Calculating rates

Steve Simon

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. en.wikipedia.org/wiki/Rate

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.