by Tom Chivers
Thursday, 23
April 2020

Beware of Covid confimation bias

by Tom Chivers
Medical firm staff work in a lab on coronavirus testing kits (Photo by Majid Saeedi/Getty Images)

The trouble with the Covid-19 crisis is that there is so much uncertainty and so many conflicting sources of evidence that you can choose whatever you like to push you towards whichever conclusions you want. It’s an absolute playground for confirmation bias.

I am aware that I’m on the optimistic end, so I ought to be careful. But this caught my eye: the first death in the US was weeks earlier than thought.

Two people died in California, one on 6 February, one on 17 February. The previous earliest known death in the US was 29 February, in Washington state; the earliest in California was 4 March.

According to CNN neither person had a travel history that suggested that he or she could have caught it outside the US; they seem to have caught it from the community. The usual lag from infection to death is about three weeks, so that implies they caught it in mid- and late January respectively and that the disease was already circulating in California then. The previous earliest known instance of community spread — the disease infecting someone who had no links to a known patient or a high-risk region — was 26 February.

To me it implies that the disease has been going around for longer and may, therefore, mean it’s infected more people (and thus killed a smaller percentage of the people it’s infected). A statistician who works on this stuff agrees: he tells me it “adds weight the argument that this started much earlier and is more widespread than we realise”.

I wanted to quickly temper that, though: some sensible voices suggest it doesn’t change very much. Plus, early serology tests in Geneva seem to suggest a smaller percentage have had the disease than some have thought, implying a higher death rate; and, as many have pointed out, there are at least 10,000 deaths so far in New York City alone, which given a population of around 8 million would imply a death rate of more than 0.1% even if literally every single person in New York City had had it (10,000 divided by 8,000,000 = 0.125%). Since that seems unlikely, it’s probably much higher, which suggests that some lower-bound estimates of the fatality rate (including my own) are probably overoptimistic.

This is what I mean. I’m not advocating radical scepticism; we can find things out. But if you’re not really careful it becomes very easy to convince yourself of anything you like. So if you find you keep reading things that convince you of stuff you already believed, then be aware that that might not be representative of the whole picture.

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