What type of control chart?

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    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? 


    Eric Maass

    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



    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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