Statistics Dictionary

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A boxplot, sometimes called a box and whisker plot, is a type of graph used to display patterns of quantitative data.

A boxplot splits the data set into quartiles. The body of the boxplot consists of a "box" (hence, the name), which goes from the first quartile (Q1) to the third quartile (Q3).

Within the box, a vertical line is drawn at the Q2, the median of the data set. Two horizontal lines, called whiskers, extend from the front and back of the box. The front whisker goes from Q1 to the smallest non-outlier in the data set, and the back whisker goes from Q3 to the largest non-outlier.

Smallest non-outlier Q1 Q2 Q3   Largest non-outlier
. .        
-600 -400 -200 0 200 400 600 800 1000 1200 1400 1600

If the data set includes one or more outliers, they are plotted separately as points on the chart. In the boxplot above, two outliers precede the first whisker (on the left side of the plot).

See also:   AP Statistics Tutorial: Boxplots (aka, Box and Whisker Plots)