Introduction
Direct standardisation is the method used in social medicine and epidemiology to compare disease frequency, mortality or other rates between populations that differ in age structure (and often sex). It removes the distorting effect of different population composition and gives one adjusted rate that can be compared directly.
Why standardisation is needed
The crude rate is the total number of events divided by the total population. It depends on age structure, because age is a strong determinant of most outcomes. An older population usually has a higher overall mortality even when its age-specific mortality is the same as, or lower than, that of a younger population. Age is therefore a confounder, and comparing crude rates between regions, countries or years can mislead.
- Crude rate mixes the true difference in risk with the difference in age composition.
- A standardised rate answers the question: what would the rate be if both populations had the same age structure?
- Typical uses: comparing mortality between districts or countries, comparing trends over time, and monitoring cancer incidence and other health indicators.