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What is p in attribute sample size formula?

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  • #51567

    newbie
    Participant

    Practically speaking, what is the “p” in the sample size formula for attribute data?
    N = (2*Z / Delta)^2 * p * q
    The proportion of what?  Defectives? And what is the rational behind the recomendation of using p = .5 when p is unknown? Assuming a 50% probability? 
    Thanks!

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    #179071

    Darth
    Participant

    The proportion is some estimate, hopefully based on previous experiences, that might represent the proportion of responses relative to the issue.  For example, I wish to sample a large group of documents to determine if they are “defective” or not.  Historically the proportion defective has been around 25%.  That becomes my p.  If I have no idea what it might be then I would use a p of .5.  Since the formula is p x (1-p) or q then the largest value of .25 will occur when p is .5.  This will give me the largest calculated sample size and thus compensate for my uncertainty.  Practically speaking, if you have no knowledge then use the .5 and take some samples.  See what the proportion is turning out to be and then recalculate sample size based on the new p.  Got it??  Merry Xmas.

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    #179076

    newbie
    Participant

    Got it.  Thanks Doc, and Merry Xmas to you too!

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