# What type of control chart?

Six Sigma – iSixSigma Forums Old Forums General What type of control chart?

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• #51401

Dillon
Participant

I have a process that has an USL of 3 and a lower spec limit of 1 and no set target but the process runs best at around 1.2 – 1.5.  No matter how I set it up in a Xbar& R chart the cpk numbers are around .4.  Is there a better way to look at this data?

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#178107

Eric Maass
Participant

Hi Doug,
My suggestion would be to think of Process Stability and Process Capability as two separate, but related, topics.
If the Xbar R chart shows that your process is in control, then keep it going…
Meanwhile, perhaps you should start an effort on improving the Process Capability:
1) Is the process (using individual data, not X-bars) normally distributed? You can check with a test like Anderson-Darling, or using a Normal Probability plot of the data. If it isn’t normal, you have at least 3 alternative approaches to consider – one is to use a transformation to normality (like the Box-Cox transformation).
2) If it is normally distributed (or is normal after a transformation) – how does the Cp look? If the Cp is also low, let’s say less than 1, then you have a varaibility problem to focus on first. If your Cp is reasonably high, say 1.5 or so, then you might want to see if you can find a way to re-center the distribution so the mean is in the middle of the spec window.
You say the process runs best at around 1.2-1.5, and yet the middle of the spec window is 2.0 …so, if the problem really is where your process is centered, and if you would get a very good Cpk by recentering, then I’d suggest you spend some time with the team thinking about why the process runs best between 1.2 and 1.5…
Best regards,
Eric MaassMotorola

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#178125

Ged
Participant

What are the units of measure (i.e. 1 to 3 what)? Why do you say the process runs better at 1.2-1.5 instead of the nominal of 2?

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