I used to be a big proponent of testing to help manage the virus. But now I am backing off that. Here is the problem.

Suppose that as a scam, I say that I have a test for the virus. But in fact, I plan to use a random number generator that 5 percent of the time will say that you have the virus and 95 percent of the time will say that you don’t.

If half the population has the virus and half does not, then my scam will be exposed very quickly. My test will be making lots of mistakes, telling people who have it that they don’t and vice-versa.

But if less than 5 percent of the population has the virus, it may not be so clear. Most of the people who “test negative” in my scam will in fact be negative, so I will have that going for me. My problem, which may not be readily apparent, is that most of my positives will be false positives and a few of my negatives will be false negatives.

I am not saying that existing tests are pure scams. But to be better than pure scams, there has to be a much lower margin of error than you might think.

The tests that we have are giving nonsensical results, such as a husband and wife with identical symptoms getting opposite results, or studies that if they were extrapolated would imply that more than 100 percent of New York state has had the virus.

I was one of those FDA-bashers who thought that requiring certification for tests was peacetime bureaucratic thinking. I have come to realize that in order to be useful, the tests have to be highly accurate. If that is where FDA was coming from, I can now appreciate that.

After I wrote the above, but before posting, a commenter pointed me to an essay by Peter Kolchinsky, which aligns with my thinking.

The meaning of getting a positive result also depends on the percent of the population that has been infected. If 50 percent of people have been infected, then a test with a 97 percent sensitivity and a 2 percent false-positive rate is still likely to be 98 percent right if it tells you you’re positive. If only 2 percent of people are infected, then such a test would be only 50 percent right if it said you’re positive.