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Beware of Covid confimation bias

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

April 23, 2020 - 2:30pm

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.

Tom Chivers is a science writer. His second book, How to Read Numbers, is out now.


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Esmon Dinucci
Esmon Dinucci
4 years ago

Even knowing about confirmation (edited from conversation via conformation) bias doesn’t always help one avoid it – it’s like trying to proof read one’s own work – fallible to say the least.

4 years ago

There is another two more problematic bias in Corvid19: the Status quo bias and the Sunk-effect bias, and currently they are hampering a proper strategy

Simon Forde
Simon Forde
4 years ago

These remarks are only tangentially about “confirmation bias” (except that I would like to believe that countries like the Netherlands, UK, and Sweden are reacting well to this crisis)… in relation to the repeated statements that Germany is doing better than the UK, partly through testing but implicitly because they are of course far superior in all respects. And I read today that in fact we should have copied Latvia whose many small, early steps show up how pathetic we have been here. So, here is my question (as a historian and not a scientist):
1) I understand that the coronavirus is not some virus that is equally spread all over the world, but one that is transmitted person to person.
2) I understand that the coronavirus largely transmitted to the Netherlands when people took their February half-term to Italy (skiing and sightseeing) and it spread particularly rapidly in Brabant and Limburg during the Carnival season that coincided with the end of this half-term holiday (Carnival being four days of partying, drinking, close proximity, etc.). And it spread rapidly in the Catholic South of the Netherlands, easily to the heavily populated provinces of North and South Holland, but very little to the highly rural, distant provinces of Friesland and Groningen.
3) incidentally, the epicentre of the German coronavirus was a tiny town at its very margins, but happened to be a couple of miles from Limburg in the Netherlands. This is not a town where tens of thousands regularly commute to major German cities. It’s as if the outbreak in the UK was centred on Minehead.
4) I cannot find details of the spread of coronavirus to Germany from skiing – certainly some were infected at Ischgl in the Tirol. But were Germans skiing in places that were relatively unaffected by the virus?
5) I also understand that spreading of the virus has been most rapid and virulent in major cities – Madrid, London, New York.

My observation is therefore that the main factors that determine the extent of the outbreak of coronavirus are (i) the total number of infections from places like North Italy and Ischgl, related to skiing and sightseeing in the February holiday – not forgetting, of course, possible direct infection from Wuhan/China; (ii) the degree of urbanisation of the places infected; (iii) the degree of mobility to these conurbations – with London and the Dutch Randstad as examples of very heavy long-distance commuting and travel.

Latvia, like Shetland, scores low on these counts. Excellent that they took urgent action to protect their population. But the fact that Latvia, Shetland, and the South Island of New Zealand have lower death rates per million than England doesn’t seem to say much about the UK’s reaction to its own circumstances. Nor does Germany seem to be a directly comparative.