# Cohort Study: Standardized Incidence Ratio (SIR)

## Standardized Incidence Ratio (SIR)

(See Aschengrau & Seage, pp 222-223)

The Standardized Incidence Ratio, or SIR, is traditionally used for age adjustment of morbidity and mortality data. As an epidemiologist, you'll often see SIR' s used in occupational data. But what is an SIR and how is one calculated? Let's see...

The SIR is a simple mathematical expression that compares the morbidity/mortality experience between the population under study and the experience of that population had they had the same morbidity/mortality experience of a comparison population (i.e., general population).

The calculation of an SIR is relatively straight-forward but requires 5 steps. Let's work through a calculation using some Epiville data. (For more information on standardization, please see Aschengrau & Seage, pp 69-72)

You have data available from the local department of health on the annual incidence rate of Susser Syndrome in Epiville. These data would allow you to calculate the standardized incidence ratio (indirect method) to determine if the incidence of the neurological disorder among SUPERCLEAN employees is higher than the incidence in the general population. Because the age distribution of the general population is quite different from the age distribution of the working population, you have to take into account the age structure of the respective groups.

[ Definition ] [ Step 1 ] [ Step 2 ] [ Step 3 ] [ Step 4 ] [ Step 5 ]