Core Indicators Work Group Recommendations: Standardization of Rates Based on Standardization of Rates paper by Nam Bains Introductory points: Both direct and indirect methods of standardization have their own merits and are appropriate to use depending on the situation. - Direct standardization produces age adjusted rates or SRATES. Indirect standardization results in Standardized Incidence Ratios (SIRs) or Standardized Morbidity/Mortality Ratios (SMRs).
- Indirect standardization is more stable when studying rates based on small numbers. The ratio of observed to expected cases is also intuitively easy to understand. Because the age-distribution of the study population is used to calculate SIRs or SMRs, it is incorrect to compare indirectly adjusted rates for different areas. The same age-specific rates (from a standard) applied to study populations with different age distributions will result in very different SMRs.
- Direct standardization preserves the consistency between different study populations; since many study populations can be adjusted to the same standard, the resulting rates can be compared against each other. This is important when comparing study populations over place or time.
Recommendations: - Examine crude rates, age-specific rates and counts before calculating adjusted rates.
- When there is little to no variation across age-specific rates or where there is no dfference in the age structure of the populations over time and geography, crude rates can be valid for comparisons over time and geography.
- Only consider direct standardization if there are 20 or more events.
- Only consider indirect standardization if there are 10 or more events.
- Consider suppressing age-adjusted rates if the Relative Standard Error is greater than 23%. RSE is similar to a coefficient of variation (CV); the larger the RSE (or the CV), the less reliable is the estimate.
- When using direct standardization, use the 1991 Canadian population structure as the standard population.
- Although there is not a recommended number of age categories to use when calculating age-adjusted rates, epidemiologists should be aware of the issues around age categories and the factors that should be considered before determining the number of age groups.
- When using direct standardization, for age strata with zero events, epidemiologists should consider combining multiple years, collapsing geographies or age strata where feasible. If this is not feasible, substitute a small number (e.g. 0.1) for zero events or impute from a higher level geography.
- When using direct standardization, confidence intervals should be calculated using the Poisson approximation.
- When using indirect standardization, confidence intervals should be calculated using the Armitage and Berry method.
Links: This page last updated: July 2, 2009 |