Stats tip: How can I be “in control” if I don’t know what it is?

Matt SavageI recently received the following question:

‘The process certification program at my company says that in order to certify a process it must be in control, be capable and be centered. Capability is measured by the process Cp and centering is measured by the Cpk. What measurement is used to determine if a process is “in control”? Is there a crisp definition of “in control”?’

“In control” is a term used to describe a process that is predictable and does not contain any special causes of variation. A special cause is something you did not expect to occur. I often refer to these as hiccups because, like a hiccup, you do not get them often.

There are many out-of-control or special cause tests you can use to help identify if the system you are evaluating appears to have special causes of variation. In general, if one of the out-of-control test rules is broken, you have license to investigate the hiccup or out-of-control point. Upon investigation, you will make a determination if the anomaly is a special cause. Then, if it is a special cause, you will determine what action to take.

In short, if you look at a control chart and it shows only common cause variation, it is said to be in-control and you should be comfortable predicting the future based on the past (“in control”) process.