number of murders in those towns in previous years, so we can determine a percent change.

Five years ago, Capital City had 42 murders while Springfield had just 29.

Subtract the old value from the new one for each city and then divide by the old values. For Capital City that means taking 50 minus 42 and dividing that result by 42. For Springfield, figure 50 minus 29 and divide that result by 29. That will show you that, over a five year period, Capital City had a 19 percent increase in murders, while Springfield's increase was more than 72 percent.

That's your lead.

Or is it? There's something else to consider when computing
percent change. Take a look at a concept called *per capita* to
find out.

Read the rest of Robert's statistics lessons for people who don't know math.

© Robert Niles. Read more in the column archive.