Box Plot

A box plot is a chart that summarises a dataset using its five-number summary — the minimum, first quartile, median, third quartile and maximum — in one compact shape.

Rather than showing every value, a box plot distils a distribution down to a handful of landmark points. This makes it ideal for comparing several groups side by side at a glance: where each sits, how spread out it is, and whether it has unusual extremes — all without plotting hundreds of individual points.

How to read a box plot

The box spans from the first quartile (25th percentile) to the third quartile (75th percentile), so it contains the middle half of the data. A line inside the box marks the median. Lines called whiskers extend from the box toward the smallest and largest values that are not considered extreme, conveying the range. Points drawn beyond the whiskers are flagged as outliers — values far enough from the rest to warrant separate attention.

A concrete example

Take the sorted values 3, 5, 7, 8, 9, 10, 12, 14, 30. The median (middle value) is 9. The lower half (3, 5, 7, 8) has median 6, the first quartile; the upper half (10, 12, 14, 30) has median 13, the third quartile. So the box runs from 6 to 13 with a line at 9. The whiskers reach toward 3 and 14, while the value 30 — sitting far above everything else — is drawn as an outlier dot. One picture, and the whole shape is clear.

Related terms

A box plot is built from quartiles and the median, shows the data's percentile structure, and explicitly flags any outlier. Its whiskers express the spread also captured by the range.