Revision as of 07:07, 5 October 2008 by Vhsieh (Talk)

The Problem:

There is a new test that will test for the HIV virus, but we are not sure of whether this test's results are usually correct or not. We are given the following information:


$ P (+ | HIV) = 0.9, P (- | HIV) = 0.1\! $ $ P (+ | no HIV) = 0.1, P (- | no HIV) = 0.9\! $


We are also given that only $ 0.5%\! $ of the population has the HIV virus. The rest do not.


Are the results usually correct, and what can you tell from the results?


The Solution:

First, we determine the probability that a random person within the population tests positive. Remember, this person will be selected at random, so we have no clue whether or not he/she actually has the virus or not.


$ P(+) = P (+ \cap HIV) + P(+ \cap no HIV)\!<\math> $

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