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November 26, 2014

November 26, 2014

Posted by **DAS** on Wednesday, February 4, 2009 at 12:56am.

a) if the second test is also positive what are his chances of actually having cancer

b) if the second test is negative what are his chances and should he get another test?

- Probability -
**economyst**, Wednesday, February 4, 2009 at 3:07pmAn excel spreadsheet is very helpful for these kinds of problems.

First calculate the probabilities for all possible outcomes. There are two kinds of people with and without cancer, the first test can have two outcomes positive and negative, and the second test has two outcomes. So, there are 8 possible outcomes.

So, the probability that a person has cancer and the two tests are positive is .015*.90*.90 = .01215. Repeat for the 7 other possibilities.

The probability that a person doesnt have cancer and the two tests are (false) positives is .985*.05*.05 =

- Probability -
**economyst**, Wednesday, February 4, 2009 at 3:10pmAn excel spreadsheet is very helpful for these kinds of problems.

First calculate the probabilities for all possible outcomes. There are two kinds of people with and without cancer, the first test can have two outcomes positive and negative, and the second test has two outcomes. So, there are 8 possible outcomes.

So, the probability that a person has cancer and the two tests are positive is .015*.90*.90 = .01215. Repeat for the 7 other possibilities.

The probability that a person doesnt have cancer and the two tests are (false) positives is .985*.05*.05 = .0024625

So, a) conditional on two positive tests, the likelihood of having cancer is .01215/(.01215+.0024625) = .831

b) take it from here.

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