
CDC: Coronavirus antibody tests may be wrong 50% of the time - walterbell
https://www.mercurynews.com/2020/05/26/cdc-coronavirus-antibody-tests-could-be-wrong-50-of-the-time/
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taeric
This wording is confusing.

With our priors of how many real positives there are, a positive result could
be wrong fifty percent of the time. The same is not necessarily true for
negatives.

If our priors are wrong, of course, the test could be wrong more or less.
Which is to say, yes, it is complicated.

Edit: and I should add that my understanding here could be wrong. Would
appreciate elaboration on the point.

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tinus_hn
No, that is not what they say.

They say that because the number of people that are actually positive is low,
the number of false positives may be high when compared to the actual number
of positives.

This does not mean it’s wrong 50% of the time, it’s a statistical trick.

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im3w1l
In jargon, the _precision_ is lower than 50%.
[https://en.wikipedia.org/wiki/Precision_and_recall](https://en.wikipedia.org/wiki/Precision_and_recall)

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contravariant
Technically that description is correct, but people might confuse that
definition of precision with the false positive rate, which usually _does not_
include the prior probabilities.

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m0llusk
This article is woefully vague. There are multiple competing antibody tests
that use different production methods. The Abbot Labs antibody test appears to
exceed 99% accuracy which makes it significantly less susceptible to this
problem. Additionally, the common expectation is that antibody tests should be
quick and cheap and easy, possibly easy enough for people to perform
themselves at home. This makes it possible to generate enough additional test
data to correct for some inaccuracies.

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tomohawk
CDC quote:

> Serologic test results should not be used to make decisions about returning
> persons to the workplace.

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m0llusk
Right, this is more about tracking clusters and finding out exactly how
effectiveness of different regimes such as lockdowns and required mask wearing
compare. We need all the data we can get to make sure that our social policies
are correct based on robust scientific analysis and not on emotions conjured
from the demon haunted world.

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dbbk
Are they referring to antibody tests in general as a concept, or a specific
test?

