## Average Chart

The average control chart
shows both the consistency of operator variability
(reproducibility) and
part-to-part variation. The averages of the measurements
for each part and each operator are displayed, as shown
in Figure A.2.

**Figure A.2:** Average Chart

For example,
in Table A.1 the average for operator Jane, part 4 is
calculated as (0.80 + 0.75) / 2 = 0.775.
Similarly computed averages are
displayed for each operator
and each part.

Operator variability can be seen
by comparing the operator averages for each part. These
averages will differ when there is variation.

The average chart also shows part-to-part differences. The
averages
should fall outside the control limits since the control
limits are based on gage error (repeatability). If the
averages do not fall outside the control limits, the
part-to-part variation is hidden in the gage variation.
The average chart shows the ability of the measurement
system to measure parts. The measurement system is generally
considered adequate if most of the averages fall outside
the limits and if the out-of-control averages tend to be
the same for each operator.

Note that this is not a standard use of the
Shewhart chart. Ordinarily the fact that the points
fall outside the control limits would raise concerns
that the process is out of control, but here the
opposite conclusion is drawn.

The average chart is created with the XCHART statement of the
SHEWHART procedure. For further information, see
Chapter 42, "XCHART Statement."

Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.