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# Probability help needed. watch

1. 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?"

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.
2. (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?"

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.
3. (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
4. 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.
Attached Images
5. IMG_20151107_0005.pdf (80.2 KB, 45 views)
6. (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
7. (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?"

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
8. 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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Updated: November 12, 2015
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