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The New Terminology by Donald J. Wheeler (SPC Press Reading Room)

Statistical Process Control has been around for over 70 years.
However, there is today considerable confusion about the nature and purpose of SPC. I believe this is because many have sought to reshape SPC according to their own background and experience. Some hear the words Statistical Process Control and immediately think of classical statistical procedures.
They try to fit SPC into this framework of parameters, distributional assumptions, tests of hypotheses, and confidence levels. Of course, when this group tries to share their version of SPC they are met with that same total lack of comprehension which is the fate of classical statistics. But statisticians are accustomed to rejection, so this is nothing new.



Others hear the words Statistical Process Control and think of process control techniques. SPC is thought of as a manual process-control technique to be used to maintain the status quo for a process. It is merely a process monitoring technique, to be used after you have already gotten the process into a satisfactory state. “And, since this is what SPC is about, wouldn’t you like to know about some of the neat algorithmic process-control techniques and process modeling techniques that have been developed in the past few years?” This group would be glad to give you a course or sell you some software. But once again, there is a hurdle of truly mathematical proportions to be overcome. The faint of heart need not apply. Those without calculus should not enter in to this door.



A third group uses the word “control” to denote conformance to specifications, and so when they hear the words Statistical Process Control they think about trying to produce product within specifications. Given this perspective, they think of SPC as a complex route to a simple objective, and accordingly they try to simplify SPC. This group tends to want to bypass the computations based on the data and instead use the specifications to set action limits. While this simplicity sells well, it has the unfortunate characteristic of completely misrepresenting what SPC is all about. Those who use these simplified approaches may meet with some limited success, but because their objective falls short of what can be done, because they do not seek to get the most out of their processes, they do not reap the benefits available from SPC. The reality falls short of the promise, and the users return to sorting as a way of life—make enough stuff and some of it is likely to be good.

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