I got a question, that i could need some help with. Below you see the picutres

The following is given: A patient can have cancer, or not. There is a test, that has two outcomes: Positive, or negative. The prior prob. over all population, that a person has cancer is 0.01. The test returns positive outcome with prob. of 0.98, where the person has indeed cancer and negative with 0.95 where the person has NOT cancer. So far, so good.

1) I should summarize the above in terms on probabilities. That is my solution: P(C) = 0.01; P(not C) = 0.99 P(TP|C) = 0.98; P(TN|not C) = 0.95

2) A patient gets tested, and the test is positive. Do we tell this patient, he has cancer? I calculated P(C|TP) and got a solution value = 0.01 being 1%. Which means for me, we don't tell him, as there is only 1% chance, that he has cancer.

3) The patient gets testet again! Both tests are independent to each other. And i should calculate the "posterior probability" of cancer and not cancer following the two tests.

And here, for 3) i am lost a bit. I would now calculate just P(not C|TP) as i did in 2). But is that actually correct? If not, what should i calculate instead and how?

Task itself Try of Task

And for reference, a similar excercise, we discussed. The solutions are said to be correct for the next 2 pictures. I was trying to apply the same methods. But for Task 3) i am unsure, how to solve it.

example Solution for example


1 Answer 1


You can tell without any calculation that your answer to $2)$ is wrong because the posterior probability is equal to the prior probability, and the test is clearly not so bad as to be completely irrelevant.

For $3)$, you need to calculate $P(\text{no cancer}\mid\text{test $1$ positive}\cap\text{test $2$ positive})$, where e.g. $P(\text{test $1$ positive}\cap\text{test $2$ positive}\mid\text{cancer})=P(\text{TP}\mid\text{C})^2$.

  • $\begingroup$ Can you show me, how you can tell? I dont get it. Also, we have probably to calculate something. Pls help. $\endgroup$ Commented Jan 9, 2020 at 12:40
  • $\begingroup$ If someone tells me, how to get pictures in here, i can show, what i did. $\endgroup$ Commented Jan 9, 2020 at 12:42
  • $\begingroup$ @user3556093: There's an image button above the editor window. But it would be better to write what you did, not least because images are not searchable. I posted a tutorial and reference for typesetting math on this site in a comment under the question. I didn't mean to imply that you shouldn't calculate anything; you certainly should. I was just saying that it doesn't require calculation that your answer to $2)$ is wrong. I already explained in the answer why. If you don't understand the explanation, please state which part you don't understand (instead of asking for a new explanation). $\endgroup$
    – joriki
    Commented Jan 9, 2020 at 12:48
  • $\begingroup$ Yes, thanks. So, you said, my probabilities are the same. Is at least my P(C|TP) = 0.01 correct? That means, that P(C) = P(C|TP) right? Maybe i did calculate the wrong thing in a whole? I just dont understand, why you say, the test is good, if it only gets correct in 1% of all cases. Probability a patient has cancer given the test is positve. That is, what i calculated. And according to the excercise, i think that is, what i should do. "new patient => Test lab is positive => should we tell him, he has cancer?" And if the probability is only 1%, that he has cancer, if the test is correct ... $\endgroup$ Commented Jan 9, 2020 at 12:59
  • $\begingroup$ I got the pictures now. Thank you for your help with that :) Hope that clarifies a bit, on what i tried and where i struggle :) $\endgroup$ Commented Jan 9, 2020 at 13:14

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