If you have been following my articles over the last few months, you have seen that even though statistical process control (SPC) charts are very powerful tools for examining a process, it turns out that there are a lot of ways to mess up SPC. This month, I am going to finish up with a few more things to watch out for as you use them, so you never have to ask, “Why doesn’t SPC work here?”

What shape are you in?

No, I am not interested in if you are working out. The basis for using control charts to help you make economical decisions comes from assumptions about the type and shape of the distribution you are dealing with. If a process is out of control, it is by definition not coming from a single distribution, so the distributional assumptions cannot be met. This does NOT mean that the control chart is useless—in fact, you use those distributional assumptions to help you identify what was unexpected so that you can spend time investigating those particular events that were unusual to the underlying process and to identify, and then eliminate, what caused them.

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