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Politics : A US National Health Care System?

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To: Lane3 who wrote (23330)	3/5/2012 9:19:05 PM
From: TimF	of 42652

"If a test to detect a disease whose prevalence is 1/1000 has a false positive rate of 5 per cent, what is the chance that a person found to have a positive result actually has the disease, assuming that you know nothing about the person's symptoms or signs?" 100000 people take the test. One in 1000 or 100 people them have the disease. 100% of them test positive, so 100 true positives. 99900 don't have the disease. 5% of them or 4995 test positive. 5095 total positives, so the chance that a positive result means you have the disease would be just under 2 percent. Reading the rest after figuring it out myself I see the article you posted gives the same answer. But there are two problems with it. One is that adding in a false positive rate can change that slightly. And you are unlikely to have zero percent false positives. A more important problem is that you may have a biased sample. If you are only testing people with symptoms its a very biased sample, and even the percent that choose to go for a test without obvious symptoms would be likely to be biased. There is no way to give a good answer for the question unless you assume a totally random sample or assume a specific known bias.

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