A man living in a country where only 1 out of 1000 people has the virus A. There is a test available that gives a positive result 5% of the time when the patient does not have virus A and a negative result 1% of the time when the patient does have Virus A. Otherwise, it gives correct results. Recall that past computation showed that the man ’s chance of having the virus, conditional on a positive test, is less than 1.9%. Assume all tests are independent.
Let the conditional probability computed 1.9% serve as the new prior. Compute the new probability that he has the virus A (new posterior) based on his receiving a second positive test.
So my thought process was the following :
P(A) = 0.0001 P(Ac) = 1- 0.0001 P(+/Ac) = 0.05 P(-/Ac) = 0.01
I am asked to calculated to P(A/+) = P(+/A)*P(A) / P(+)
P(+) = P(+/A)*P(A) + P(+/Ac)*P(Ac)
My question is I am still confused on how I can get P(+/A).
Could you please help me?