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Particularly when numeric data do not tend to concentrate around a unique central value (e.g. fairly uniform distributions), more than one descriptive measure is needed to summarize the data distribution. These measures are called quantiles.

The most common quantiles are quartiles, which are three values (first quartile Q₁, second quartile Q₂, and third quartile Q₃) corresponding to specific positions in the sorted list of data values (Figure 1.7).

 75% of the data are less than Q3 and 25% are greater than Q3.

 50% of the data are less than Q2 and 50% are greater than Q2.

 25% of the data are less than Q1 and 75% are greater than Q1.

Figure 1.7 Quartiles.

The first quartile is also known as the 25th percentile, the median as the 50th percentile, and the third quartile as the 75th percentile.