SOLUTION: I have a medical statistics problem. I have listed it below and the WRONG answers that I got to the three part problem: Suppose that a certain HIV test has both a sensitivity a

Question 1042368: I have a medical statistics problem.
I have listed it below and the WRONG answers that I got to the three part problem:
Suppose that a certain HIV test has both a sensitivity and specificity of 99.9%. This test is applied to a population of 1,000,000 people.
Suppose that 1% of the population is actually infected with HIV.
(a) Calculate the PPV.

91.7

Incorrect: Your answer is incorrect.

(b) Calculate the NPV. (Round your answer to three decimal places.)

99.989

Incorrect: Your answer is incorrect.
(c) How many people will test positive who are, in fact, disease-free?

900

Incorrect: Your answer is incorrect.
Can anyone tell me the correct answer to these three parts and how you got them. I made a table but somehow I messed up along the way. I don't have an example in my book for solving this.
Answer by Boreal(15235) (Show Source): You can put this solution on YOUR website!
=======Test +========Test-=======Total
HIV+----------------------------------------1000
HIV-
Total-------------------------------------1,000,000
You know this first, and then you can put in the 99.9% sensitivity and specificity.
=======Test +========Test-=======Total
HIV+-----999--------------1--------------1000
HIV------999------------998001---------999000
Total-----1998----------998002---------1,000,000
The positive predictive value is 50%, 999/1998
The negative predictive value is 998001/998002 or 99.999+%
The number of people who test positive and are disease free is 999.
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