# Attribute : Negative LCL

Six Sigma – iSixSigma Forums Old Forums General 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

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#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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