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Homework Help: Conditional Probability

  1. Apr 29, 2013 #1
    1. The problem statement, all variables and given/known data

    In 1988, the state of Illinois required HIV testing for a couple to obtain a marriage
    license. The HIV testing at the time consisted of two separate tests, the ELISA
    test and the Western Blot test. The Elisa test was signi cantly less expensive. A
    person who is HIV positive would test positive under the ELISA test 95% of the
    time. A person who is HIV negative would test positive under the ELISA test 99%
    of the time. In 1988, it was estimated that the percentage of people applying
    for a marriage license that were actually HIV positive was 1%.

    If someone test positive on the ELISHA test, then that person is given the West-
    ern Blot test. A person who is HIV positive will test positive on the Western
    Blot test 99% of the time, while a person who is HIV negative will test positive
    on the Western Blot test 5% of the time. What is the conditional probability that someone is HIV positive given that the person tests positive on both the Western Blot and the ELISHA test?

    3. The attempt at a solution

    Basically I drew a tree diagram to figure this out, but it seems very low. I would appreciate it if someone could tell me where I'm going wrong.

    If anyone has any questions on notation, please ask

    Attached Files:

  2. jcsd
  3. Apr 29, 2013 #2


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    2017 Award

    Staff: Mentor

    I think this should be "negative" (or 1% instead of 99%), otherwise the test would be very strange.
    While this should change your result to a larger value, small probabilities to have some rare disease are not uncommon in medical tests in general.
  4. Apr 29, 2013 #3
    Yes, I think you're right. I completely misread the question. Although, I think my confusion is rather valid.
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