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Sample Size Calculation

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  • #54753

    Sahab
    Participant

    Hi
    I am looking on how to do a sample size calculation on attribute data

    Population: 1000 tests per month
    Pass Rate: 85% of tests are correct

    How can I determine of the 1000 tests per month, how many tests i would need to do at 95% CI based on 85% of the tests being correct every month. I don’t want to test all 1000 every month, just a sample of them

    also how can I do this calculation in Minitab?

    thanks

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    #196989

    Katie Barry
    Participant

    @singhsahab — Consider describing how you would approach this problem. What you’ve tried, where you’re getting stuck, what specifically you don’t understand. iSixSigma readers are likely to help but they’re not here to do your work for you.

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    #196990

    Prabhu V
    Participant

    Hi,

    Please use the below guidelines for your case.

    For discrete data

    N=(Z/E)^2*p(1-p)

    Z – Constant for confidence level (like 1.645 – 90%, 1.96 – 95%, 2.575 – 99%)
    E – Precision or Error – level of precision desired from the sample in units of proportion (say 5% or 0.05, if more critical then use 1% or 0.01)
    p- Proportion(like 15% or 0.15 defect rate in your case)
    Please feel free to post your queries in case if you want for clarifications.

    Thanks and regards
    Prabhu V.

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    #196991

    Chris Seider
    Participant

    Stat>Power and Sample Size>1 proportion

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    #197004

    shubin
    Guest

    If 95% confidence (two sided) @ 85% proportion with 5% margin of error, some resuls for your reference.
    1. Per Prabhuv’s formula (Central limit therom), Sample Sze=195
    2. Minitab (Stat-Sample size & power-Sample size for Estimation), sample size=254.
    If 95% confidence (one sided, say upper bound), Sample size=124.

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    #197079

    Russell
    Participant

    Many great free calculators on the internet too…..

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