Sorry I have to disagree with you.
We have that opportunity here!
By definition and by the laws of physics the two are unrelated. You choose to use spec limits as the anchor from which you set the value at which you address tool wear. Allowing the systemic drift to continue until it comes "too close" to the spec limits. You set these "action limits" using the statistical knowledge of the variation about any location of the process.
Yes, this is utilizing the distribution of the most significant variation for that process - tool wear. We set the control limits to address
that variation and decision on the process based on
them.
We can set statistical control limits about any local average in a situation where there is a systemic drift. in essence we are subtracting the drift from the simultaneous variation about the drift line... Some call these sloping limits as the are above and below the 'sloping' drift line. It is these limits that are the statistical process control limits.
They are
another opportunity for control. This approach
removes the most significant variation and seeks to observe the remaining variations. Unfortunately, the remaining variations are
insignificant for the control of that process, by the definition of the process. That chart would yield, at best, rare special causes. At lot of work for little return. And, it is not the
only process control based on statistics, for sure. The uniform distribution is a statistical tool as effective as the normal curve. The data shows it to be true.
I think you are talking about two types of 'errors'; one is what I would term a mistake or an incorrect human choice - choosing to measure the wrong thing. The other is the classical thing we think of when using the phrase "measurement error": this is the variation of the measurement system itself as quantified by the standard deviation. It is not a human choice but inherent in the physical system.
The point you made was understood, but I have found that the "human error" does enter into the standard deviation, as does
all error in the process add up to the total deviation. Addressing that error has provided much clearer data, leading to better decisions. That is why I added that comment.
Clarifying my point further, I never meant to say that this error is never big enough to interfere with our ability to see true process variation, I meant that it often isn't as impactful as commonly believed. This misperception is primarily driven by the popular approach to gage R&R which expresses error as a % of the tolerance. This approach has been shown to be statistically and mathematically incorrect and often dramatically overstates the effect of measurement error. Instead of employing a critical assessment of their measurement error, too many people take the path of least resistance and succumb to the popular myths of measurement error. I rail against this in the same way you rail against the "normal centrists" as you call them...
I am aware of this contention. I have read Dr. Wheeler's writings on that topic. The worst case is the people that take an even
lesser path of resistance and ignore resolution
entirely, and its effect of measurement in SPC. I sure is handy to have statistically significant categories of resolution that will ensure the plotted points are of some validity and that the range is not from the measurement error (of either type), but rather the other participants of the total variation. Total ignorance of the problem is why we see control charts with 10 categories between the control limits, but dots on every other one. They miss the point of the most basic need for resolution - forget statistically significant resolution. I would be satisfied to rid the world of that problem. Again, even with the mathematical problems of ndc, its need to be calculated to the control limits rather than the specification, et.al., just
thinking about the need for statistically significant categories is far and above ignoring it.
....not that I ever said you would have anyone ignore that...
I rail against this in the same way you rail against the "normal centrists" as you call them...
Clearly, we both have a ways to go....
