StATS: Calculating rates (created 2007-04-06).

Someone on the MedStats discussion group asked how to calculate a rate of needlestick incidents. The answer is quite simple, but there are a variety of possible responses.

The formula for a rate is x/y, or simple division. The Wikipedia definition of a rate is helpful.

A rate is a special kind of ratio, indicating a relationship between two measurements with different units, such as miles to gallons or cents to pounds. For example, suppose one spends 9 dollars on 2 pounds of candy. The rate $9 / 2 pounds compares the money spent to the number of pounds of candy.

In this particular case, the numerator is the number of needlestick incidents and the denominator is some other type of measurement. Typically the denominator is a measure of workload, area, volume or time. So one possible denominator is simply time itself. Divide the 6 needlesticks by the 30 days in a month to produce a rate of 0.2 needlesticks per day.

I'm also starting to pull together some of the formulas needed for confidence intervals for count data.

This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Poisson regression.