Page 432 - Six Sigma Demystified

412 Six SigMa DemystifieD

find the process elements that contribute to sporadic changes in process loca-
tion. To predict the capability of the process after special causes have been
eliminated, you should remove the out-of-control points from the analysis,
which will remove the statistical bias of the out-of-control points by dropping
them from the calculations of X-doublebar and the X-bar control limits.
Look for obviously nonrandom behavior. Use the run-test rules, which apply
statistical themselves for trends to the plotted points.
If the process shows control relative to the statistical limits and run tests for
a sufficient period of time (long enough to see all potential special causes), then
we can analyze its capability relative to requirements. Capability is only mean-
ingful when the process is stable, because we cannot predict the outcome of an
unstable process.