Understanding the Box-and-Whisker Plot
What is a box-and-whisker plot?
A box-and-whisker plot is a useful graphical tool for displaying multiple years of data with and the
distribution of the data points within each year. For example the graph below, summarizes more than
550 data points across five years! Each year is represented by two “whiskers”, a “box”, and the
median.
What are the “Whiskers”?
66.9%
The “whiskers” span the total range of data
points, from the lowest measured or
observed value, to the highest measured or
observed value.
In the example below, the “whiskers” on
the graph represent a range from 27.8% to
66.9%.
27.8%
Understanding the Box-and-Whisker Plot
What is the “Box”?
The “box” is made up of three components:
The top of the “box” represents the 75th
percentile and the bottom of the “box” represents
the 25th percentile. The “box” itself represents
the middle 50 percent of cases. This portion of the
graph identifies the range among the majority of
the data points.
The “shorter” the box is, the smaller the middle
50% range and the more clustered these data
points are. A “taller” box implies more variation in
the data.
The median line within the “box” represents the
“typical” or “middle” value among the
observations.
Putting it together
One box-and-whisker plots conveys the same information
as a histogram. The box-and-whisker plot provides a more
efficient method of conveying multiple distributions on the
same graph
A useful way to visualize what the box-and-whisker displays
is to imagine a histogram next to each one.
As can be seen in the example on the right, the more
common values on the histogram tend to fall within the
“box” region of the box-and-whisker.