What types of questions are best addressed by maps showing incident cancer case counts, unadjusted (crude) rates, direct age-adjusted rates, or indirect age-adjusted rates?
Presenting cancer case counts in a table with the geographic unit (eg, county) as the row can be a useful starting point for cancer prevention and control discussions. On the basis of information in the table, a choropleth map of case counts can be developed. However, because case counts are often proportional to population size, decision makers may ask questions that require tables showing the case-to-population ratio or rate by geographic unit and choropleth maps designed on the basis of that information. Rates for many cancers increase with age, and differences in the population age distribution in different areas can influence the observed crude cancer rates in each area. To control for such differences, direct and indirect methods can be used for age adjustment (sometimes referred to as age standardization) (11,24,25). The direct age-adjusted rate is calculated by multiplying the age-specific crude rates for the local study population (eg, for a county) by the corresponding age-sp
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