In the 1980s, demand for SPC classes outstripped the supply of competent instructors. Novices were teaching neophytes, and misinformation could be found everywhere. Out of this chaos, many incorrect ideas about process behavior charts became widely circulated. These ideas continue to be spread by those who don’t know any better and by those who use them to try to discredit SPC and sell more-complex techniques. Since these ideas can lead to the misuse of data, they’re hazardous in practice.

My colleague Allen Scott provided me with a list of many of these hazardous ideas that he found currently in circulation. Eight of these ideas are listed below, along with a brief explanation of the fallacies contained therein.

‘SPC is not needed with stable process’

It’s been said that once we have reengineered our process and gotten things working smoothly we no longer need to use SPC to monitor the process.

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