KVPY 2020-21 SB SX Exam Previous Papers

In medical diagnostics for a disease, sensitivity (denoted a) of a test refers to the probability that a test result is positive for a person with the disease whereas specificity (denoted b) refers to the probability that a person without the disease test negative. A diagnostic test for influenza has the values of a = 0.9 and b = 0.9.
Assume that the prevalence of influenza in a population in 50%. If a randomly chosen person tests negative, what is the probability that the person actually has influenza ?