In: Statistics and Probability
A disease strikes 1 in 25 students. A new test correctly diagnoses if one has it 95% of the time and correctly confirms that one does not have it 97% of the time. If you tested positively for the it, what is the probability that you actually have it?
P(have the disease) = 1/25 = 0.04
P(test positive) = P(test positive | have the disease) * P(have the disease) + P(test positive | does not have the disease) * P(does not have the disease)
= 0.95 * 0.04 + 0.97 * (1 - 0.04)
= 0.9692
P(have the disease | test positive) = P(test positive | have the disease) * P(have the disease)/P(test positive)
= (0.95 * 0.04)/0.9692
= 0.0392