Box Plot (1 of 2)

A box plot provides an excellent visual summary of many important aspects of a distribution. The box stretches from the lower hinge (defined as the 25th percentile) to the upper hinge (the 75th percentile) and therefore contains the middle half of the scores in the distribution.

The median is shown as a line across the box. Therefore 1/4 of the distribution is between this line and the top of the box and 1/4 of the distribution is between this line and the bottom of the box.

The "H-spread" is defined as the difference between the hinges and a "step" is defined as 1.5 times the H-spread.

Inner fences are 1 step beyond the hinges. Outer fences are 2 steps beyond the hinges.

There are two adjacent values: the largest value below the upper inner fence and the smallest value above the lower inner fence. For the data plotted in the figure, the minimum value is above the lower inner fence and is therefore the lower adjacent value. The maximum value is the inner fences so it is not the upper adjacent value.

As shown in the figure, a line is drawn from the upper hinge to the upper adjacent value and from the lower hinge to the lower adjacent value.