Box-and-whisker diagrams, or Box Plots, use the concept of breaking a data set into fourths,
or quartiles, to create a display. The box part of the diagram is based on the middle (the
second and third quartiles) of the data set. The whiskers are lines that extend from either
side of the box. The maximum length of the whiskers is calculated based on the length of the
box. The actual length of each whisker is determined after considering the data points in the
first and the fourth quartiles.
Although box-and-whisker diagrams present less information than histograms or dot plots,
they do say a lot about distribution, location and spread of the represented data. They are
particularly valuable because several box plots can be placed next to each other in a single
diagram for easy comparison of multiple data sets.
What can it do for you?
If your improvement project involves a relatively limited amount of individual quantitative
data, a box-and-whisker diagram can give you an instant picture of the shape of variation in
your process. Often this can provide an immediate insight into the search strategies you
could use to find the cause of that variation.
Box-and-whisker diagrams are especially valuable to compare the output of two processes
creating the same characteristic or to track improvement in a single process. They can be
used throughout the phases of the Lean Six Sigma methodology, but you will find box-and-
whisker diagrams particularly useful in the analyze phase.
How do you do it?
Data Table divided into quartiles
1. Decide which Critical-To-Quality (CTQ) characteristic you wish to examine. This CTQ
must be measurable on a linear scale. That is, the incremental value between units of
measurement must be the same. For