Question

100,000 random people were tested for Condition A. Roberto's doctor told him he tested positive for Condition A. If the test is 99% accurate and Condition A is rare—one out of every thousand people have it. What is the probability that testing positive means Roberto has Condition A?
answer choices

99.0%

1%

9%

99.9%

Answer 1

The answer would be: 99.0%

The disease is rare with a prevalence of one out of every thousand people have it. That means, the chance of Roberto has the disease without any test would be 1/1000 or 0.1%
The test sensitivity is 99%, which mean 99% of people with positive test result would have the disease. The chance is should not be influenced by the disease prevalence. 

Answer 2

He would have a 9% of having the rare condition.

Hope this helps :)