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    Ravi Khare

    I feel fascinated by PreControl, and it’s simple implementation.
    Can anyone help me discuss how PreControl behaves when the process is not Normally Distributed.


    Mike Carnell

    There are several versions of Precontrol rolling around. In its typical form where it is calculated from Spec Limits rather than Control Limits. If yo are not normally distributed it will affect the probabilities of the points landing in the red, yellow, and green zones. If your distribution isn’t within the spec limits to begin with you are signing up for a lot of tweeking or down time to begin with.
    There are a lot of variations to Precontrol. Do little research and choose one that fits what you are looking for. Breyfogle has some of the versions in his book. 


    Shree Phadnis

    Dear Ravi,
    In fact precontrol is very simple and to get overwhelmed by it is easy, however there are certain points very important in its application.
    1. Applicable to Normal distributions only as the theory of precontrol is based on Probability theory.
    2. Apply precontrol only when you are sure that the process capability of the process is way above 1.33 min In case you do not verify this you can run an extermely large Alpha risk and might actually tamper the process rather than improve it.The rule of 5 consequetive pieces in green means 1.33 CPK should not be used for the determination of process capability
    3. The biggest drawback of Precontrol is that it cannot help us distinguish between the Special causes and the common causes.
    4.The best area to apply precontrol is on the X of the processes after the DOE and after verification of the process capability of the X .
    5. In fact for the application mentioned in point no 4 also it would be more appropriate to apply EWMA or CUSUM Charts.
    In case you need a detailed discussion pls refer to ASQ Publication some where last year in Technometrics.

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