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Homework Help: Confidence interval, proportions

  1. Mar 7, 2010 #1
    I a survey, out of 530 women and 509 men, 10.4% of the women and 21.6% of the men answered yes to a question.

    Calculate a 99% CI for the difference between two times the population proportion women and the population proportion men who answered "yes".

    Answer from book: (-0.091;0.075)

    What I did was
    p hat 0 = (530 * 0.208 + 509 * 0.216)/ (509 + 530)
    and then find
    s^2 (p hat x - p hat y) = P hat 0 * (1- p hat 0) * (1/509 + 1/530)
    and finally -0008 +- the square root of that

    But the answer I get is wrong and I can't see where I made a mistake. Any help please?
  2. jcsd
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