Quantile

From Wiki.GIS.com

When classifying data for symbolizing quantities on a map, the Quantile classification method divides the range of possible attribute values into unequal-sized intervals so that the number of values is the same in each class.

Classes at the extremes and middle have the same number of values. Because the intervals are generally wider at the extremes, this option is useful to highlight changes in the middle values of the distribution.

Other methods of data classification used in GIS include Jenks Natural Breaks, Equal, Defined, or Geometric Interval, and Standard Deviation.