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Process Capability

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  • #50655

    Deka
    Participant

    A basic rule for Process Capability is that the Process must be in control.  As far as I can see, the only reason for this is that if the process is not in control and Process Capability is calculated the results may be higher that reality.  If fact, you may still have a capable process, just wider that reality.  Is that the only reasoning behind this rule?

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    #174444

    Deka
    Participant

    I mean higher than reality..sorry

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    #174451

    Michael Mead
    Participant

    Hello Deka, basically that is it.
    Process capability relies on a predictable process. When calculating Cp and CpK we use the range to estimate of the standard deviation If the range is not predicatable, neither are the results.
    Another assumption that is made in capabality analysis is the normal distribution. You can calculate the standard deviation from any shape of distribution, but estimates of the proportion in the tails can be way off.
    If the process is not in control, you have made a histogram of the sample data, without making any real assessment of the process or the process potential.
    Good luck. 

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    #174461

    Deka
    Participant

    Thank You

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    #174462

    Remi
    Participant

    Hai Deka,
    the most used intention of Cp(k) is to give a quality verdict for the future: what can the customer expect from my process compared to his wishes (and yes sometimes it is used to evaluate the past).If my process is not stable any value I calculate from data over a certain time period has almost no connection to the quality level my customer can expect to recieve in the future.So the Cp(k) can always be calculated but sometimes it has no meaning or it leads to wrong conclusions.
    Success.

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    #174603

    Chris Whittaker
    Participant

    The last poster is not quite right ,Pp and  Ppk is the long term
    It is very common to have a process that is in control but not capable.
    The capability is set by the CUSTOMER who assignes the Upper and Lower specification limits, the PROCESS defines the capability.
    The CP is the ability of your process to mmet the Specifiocations laid down by the customer in the short term , the Pp is the ability of your process to meet the specifications of your customer in the long term.
    In other words can my process meet the customers specifications.
    You can have a process that is in control but is not capable of meeting the specs, conversely you can have a process that is out of control but still meets specs.  the former means you will loose money through lost business , the latter means you will lose money from an ineffcient production process.
    Cpk /Ppk is the way your current process FITS the customers specifications is it to much to the left or right of the bell curve idicates how much adjustment to your process is required to fit your process to the customers specs.
    It can sometimes also happen that you may have a brilliant process but will never meet the customers specs regardless of what you do which is when you start to look at new machinery or processes that will.

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