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Z proportions

  • 06-09-2011 7:06pm
    #1
    Maybe_Memories
    Registered Users, Registered Users 2 Posts: 927 ✭✭✭


    IG
    IB
    Non-Germinated ---405
    341
    Germinated
    386
    180

    Carry out the Z-test to compare the proportions of seeds germinating for the IG and IB columns and interpret the result.


    I don't know what I'm doing wrong but I'm assuming it's something because my answer is too big for the Z-table

    Z = (p1-p2)/root([(p1(1-p1)/n1)+[(p2(1-p2)/n2)

    P1 = 386/791
    P2 = 341/521

    Then throw everything into the Z formula. What's wrong with this?


Comments

  • #2
    MathsManiac
    Registered Users, Registered Users 2 Posts: 1,595 ✭✭✭MathsManiac


    Not sure what resource you're using for the hypothesis test, but I think the more usual version for the standard error for the difference of two proportions is to first get the "pooled proportion" p from the sample (in this case, 566/1312) and use that to get the standard error = sqrt(p(1-p)/n1 + p(1-p)/n2). [=0.028 here, I think.]

    The test statistic is then z = (p1 - p2)/SE.

    I get z = 5.95, indicating that the observed results are highly significant.

    (This is all on the assumption that you've been asked to test the null hypothesis that the population proportions are the same, against the alternative hypothesis that they are different.)


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