If your calculation is correct, and you should check it, then what it is saying is that the process mean is outside the spec limits. But if it is, you don’t need Cp or Cpk or even a control chart to tell you that. You have a serious problem that needs to be addressed. And it is an obvious problem!
If your process mean is within the spec limits, and that is pretty easy to test, and your Cpk is negative, then your calculation for Cpk is wrong. It is fairly easy in Excel to get the sign wrong!
So look to your process mean. If it is outside the specification limits, you have an obvious problem and you should forget about Cpk and control charts and all that nonsense until you at least get the process onto the right planet.
When quality guys talk about a process being “in control”, they are usually referring to the Shewhart tests for a control chart. But the assumption here is that generally, the process mean is somewhere near where it needs to be. If your specification is 1 to 3, and you are producing product at 10, you don’t need a control chart to tell you that there is a problem.
However, if your specs are to 3, and your process is running around 2.5, then your Cpk can’t be negative. A negative Cpk indicates a problem in your spreadsheet.
Remember that for Cpk, you need the distance to the nearest spec limit. So if X is the process mean, and LL and UL are the lower and upper spec limits, you calculate X-LL and UL-X and take the smaller of the two. If X is between LL and UL, this result will always be positive. Pretty easy in Excel to get one or both of those swapped.
–McD
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