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Houshalter comments on Open thread, Sep. 26 - Oct. 02, 2016 - Less Wrong Discussion
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I was at the vet a while back; one of my dogs wasn't well (she's better now). The vet took her back, and after waiting for a few minutes, the vet came back with her.
Apparently there were two possible diagnosis: let's call them x and y, as the specifics aren't important for this anecdote.
The vet specifies that, based on the tests she's run, she cannot tell which diagnosis is accurate.
So I ask the vet: which diagnosis has the higher base rate among dogs of my dog's age and breed?
The vet gives me a funny look.
I rephrase: about how many dogs of my dog's breed and age get diagnosis x versus diagnosis y, without running the tests you did?
The vet gives me another funny look, and eventually replies: that doesn't matter.
My question for Lesswrong: Is there a better way to put this? Because I was kind of speechless after that.
"Base rate" is statistics jargon. I would ask something like "which disease is more common?" And then if they still don't understand, you can explain that its probably the disease that is most common, without explaining Bayes rule.
Mightn't the vet have already factored the base rate in? Suppose x is the more common disease, but y is more strongly indicated by the diagnostics. In such a case it seems like the vet could be justified in saying that she cannot tell which diagnosis is accurate. For you to then infer that the dog most likely has x just because x is the more common disease would be putting undue weight on the Bayesian priors.