A major task in epidemiology is to compare event frequencies for groups of people. Both rate and risk are commonly used to measure event frequency in the comparison. Rate is a measure of change in one quantity per unit of another quantity. An event rate measures how fast the events are occurring. In contrast, an event risk is the probability that an event occurs over a specified followup time period.
An event rate of a population over a specified time period can be defined as the number of new events divided by the populationtime of the population over the same time period,

where d is the number of events and is the populationtime that is computed by adding up the time contributed by each subject in the population over the specified time period.
For a general population, the subsets (strata) might not be homogeneous enough to have a similar rate. Thus, the rate for each stratum should be computed separately to reflect this discrepancy. For a population that consists of K homogeneous strata (such as different age groups), the stratumspecific rate for the jth stratum in a population is computed as

where is the number of events and is the populationtime for subjects in the jth stratum of the population.
Assuming the number of events in the jth stratum, , has a Poisson distribution, the variance of is

By using the method of statistical differentials (ElandtJohnson and Johnson, 1980, pp. 70–71), the variance of the logarithm of rate can be estimated by

Because the rate value can be very small, especially for rare events, it is sometimes expressed in terms of the product of a multiplier and the rate itself. For example, a rate can be expressed as the number of events per 100,000 personyears.
A confidence interval for based on a normal distribution is given by

where is the quantile of the standard normal distribution.
A confidence interval for based on a normal distribution is given by

where is the quantile of the standard normal distribution and the variance .
Thus, a confidence interval for based on a lognormal distribution is given by

Denote the quantile for the distribution with degrees of freedom by

Denote the quantiles for the distribution with degrees of freedom by

Then a confidence interval for based on the distribution is given by

For rate estimates from two independent samples, and , a confidence interval for the rate difference is

where is the quantile of the standard normal distribution and the variance

For rate estimates from two independent samples, and , a confidence interval for the log rate ratio statistic is

where is the quantile of the standard normal distribution and the variance

Thus, a confidence interval for the rate ratio statistic is given by

At stratum j, a stratumspecific standardized morbidity/mortality ratio is

where is the expected number of events.
With the rate

SMR can be expressed as

Thus, a confidence interval for is given by

where is a confidence interval for the rate .