The ninth revision came into general use in 1979, and has now been superseded by the 10th revision for many applications. When the classification alters from one revision to the next, disease rates may not be directly comparable before and after the change. For example, the eighth revision included separate categories for gastric ulcer and for peptic ulcer of unspecified sites, whereas in the seventh revision this distinction was not made. In this situation some aggregation of categories is needed before valid comparisons can be made. Measures of association Several measures are commonly used to summarise comparisons of disease rates between populations, each with its special applications.
Attributable risk is the disease rate in exposed persons minus that in unexposed persons.
It is the measure of association that is most relevant when making decisions for individuals. For example, in deciding whether or not to indulge in a dangerous sport such as rock climbing, it is the attributable risk of injury which must be weighed against the pleasures of participation. Relative risk is the ratio of the disease rate in exposed persons to that in people who are unexposed. For example, given a choice between a doubling in their risk of death from bronchial carcinoma and a doubling in their risk of death from oral cancer, most informed people would opt for the latter. Nevertheless, relative risk is the measure of association most often used by epidemiologists.