Source: Science and Human Rights Program, American Association for the Advancement of Science.

A box plot is a graph that characterises the pattern of variation of the data. The plot simultaneously displays several measures of central tendency (what the “average” or “middle point” of the data is) and dispersion of the data (how spread out the data are).

The box plot provides the following information:

1. the position of the median;
2. the 25th and 75th percentiles (the 1st and 3rd quartiles; these are the sides of the box); and
3. lines extending from the sides of the box as far as the minimum and maximum values.

Sometimes these lines will extend no further than one (or 1.5) inter-quartile range below the 1st quartile and above the 3d quartile; in this case, points outside the lines will be individually identified.