As part of the Cape Cod Breast Cancer and Environment Study, we examined breast cancer incidence on Cape Cod for the period 1982 through 1994. To describe patterns of breast cancer incidence, we used the Standardized Incidence Ratio , or SIR , a common tool for monitoring disease rates.
Incidence is the number of newly diagnosed cases in a given location during a given time period.
An SIR compares the actual number of cases for a given place and time to the number that would be expected based on cancer rates in some comparison area. In this study, we compared breast cancer incidence on Cape Cod to incidence in the rest of Massachusetts. An SIR of 100 means that the actual number of cases equals the expected number. This SIR is statistically significant , which tells us that this result is unlikely to have occurred by chance. To get a look at changes over time, we calculated SIRs for a series of short, overlapping time periods.