R&R Analysis Using Graphical Techniques
Graphs of measurement results provide a visual
means to analyze measurement systems.
Appropriate graphical techniques include:
- Control charts used to monitor test equipment
variation and appraiser variation.
- Scatter plots to look at how consistent
different appraisers are to each other as well
as their consistency taking measurements of the
same part or sample.
- Whiskers charts to view how consistent each
appraiser is in measuring the same part as well
as to check the consistency between appraisers.
Control Charts for MSA
The x-axis is used differently.
- When used to analyze the repeatability and
reproducibility of the measurement system, the
x-axis of the control chart does not represent
- The parts or samples tested by each appraiser
are tracked on the x-axis.
As usual, check the Range chart first.
- Any point outside the upper control limit on
the range chart or any unusual patterns of
variation (such as all of the range values for
one appraiser being below the average range) are
a sign that the measurement system is
out-of-control. It should be investigated and
With the Averages (X-bar) chart, we “want” to see
points outside of the control limits.
- This is obviously different than our goal with
a standard averages chart. With standard
averages charts, we want all points to be
in-control. We do not want that here.
- An X-bar & R control chart on the measurement system
analysis results must have range values for each
part/sample and operator in-control. But unless
the majority of the values on the X-bar chart are
outside of the control limits, then the
measurement system is not adequate.
- Unless more than half the averages are outside
of the control limits, the measurement system
has considerably more variation than the total
part, or process, variation so it cannot
adequately discriminate between good and bad
- The X-bar chart should have roughly the same pattern
for all of the appraisers. It the patterns of
the appraisers are very different, there might
be some differences in their techniques that
should be investigated.
Checking for appraiser bias.
- A separate control chart for the grand
averages for all appraisers should also be set
up to show test differences between the
appraisers. Only the averages chart is used for
this view of the measurement system.
- If any of the grand averages fall outside the
control limits, the we must investigate the
differences between that appraiser and the
Scatter Plots for MSA
Whiskers Charts for MSA
A Whiskers Chart plots the range of the
measurements for each part and for each
This provides a view of how consistent each
appraiser is in making measurements on the same
part/sample. It can also be used to look at
consistency between appraisers.