There are more than 1,100 textbooks referring to “short-term process capability,” as distinct from “long term.” Surely 1,100 textbooks can’t be wrong? Let’s apply the first Bull Wrangler test. Does short- and long-term process capability make common sense?
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What is capability?
According to ISO 15504—“Information technology—Process assessment,” process capability is the capability of a process to do what it’s supposed to do, or “to meet its purpose.” In Juran’s Quality Control Handbook (McGraw-Hill, 1988), Joseph Juran called it “a competence to produce quality products.” In order to be assured that our product or service will be capable of meeting its purpose in the future, we must have some means of forecasting the future. Walter Shewhart gave us a tool to do just that. It allowed us to predict what was going to happen to our process. The tool is the Shewhart Chart, which says that if a process is in control, it is predictable. The Shewhart Chart is the only such process-behavior forecasting tool available.
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Comments
The Terminal Man
It's the title of a 1972 Michael Crichton's novel, certainly worth reading, if not done, so far. It seems it may have little to do with stats formulas, but for one thing: I do strongly share both QS 9000 and ISO/TS 16949 - and their likes - approaches that STATS must be shop-floor business, first of all, not ivory-towers foggy exercises. Thank you.
Love It!!
For what it's worth, I've used short term capabilty to refer to a single sample set, perhaps one production run, whilst a long term capability would include additional data or production runs. The primary point of that would be to convey that the initial Ck / Cpk values were from a short term or small sample size. Of course, we would always subtract 1.5 from the value for long term estimates! (j/k! - never have seen or agreed with that seemingly arbitrary number)
As you lovingly point out, a Shewhart chart will offer that same information over time.
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