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    Hello all.

    I am a little confused by this question. I think it's conditional probability but not sure. Just cannot get my head around it

    The question is

    "A laboratory blood test is 80% effective in detecting a certain disease when it is present in the person tested. However, the test also gives a 'false positive' result in 5% of those tested who do not have the disease. Suppose that 0.4% of the population actually have the disease. What is the probability that a person with a positive test result actually has the disease?"

    Enter your answer in the box below, to 3 decimal places.

    I put 0.004 but I certain this is wrong.

    Any help would be appreciated as well as an explanation if they have time.

    Thanks again.
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    (Original post by Mr XcX)
    Hello all.

    I am a little confused by this question. I think it's conditional probability but not sure. Just cannot get my head around it

    The question is

    "A laboratory blood test is 80% effective in detecting a certain disease when it is present in the person tested. However, the test also gives a 'false positive' result in 5% of those tested who do not have the disease. Suppose that 0.4% of the population actually have the disease. What is the probability that a person with a positive test result actually has the disease?"

    Enter your answer in the box below, to 3 decimal places.

    I put 0.004 but I certain this is wrong.

    Any help would be appreciated as well as an explanation if they have time.

    Thanks again.
    Draw a tree diagram with branches: Diseased and not diseased and from each of these branches draw another two for positive result and negative result - then think of conditional probability.
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    (Original post by Zacken)
    Draw a tree diagram with branches: Diseased and not diseased and from each of these branches draw another two for positive result and negative result - then think of conditional probability.
    Okay thanks
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    it is conditional probability.

    i used this tree diagram:

    since it is P ( positive | has disease )

    you put any of A, B, C, D underneath ( added ) as the denominator, and one of them on top.
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    (Original post by the bear)
    it is conditional probability.

    i used this tree diagram:

    since it is P ( positive | has disease )

    you put any of A, B, C, D underneath ( added ) as the denominator, and one of them on top.
    Thank you
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    (Original post by Mr XcX)
    Hello all.

    I am a little confused by this question. I think it's conditional probability but not sure. Just cannot get my head around it

    The question is

    "A laboratory blood test is 80% effective in detecting a certain disease when it is present in the person tested. However, the test also gives a 'false positive' result in 5% of those tested who do not have the disease. Suppose that 0.4% of the population actually have the disease. What is the probability that a person with a positive test result actually has the disease?"

    Enter your answer in the box below, to 3 decimal places.

    I put 0.004 but I certain this is wrong.

    Any help would be appreciated as well as an explanation if they have time.

    Thanks again.
    You can also use bayes theorem if you know it
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    Can anyone answer this??? Suppose that A, B and C are events in a sample space S, such that A∪B∪C = S and A∩B∩C= ∅ If P(A) = 0.139, P(B) = 0.268, P(A∩B)= 0.045, P(B∩C)= 0.204 and P(A∩C) = 0.055, find P(C), and P(A|C).
 
 
 
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