# Confidence Intervals Versus Confidence Limits

A confidence interval is a range of values about a point estimate that indicates the degree of statistical precision that describes the estimate. The level of confidence is set arbitrarily, but for any given level of confidence, the width of the interval expresses the precision of the measurement. A wider interval implies less precision, and a narrower interval implies more precision. The upper and lower boundaries of the interval are the confidence limits.

# Cohort Studies with Incidence Rate Data

For cohort studies that measure incidence rates, we use the following notation:

 Exposed Unexposed Cases a b People-time at risk PT1 PT0

The incidence rate among exposed is a/PT, and that among unexposed is b/PT0. To obtain confidence intervals for the incidence rate difference (ID), a/PT1 — b/PT0, and the incidence rate ratio (IR), (a/PT1)/(b/PTg), we use the following equations for the standard error of the rate difference and the logarithm of the incidence rate ratio: 