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Attribute : Negative LCL

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  • #28556

    Kenny
    Participant

    Hi,
      I use some attribute data to calculate the LCL for p chart.  My formula is LCLp = pBar-3*(pBar(1-pBar))^0.5/pBar^0.5
      I’m sure my calculation is OK, but I get a negative LCL value.
      Shall i     1) use this negative value ? or     2) convert it to zero ? or    3) assume no LCL at all ?
    Thanks in advanceKenny

    0
    #71333

    Erik L
    Participant

    Three things.  Your formula is incorrect.  The formula for LCLp is pbar-3*(pbar(1-pbar)/n)^.5.  Second, LCL for p-chart only has a meaning if it yields a postive value.  You’d be safe to make it 0.  Third, are you sure you have a count data situation that satisfies the requirements of the binomial?
    Regards,Erik
     

    0
    #71346

    Kenny
    Participant

    Thanks Erik.
    My mistake, the formula should be
      LCLp = pBar-3*(pBar(1-pBar))^0.5/n^0.5
    From the formula, LCL can be negative if pBar is small or n is small.
    Just like to know, if LCL is negative, what can i do ?
    What is requirement of binomial ?
    Kenny

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