3 ms·
A test exists. It's good but not perfect. If you have the disease it will return positive 99% of the time. If you do not have the disease it will return positiv
by DanBC 7y ago
A test exists. It's good but not perfect. If you have the disease it will return positive 99% of the time. If you do not have the disease it will return positive about 1% of the time. About 0.1% of the population has the disease. You take the test. It returns positive.
What are the chances you have the disease?
People on HN may be able to work this out, but I guarantee that many people (including doctors and nurses) can't.
See Gerd Gigerenzer's work on this in Reckoning With Risk or Risk Savvy.
- DanBC 7y agoWe can re-word this to make it easier. 1000 people take a test. 1 of them has a disease, and the test will probably return positive for that person. 999 of them do not have the disease, and the test will return positive for about 9 of them.
- nootropicat 7y agoThat 0.1% includes newborns, the distribution of hiv is definitely not uniform. What are the chances if the person in question is a gay man that uses the test before unprotected receptive anal sex with newly met strangers?
- DanBC 7y agoBut that scenario makes the test pointless: there are a wide range of things to be prevented and safer sex is a better solution than testing.