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How Do You Calculate Sample Size?

Six Sigma – iSixSigma Forums General Forums Methodology How Do You Calculate Sample Size?

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  • #54078

    Scott Lawley
    Member

    Question # 71:

    “When sigma is 10, what sample size is needed to specify a 95% confidence interval of +/- 3 units for the mean?

    a) 7
    b) 11
    c) 32
    d) 43

    I’ve done it a couple of times using what I thought were correct formulae and came up with 30.
    I even used different calculators

    Thanks,

    0
    #193422

    Straydog
    Guest

    Are you dealing with proportional or continuous data? What is your confidence that the sigma is 10? It seems like you’re looking for the answer to an exam question and the examiners did not provide essential information, assuming that you’d use an expected formula.

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    #193423

    Robert Butler
    Participant

    What you are looking for it t*s/sqrt(n) = 3 You know s and it looks like the homework problem is assuming t for infinity therefore your answer should be d)

    0
    #193428

    Scott Lawley
    Member

    @rbutler Thanks, you’re right. I get it now.

    0
    #193824

    sriram
    Member

    Sample size determination:

    (Z*S)^2
    n = ——-
    E^2
    where Z is the confidence level(90%,95%,99%) values (1.645, 1.96, 2.575)
    S – Standard deviation,
    E = Estimated Mean – True Mean (Xbar – Mu).
    Hope this clarifies…

    0
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