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Planet Money Investigates The Base Rate Fallacy As It Pertains To The Pandemic


When we try to follow the shifting path of the pandemic, it's easy to get lost in the numbers. Not all of us are statisticians. Amanda Aronczyk of NPR's Planet Money podcast brings us a fallacy to watch out for.

AMANDA ARONCZYK, BYLINE: At the end of July, I went to Iceland for the same reason that lots of people have been travelling there. They've had very low COVID rates and very high vaccination rates. So Iceland said to potential tourists, come to Iceland, where you can pay big bucks to ride little horses. But as we are packing to go on our trip, there was this COVID spike there. And this was very concerning because most of the people getting infected were fully vaccinated. You might have heard similar worries about Israel or the U.K. or about that outbreak in Provincetown, Mass. Katrine Wallace, an epidemiologist at University of Illinois at Chicago, ran the Iceland numbers for me. She found that over one month this summer, 67% of COVID infections there were in people who were fully vaccinated, which sounds really bad.

KATRINE WALLACE: So when you first look at that, at first blush, you say, oh, that means most people that are vaccinated will end up getting COVID. So we shouldn't use the vaccine, right?

ARONCZYK: But that 67% does not mean that the vaccine isn't working. It's a misreading of what's actually happening.

WALLACE: It is just people committing the base rate fallacy. They're not considering the whole context of the data.

ARONCZYK: The base rate fallacy. That means a very critical piece of information is missing, the base rate. The base rate is basically how common some characteristic is in a group. So in this case, the base rate that we care about is what percentage of the country's population has already been vaccinated. And the answer is 71%. Seventy-one percent of the total population of Iceland has been vaccinated. So that's a lot of people.

WALLACE: For context, in the U.S., we only have about 50.2% of our population fully vaccinated at this time.

ARONCZYK: In Iceland, almost everyone who could get a vaccine got a vaccine. So it's not surprising then that when COVID cases do happen in Iceland, most of them happen in vaccinated people. A thought experiment - imagine a world where everybody has the vaccine, where 100% of people are vaccinated. In that world, if there's any COVID left, every single case - 100% of cases - would be in vaccinated people. So the more useful question to ask is, how big was the risk of catching COVID if you were vaccinated? And how big was the risk if you were unvaccinated? Wallace says, in the past six months, six of every thousand vaccinated people in Iceland caught COVID. But for the unvaccinated, the rate of infection was triple that, 18 of every thousand. So clearly, it was better to be vaccinated. And it's not just COVID. Wallace says this base rate fallacy is useful to keep in mind whenever you hear people throwing numbers around to make a point.

WALLACE: What I would advise is just anytime you see, like, X percent of this or, you know, five out of 10 that, I would just say, what's the broader context here? What are we talking about?

ARONCZYK: Because once you consider the base rate, you get a very different story.

Amanda Aronczyk, NPR News. Transcript provided by NPR, Copyright NPR.

Amanda Aronczyk (she/her) is a co-host and reporter for Planet Money, NPR's award-winning podcast that finds creative, entertaining ways to make sense of the big, complicated forces that move our economy. She joined the team in October 2019.