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P Chart Sample Size

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  • #38267

    jimbo
    Participant

    I’m constructing a control chart where I’m tracking the number and results of electrical tests during manufacturing.  My entire population size is 20 – 30 tests per month and the outcome of the test is pass/fail (attribute).  My question is this:  what is the minimum sample size for a p chart?  I have a copy of the Western Electric book and I’m looking at Part A on Process Control Charts and the only reference I can find on sample size for a P control chart does set a lower limit on the sample size. 
    I have a colleague that wants control charts to be more responsive to changes in the process as it happens on the manufacturing floor — I also agree, buy this would mean posting the data every week instead of every month.  Doing this would lower my sample (population) size to about 7 tests per week.  My position is that such a small sample size would very sensitive to just one failure. 
    Any advice?
    Jimbo

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    #114370

    Bob J
    Participant

    Jimbo,
    I can’t cite my reference but I was taught to use a minimum sample size that gave me np>5….
    Best Regards,
    Bob J

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