Scatter Diagram

[Steven Bonacorsi]

A scatter diagram shows the correlation between two variables in a
process. These variables could be a Critical To Quality (CTQ)
characteristic and a factor affecting it, two factors affecting a CTQ
or two related quality characteristics. Dots representing data points
are scattered on the diagram. The extent to which the dots cluster
together in a line across the diagram shows the strength with which
the two factors are related.

Sometimes the scatter plot may show little correlation when all the
data are considered at once. Stratifying the data, that is, breaking
it into two or more groups based on some difference such as the
equipment used, the time of day, some variation in materials or
differences in the people involved may show surprising results.

The reverse can also be true. A significant correlation could appear
to exist when all the data are considered at once, but stratification
could show that there is no correlation. One way to look for
stratification effects is to make your dots in different colors or use
different symbols when you suspect that there may be differences in
stratified data.

Why Use Scatter Diagrams?

To study and identify the possible relationship between the changes
observed in two different sets of variables. Scatter diagrams are
especially useful in the measure and analyze phases of the Lean Six
Sigma methodology.

What Does Scatter Diagrams Do?

- Supplies the data to confirm a hypothesis that two variables are
related.

- Provides both a visual and statistical means to test the strength of
a potential relationship

- Provides a good follow-up to a Cause & Effect Diagram to find out if
there is more than just a consensus connection between the causes and
the effect. You must take care, however, so that you do not
misinterpret what your diagram means.

What Scatter Diagrams Can Do For You?

To control variation in any process, it is absolutely essential that
you understand which causes are generating which effects. A cause and
effect diagram can help you identify probable causes. Scatter diagrams
can help you test them. By knowing which elements of your process are
related and how they are related, you will know what to control or
what to vary to affect a quality characteristic.

Scatter diagrams can be used to prove that a suspected cause-and-
effect relationship exist between two process variables. With the
results from these diagrams, you will be able to design experiments or
make adjustments to help center your processes and control variation.


Steven Bonacorsi is a Certified Lean Six Sigma Master Black Belt
instructor and coach. Steven Bonacorsi has trained hundreds of Master
Black Belts, Black Belts, Green Belts, and Project Sponsors and
Executive Leaders in Lean Six Sigma DMAIC and Design for Lean Six
Sigma process improvement methodologies.

Author for the Process Excellence Network (PEX Network / IQPC)

Process Excellence Network
Steven Bonacorsi, President of International Standard for Lean Six
Sigma(ISLSS)
Certified Lean Six Sigma Master Black Belt
47 Seasons Lane
Londonderry, NH 03053
Phone: +(1) (603) 401-7047
E-mail: sbona...@islss.com
Process Excellence Network: http://bit.ly/n4hBwu

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