Many people go through a point in their lives where they question the beliefs they hold most dear. Then there are those who question the entire basis for statistical process control (SPC) once they have learned the statistical basis for them. I can’t help you with the former, but I have something to say to the latter after the break.

There are lots of ways to mess up control charts, but today I’d like to discuss a concern I hear from students and clients after they have learned some statistics and encounter one of these situations. Interestingly, although one example is with a very large sample size (for continuous data) and the other a fairly small one (for discrete data), the root of the problem is the same misunderstanding about control charts. In both cases, you might be tempted to either spend money trying to control a process only to find yourself losing more due to increased variation, or you might abandon control charts altogether as tools that don’t work for you.

Let me give you two scenarios.

Scenario 1

You have a high-volume process that automatically samples 50 units for your control chart. You decide to use an x-bar and s chart because you know s is a better chart to use when n > 7. This is what you get:

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