A box plot, or box and whisker plot, can be used to illustrate the distribution of a data set. A box plot has three parts.

• A rectangular box that extends from the first to the third quartiles (Q_1 and Q_3) with a line between Q_1 and Q_3 indicating the position of the median.
• A segment attached to the left of the box that extends from the first quartile to the minimum of the data set.
• A segment attached to the right of the box that extends from the third quartile to the maximum of the data set. The two segments are called whiskers.

If a data set has outliers, they are marked as separate points to the left and/or right of the whiskers. A box plot is a scaled figure and is usually presented above a number line. The set of numbers used to draw the box plot is called the five-number summary of the data set. Each of the five numbers is labeled below.

A box plot provides a visual illustration of the distribution of a data set. Each segment of the plot contains one quarter, or 25 %, of the data, and the center 50 % of the data lies inside the box. The further apart the segments are, the greater the spread is for that quarter of the data.