Are Breast Cancer Rates Higher Because the Capes Population is Older than the General Population of Massachusetts?
Breast cancer is more common in older women than in younger women. Cape Cod is home to many retirees, and the Cape’s population is older on the whole than the population of the rest of Massachusetts. However, this difference does not explain the excess breast cancer incidence on the Cape. In calculating SIRs, we correct for age differences — that’s what “standardized” means in Standardized Incidence Ratio. Remember that an SIR is a ratio, or fraction, with the actual number of cases in the numerator and the expected number in the denominator: SIR = actual number of cases / expected number of cases In calculating the expected number of breast cancer cases, we look at each age group separately, precisely because cancer rates are different at different ages. The following table shows how the expected number of cases is calculated for a town on Cape Cod. We first calculate the breast cancer rate for each age group in non-Cape Massachusetts, our comparison area. We then estimate the number
