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    basirou's Avatar
    basirou Posts: 12, Reputation: 1
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    #1

    Sep 3, 2010, 11:53 AM
    can any one help me with this question please
    Suppose that for the general population, 1 in 5000 people carries the human immunodeficiency virus (HIV). A test for the presence oh HIV yields either a positive (+) or negative (-) response. Suppose the test gives the correct answer 99% of the time. What is P[-|H], the conditional probability that a person test negative given the person does have the HIV? What is P[H|+], the conditional probability that a random chosen person has the HIV given that the person tests positive?
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Sep 3, 2010, 12:18 PM

    You need to draw a probability tree to help you. Having done this, you'll get:

    P(+) = 1/5000
    P(-) = 4999/5000
    P(H|+) = 1/5000 x 99/100
    P(H|-) = 4999/5000 x 1/100
    P(no H|+) = 1/5000 x 1/100
    P(no H|-) = 4999/5000 x 99/100

    From those, you should be able to work out the rest.

    Do you know about probability trees?

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