- #1
wolfmanzak
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Homework Statement
The proportion of people in a given community who have a certain disease is 0.005. A test is available to diagnose the disease. If a person has the disease, the probability that the test will produce a positive signal is 0.98. If a person does not have the disease, the probability that the test will produce a positive signal is 0.02.
If a person tests negative, what is the probability that the person actually has the disease?
Homework Equations
I'm at a loss for the relevant equation. I've scanned through my book several times.
The Attempt at a Solution
I set the problem up as follows:
Given the following:
Probability that one has the disease: P(D)=.005
Probability that given one has the disease, they test positive: P(+|D)=0.98
Probability that given one does not have the disease, they test positive: P(+|D^c)=.02
I've used the notation P(-) for test is negative which is equal to P(+^c)
I'm trying to find P(D|-) based on the problem statement. I can't find any way to relate this to what I've been given. The solution says to use P(+|D^c)P(D)=.02*.005=1.0E-4 but I have no idea where that relation came from or where to find a proof of such online.
If anyone could explain to me why P(D|-)=P(+|D^c)P(D) it would really help my understanding of the problem.