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.
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.
fences are 1 step beyond the hinges. Outer fences are 2 steps beyond
There are two adjacent values: the largest value below
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.