Interval Type

Quantile Intervals

The site uses quantile intervals to focus attention on relative rankings - the highest set of areas, the next highest set, and so on to the lowest set. With quantile intervals, the same number of areas is assigned to each group. If the number of intervals is set to four, then the groups are quartiles, five intervals are quintiles, etc. With quantile maps, outlier values are less visible since they are grouped with other high or low value areas.

Additional information about interval types

There is no single best interval type (also known as a classification method). For additional details, see: https://www.cdc.gov/brfss/gis/gis_maps_faq.htm and this journal article: Brewer, C. A. and L. Pickle, 2002. Evaluation of Methods for Classifying Epidemiological Data on Choropleth Maps in Series. Annals of the Association of American Geographers, Vol. 92, No. 4, pp. 662-681.