Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: What’s Up With the CDC Nowcast?, published by Zvi on December 22, 2021 on LessWrong.
How’s it going?
The CDC nowcast last week was 2.7% Omicron. That seemed like a reasonable guess.
The CDC nowcast this week is 73% Omicron, and last week’s nowcast got revised from 2.7% to 12.6%.
That’s two retroactive extra doublings last week, and then four more in the following seven days relative to Delta, for a doubling time of less than two days.
That report came out right after I put out Omicron Post #8, and I quickly noticed I was confused.
One possibility is, hey, large error bars, so I guess there’s that?
This conversation provided potential factors, but did not clear up the confusion.
This provides a clear alternate hypothesis, and I trust the source quite a bit.
Censoring the past ten days is quite a high price to pay given how fast things are moving. That’s a lot of data to give up, and it’s worth noting that 10 days later the difference between 40% and 73% isn’t all that big in context. But if Trevor is right about the speed of submission, and the Nowcast isn’t adjusting, it’s going to give out a nonsense answer.
Which is exactly what it is giving. There are three huge problems with the nowcast’s answer, on top of the revision being rather large and not inspiring confidence.
Where are all the cases?
The regional numbers make even less sense.
Also, it’s averaging over a week so it’s implying even higher rates now.
The third problem I only noticed later, but if it’s 73% for the whole week, and 12% for last week, where did we enter the week, and therefore how high did we have to get to balance out the first few days? Not that this problem is necessary to notice the flaws.
The first issue jumped out at me right away. We know that there wasn’t a dramatic rise in the number of cases overall. We know there wasn’t a rise in the positive test percentage. Yet the claim is there was suddenly, over the whole week, three times as many Omicron cases as Delta. Does that mean that the number of Delta cases was down by more than half inside of a week? Does that seem remotely possible?
Testing capacity is a limiting factor, but if it was having a big effect, we’d presumably be seeing a much bigger jump in the positive test rate. I can imagine a world where that’s not true, but it doesn’t match the data from earlier in the pandemic.
This gets far more extreme if you go to the regional level, and the implications get bonkers.
This was a good visualization of the nowcast by region.
This means that in two regions covering ten states, we had more than 95% Omicron cases, so twenty Omicron cases for every Delta case, whereas a week ago Omicron was a clear minority of cases. Does that possibly live in the same world as our case counts?
This is another good explanation of some of the reasons the data doesn’t add up. This is how one should approach the situation when algorithms produce obvious nonsense. As he notes, this isn’t a knock on the CDC. I can be harsh on the CDC, but this isn’t the time and place for that. It is an example of them doing their best to be helpful, and failing to notice that their algorithm had produced nonsense because they didn’t have a human look at it.
And sure, given how many eyes were on the forecast they should have had a human do a sanity check and put in a warning note. But they also shouldn’t have had to. Everyone else should have also noticed they were confused and that the number didn’t make sense, rather than reporting an obvious nonsense projection with huge error bars as if it were a fact. If I were at the CDC, I’d fix the ‘no human sanity check’ issue but also would be muttering about how this is why we can’t have nice things.
It would be better to not give obvious nonsense as an output, but that’s a lot to ask here. It’s really really hard to create an algor...