In: Statistics and Probability
One in a thousand people are afflicted with the dreaded zigma disease. A pharmaceutical company develops a test for this disease and brags that only 5% of those with the disease test negative for it and 1% of those who do not have zigma will test positive for it.
What is the probability that a person who tests positive actually has zigma?
9.99%
1.09%
8.68%
5.01%
9.51%
P(having the disease) = 0.001
P(tests negative | have the disease) = 0.05
P(tests positive | don't have the disease) = 0.01
P(tests positive | have the disease) = 1 - P(tests negative | have the disease) = 1 - 0.05 = 0.95
P(tests positive) = P(tests positive | have the disease) * P(have the disease) + P(tests positive | don't have the disease) * P(don't have the disease)
= 0.95 * 0.001 + 0.01 * (1 - 0.001)
= 0.01094
P(have the disease | tests positive) = P(tests positive | have the disease) * P(have the disease) / P(tests positive)
= 0.95 * 0.001 / 0.01094
= 0.0868
= 8.68% (ans)