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Math Help - Confidence interval

  1. #1
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    Confidence interval

    In a test of the effect of dampness on electric connections, 100 electric connections were tested under damp conditions and 150 were tested under dry conditions. Twenty of the damp connections failed and only 10 of the dry ones failed. Find a 95% confidence interval for the difference between the proportions of connections that fail when damp as opposed to dry.

    I am having a very hard time getting through this problem.
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  2. #2
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    what answer did you get?
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  3. #3
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    i don't know how to start it
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  4. #4
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    95% confidence interval for the difference between the proportions is


    (p_1 - p_2)\pm Z_{\alpha /2}\times \sqrt{\frac{p_1 q_1}{N_1}+\frac{p_2 q_2}{N_2}}


    here your

    p_1=\frac{n_1}{N_1}

    p_2=\frac{n_2}{N_2}

    N_1=100, N_2=150
    n_1=20, n_2=10
    Z_{\frac{0.05}{2}}=1.96


    So solve accordingly.


    PS: Check the correction i made.
    Last edited by amul28; May 5th 2011 at 08:59 PM. Reason: changed n1 and n2 values correctly
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  5. #5
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    how do i calculate q?

    1-p=q
    lost my mind for a sec

    so i got 1.225, -.5668
    does that seem right?
    Last edited by mr fantastic; May 6th 2011 at 03:30 AM. Reason: Merged posts.
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  6. #6
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    jabba1,

    My bad. I guess even you dint realize that i have made a mistake in giving the values for n_1 and n_2.

    And yes q=1-p.
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