Question

Note: This is using Bayesian statistics (a) Suppose that in a population, the probability of having...

Note: This is using Bayesian statistics

(a) Suppose that in a population, the probability of having a rare disease is 1 in 1000. We use θ to denote the true probability of having the disease. A diagnostic test for this disease has a sensitivity of 99% and a specificity of 95%. A randomly selected person from the population is administered the test and the test and it comes up positive (the test suggests that the person has the disease). What is the probability that the person indeed is suffering from the rare disease?

I know the answer to this question is 1.94%, but have posted this part of the question because the next few parts rely on this part.

(b) Suppose that the person in part (a) had a second test using the same diagnostic tool (a re-test to confirm earlier results). Suppose further that the re-test is negative. What is the updated probability that the person has the disease now that the second test is available?

(c) Consider a different person from the same population as above (part (a)). The person is also tested twice for the presence of the disease. The first test is negative, but the second test is positive. What is the probability that this second person is suffering from the disease and how does this probability compare with the probability you computed in part (b)?

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