Page 216 - The Handbook for Quality Management a Complete Guide to Operational Excellence

202 P r o c e s s C o n t r o l Q u a n t i f y i n g P r o c e s s Va r i a t i o n 203

Control Chart Method: Attributes Data
1. Collect samples from 25 or more subgroups of consecutively
produced units. Follow the guidelines presented in steps 1–10 above.
Plot the results on the appropriate control chart (e.g., c chart).
If all groups are in statistical control, go to step #3. Otherwise,
identify the special cause of variation and take action to eliminate
it. Note that a special cause might be beneficial. Beneficial activities
can be “elimi nated” as special causes by doing them all of the
time. A special cause is “special” only because it comes and goes,
not because its impact is either good or bad.
Using the control limits from the preceding step (called
operation control limits), put the control chart to use for a period
of time. Once you are satisfied that sufficient time has passed for
most special causes to have been identified and eliminated, as
verified by the control charts, go to step #4.
2. The process capability is estimated as the control chart centerline.
The centerline on attribute charts is the long-term expected quality
level of the process, for example, the average proportion defective.
This is the level created by the common causes of variation.
If the process capability doesn’t meet management requirements, take
immediate action to modify the process for the better. “Problem solving”
(e.g., studying each defective) won’t help, and it may result in tampering.
Whether it meets requirements or not, always be on the lookout for pos-
sible process improvements. The control charts will provide verification
of improvement.

Control Chart Method: Variables Data

1. Collect samples from 25 or more subgroups of consecutively
produced units, following the 10-step plan described above.
Plot the results on the appropriate control chart (e.g., X and
R chart). If all groups are in statistical control, go to the step #3.
Otherwise, identify the special cause of variation and take action
to eliminate it.
2. Using the control limits from the preceding step (called operation
control limits), put the control chart to use for a period of time. Once
you are satisfied that sufficient time has passed for most special
causes to have been identified and eliminated, as verified by the
control charts, estimate process capability as described below.
The process capability is estimated from the process average and stan-
dard deviation, where the standard deviation is computed based on the
average range or average standard deviation. When statistical control has

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