You are tested for a rare flesh-eating virus that afflicts 1% of the population. If you have the virus, the test will always come back positive, but if you do not, there is a 20% chance of a false positive. You are told that your test came back positive. What is the probability that you have the disease?
September 6th, 2011 | Category: 
Warning: Use of undefined constant bfa_comments - assumed 'bfa_comments' (this will throw an Error in a future version of PHP) in /home/customer/www/riddledude.com/public_html/wp-content/themes/atahualpa/comments.php on line 132
20%
No, keep guessing.
5%
5% is correct, Eliza Beth.
n=100 was used and numbers are rounded to whole people.
Of 100 people, 1 true positive and 20 false positive results are reported.
A total of 21 positives are reported and thus, 1/21 positives are true positives.
1/21 x 100% = 5%
That is the answer, of the 21 positive results, he has a 5% chance of being one of the fortunate 20.
Great job! You’re today’s winner.