SPC: From Chaos To Wiping the Floor by Lynne Hare (who also was the 1997 Hunter Award winner)
Shewhart based control chart limits more on the economics of change than on underlying probabilities. Ever the empiricist, Shewhart seems not to have trusted probability limits alone.
Setting control limits at 3 standard deviations is a decision based on experience. Shewhart, Deming and others determined it was sensible to take resources to look for a special cause was most effective for results more than 3 standard deviations from the mean - it is not a mathematical conclusion but a empirical conclusion.
It is disappointing to see some users place specification limits on control charts. Processes don’t know or even care about specifications. The presence of specification limits on control charts encourages users to adjust on the basis of them instead of the calculated limits. The resulting miscued adjustments are likely to result in increased process variation, which is the opposite of the intent.
True.
SPC grew into everything we do. It changed the way we think, work and act, and it evolved into total quality management (TQM). But there is an ebb and flow to new technologies in our society, and TQM’s star faded in the presence of reengineering, which faded with the advent of Six Sigma. All the while, the basic SPC tools have been refined and augmented, and SPC serves in muted presence to underpin the newer, expanded technologies.
Another insightful statement.
December 6th, 2006 at 3:24 pm
[...] If you would like to reduce the variation picking the highest data values (within the control limits) and trying to study them to figure out why they are so high is not effective. Instead you should study the whole system and figure out what systemic changes to make. [...]