- #1
neubreed
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Homework Statement
The test for an illness is 90% accurate (meaning it gives a positive result 90% of the time if the patient has gingivitis and a negative reading 90% of the time if the patient doesn't). 4% of all people have this illness.
a) calculate the probability the test will be positive for a random patient
b) once the patient tests positive, what is the probability that he actually has the illness?
Homework Equations
The Attempt at a Solution
a. P(positive):
b. P(has disease given test is positive):
I'm completely lost.
For a, I would think that the chances of the patient being actually ill are 0.04 and then him test positive is .9 of that, sooo the answer would be 0.036, but I have no idea how to make that look like an actual probability problem or if the answer is even correct...
For b, I don't even know where to start.
If someone could point me in the right direction, I would be much obliged. Thanks!