Look before you leap: Stratify before you standardize

This paper presents a mathematical model to show the conditions in which ag e standardization can be used to summarize age-specific rates for compariso n purposes over calendar time. It shows that the conditions for valid compa rison depend on the type of measure used for comparison, that is, differenc e, ratio, or percent change, If the measure for comparison is a difference of the standardized rates at two time points, then the age-specific rates n eed to maintain a constant rate difference over time for the comparison to be valid. If the measure for comparison is a ratio or percent change of the standardized rates at two time points, then the age-specific rates need to maintain a constant rate ratio over time for the comparison to be valid. S ince in reality, as shown by our Canadian empirical data, age-specific rate s do not always maintain a consistent pattern over time, it is recommended that one should always stratify the data to look at patterns of age-specifi c rates before applying age standardization.