Quantiles: Use 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.
Equal Intervals: Use equal intervals to highlight differences in the rate values between areas. Areas with similar rates will be the same color regardless of how many areas fall into that group. For equal interval maps, the range of values is divided into the specified number of groups with each interval having the same width. With equal interval maps, the distribution of values including outliers is more easily seen.
Additional information about interval types
There is no single best interval type (also known as a classification method). The interval types provided in the State Cancer Profiles Maps section enables the user to choose the interval type that they feel is most appropriate for their purpose and audience. For additional details, see: http://www.cdc.gov/BRFSS/maps/faqs.htm#9 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.