Appearances vs. Reality

I’m looking at a form labeled as an X-Bar / R-Bar SPC chart. It is filled out pretty consistently by the operators.

But the “control limits” are actually specification limits, and the “standard deviation” looks like it is calculated as 1/3 x the spec limits to force the lines to where they want them from the mean… er ah… nominal.

When I asked about it, at least one company leader was under no illusion that this was a statistical process control, it is simply a means to develop a run chart of actual parameters vs. a nominal value and the specification limits.

I’ve seen far worse, like the time an engineer didn’t understand the difference between “in control” and “in specification.”

But what do they say in their everyday conversations? Do they talk about this process being “within specification” or “in control?” I expect the later.

One Reply to “Appearances vs. Reality”

  1. If I had a dollar for every engineer that didn’t understand the difference between control and spec, I’d be close to retirement. I’m also constantly amazed at ops and engineering managers that don’t understand capability and the power it can have in improvement when applied correctly.

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