What method should be used for grouping age-adjusted cancer incidence rates into categories for map display?
The choice of categories for grouping rates for mapping can affect the visual appearance of the map and thereby interpretation of mapped results. Selecting the interval may depend on the purpose of the map, the underlying data distribution, and the intended audience for the map (6-8). Map legends automatically generated by geographic information system (GIS) software programs typically provide information about the range in rate values for each rate category along with the color assigned to that category. Adding details on the number of geographic units included in each class interval may help map readers understand the distribution of the information mapped. The counts for each category can be manually added next to each category on the map legend or displayed in a frequency distribution graph included on the map. Four examples of classification methods (6) are the following: • Quantiles: The rates for the areas of interest (eg, counties) are first rank-ordered, and then an equal numb
