Never say “never”? I guess I overlooked that memo. In case you missed it, the last two columns were written about what you should never do with control limits. These three nevers, if avoided, will ensure that your control charts are useful, reliable tools.

The first two nevers of control limits are:

1.

Never allow control limits to be typed in.

2.

Never allow control limits to be automatically recalculated.

If any of the nevers is undertaken, you could be left with a control chart that doesn’t identify important process changes. But isn’t that the whole idea? Shouldn’t control limits be set so that they alert users to unusual circumstances? Well, yes. That’s the idea, and that’s why control charts are so extraordinarily useful.

Lots of different alarm rules can be applied on a control chart. But this column is written to specifically address the most commonly applied Western Electric rule of all: points that fall outside of control limits.

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