On Friday, April 17, 2020 at 8:22:13 PM UTC-7, John Cochrane wrote:
A test that is wrong 50% of the time can be replaced with a coin toss.


Not true actually, a test with a 50% chance of being wrong can be quite useful.

Suppose 2% of the population has the disease. Take a test that has 100% chance of positive if you do have it, and 50% chance of positive if you don't have it. It tells the 2 people who do have it they have it, and it tells one other healthy person he has it too. Three people stay home, 97 go soaring in perfect safety.

Take a test that has 100% chance of negative if you don't have it, but 50% chance of positive if you do have it. 98 people are correctly cleared, 1 person is incorrectly cleared, 1 stays home. Well, we got half the sick people out of the population and reduced the reproduction rate by half.

John Cochrane.

John, I'm afraid I don't understand the first part of your example. If you test 100 people with a test that has 50% false positives, and 2 actually have the disease, you will get 51 positive results - the two that have it and half of the remaining 98.

Fortunately, the coronavirus test is much more accurate than Bob suggests. The main problem with it is its scarcity.