Example Categorical Maps

The two maps below present the same source data in the same number of classes (four), but the first is an equal-interval map while the second is equal-number (quantile). The equal-interval scheme divides the numeric range into subranges of equal size; the equal-number scheme places the states into classes of the same size (in this case, four classes with roughly 14 states / territories in each).

Note that the equal-interval map highlights that South Dakota and Ohio are outliers.  The decision to use equal-interval vs. equal-number classification depends on the content owner’s public health message regarding the data.