MedCalc

# Free statistical calculators

## Confidence interval for a rate

 Numerator (e.g. number or events counted): Denominator (e.g. total person-years): Express result as 1:X Express result as 1:X

## Description

A rate is a ratio between a count and another measurement, for example the ratio of a number of events observed and the total number of person-years of observation.

## Required input

• Numerator: the observed number of events.
• Denominator: for example the total person-years.
• Option Express result as 1:X: when this option is selected the rate R will be displayed as 1:(1/R), e.g. the rate 10/200 equals 0.05 and can be represented as 1:20.

## Results

MedCalc displays:

• The Poisson 95% Confidence Interval for the number counted (the Numerator).
• The incidence rate.
• The 95% Confidence Interval for the incidence rate.

## Literature

• Daly LE (1998) Confidence limits made easy: interval estimation using a substitution method. American Journal of Epidemiology 147:783-790.