4 ms·
There is a 1/10000 (the OP said 0.01%) chance of having the disease. That is, in 10000 people, one person will have the disease. But also, if you test everyone
by hervature 5y ago
There is a 1/10000 (the OP said 0.01%) chance of having the disease. That is, in 10000 people, one person will have the disease. But also, if you test everyone with a 99% accurate test, 100 people will test positive. If you test them again, 1 person will get another positive test. So, at the end of the day, your test will identify the one person with the disease with a 99% certainty (or 98.01 if you require two tests to be positive). But also identify another individual as being doubly positive. So, after two rounds of tests, 50% of the marked individuals truly have the disease.
The silly thing with this demonstration is that accuracy is poorly defined. Tests are normally conditional on the actual presence of the disease or not.