# Confidence interval for a rate

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

The program 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.