# multifoliaterose comments on Why We Can't Take Expected Value Estimates Literally (Even When They're Unbiased) - Less Wrong

67 18 August 2011 11:34PM

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Comment author: 19 August 2011 12:18:27AM *  2 points [-]

Upvoted.

This jumped out instantly when I looked at the charts: Your prior and evidence can't possibly both be correct at the same time. Everywhere the prior has non-negligible density has negligible likelihood. Everywhere that has substantial likelihood has negligible prior density. If you try multiplying the two together to get a compromise probability estimate instead of saying "I notice that I am confused", I would hold this up as a pretty strong example of the real sin that I think this post should be arguing against, namely that of trying to use math too blindly without sanity-checking its meaning.

(I deleted my response to this following othonormal's comments; see this one for my revised thought here.)

Of course, I believe this because I think the creation of smarter-than-human intelligence has a (very) large probability of an (extremely) large impact, and that most of the probability mass there is concentrated into AI, and I don't think there's nothing that can be done about that, either.

Why do you think that there's something that can be done about it?

Comment author: 19 August 2011 12:48:13AM 15 points [-]

I disagree. It can be rational to shift subjective probabilities by many orders of magnitude in response to very little new information.

What your example looks like is a nearly uniform prior over a very large space- nothing's wrong when we quickly update to believe that yesterday's lottery numbers are 04-15-21-31-36.

But the point where you need to halt, melt, and catch fire is if your prior assigns the vast majority of the probability mass to a small compact region, and then the evidence comes along and lands outside that region. That's the equivalent of starting out 99.99% confident that you know tomorrow's lottery numbers will begin with 01-02-03, and being proven wrong.

Comment author: 19 August 2011 12:57:29AM 4 points [-]

Yes, you're right, I wasn't thinking clearly, thanks for catching me. I think there's something to what I was trying to say, but I need to think about it through more carefully. I find the explanation that you give in your other comment convincing (that the point of the graphs is to clearly illustrate the principle).