A boxplot is a standard graphical summary of one or more distributions of values. As the diagram below illustrates,
each 'box' in a boxplot displays:
a rectangle representing the range of values between the 25th percentile (Q1) and the 75th percentile (Q3), called the interquartile range (IQR)
a line within that rectangle representing the median value
a lower whisker indicating the greater of the minimum value or Q1 - 1.5 * IQR
an upper whisker indicating the lesser of the maximum value or Q3 + 1.5 * IQR
an 'o' symbol for each outlier between 1.5 and 3 times the IQR below Q1
an 'o' symbol for each outlier between 1.5 and 3 times the IQR above Q3
an 'x' symbol for each outlier below Q1 - 3 * IQR
an 'x' symbol for each outlier above Q3 + 3 * IQR
Outliers and extreme outliers often appear for some boxes but not others, reflecting the varying breadths of the
corresponding distributions. In the example below, the August 2014 distribution contains no outliers, the
September 2014 distribution contains one high outlier, the October 2014 distribution contains a few high
outliers, and the November 2014 distribution contains a few low outliers, many high outliers, and a few
extreme high outliers:
