2.3.1 Standardised Incidence Ratio

In comparing cancer incidence between areas or over time, three important factors must be considered—the number of people at risk, their sex and their age. In this report, cancer incidence for men and women was considered separately, which deals with possible differences between sexes. The reason for correcting for the number of people at risk is obvious; the number of cases is divided by the number of people resident in the area during a specified period (as reported by the census) to produce an incidence rate.

Since the risk of developing cancer doubles with every eight or nine years of life, an area with an older population would be expected, all else being equal, to have more incident cancer cases than an area with a younger population. There are several different approaches available to adjust for differences in age; this atlas has used indirect standardization, which is the most appropriate method for small area comparisons, as it provides more stable rates than other standardization techniques, and works even if there is no population-at-risk in some age groups within the area (Estève et al., 1994). For each small area i, the national incidence rates for each age group j were applied to the population counts (N) in each age group, to calculate the total expected number of cancers (E) in the area. This can be compared to the number actually observed (O) in the area, in the form of an observed to expected ratio, or percentage. This is called the standardised incidence ratio, abbreviated to SIR. The SIR for any cancer for either men or women for Ireland as a whole is, by definition, 1 (or 100%), where for any small area (ED or ward) i:

     where