Nisan comments on Inherited Improbabilities: Transferring the Burden of Proof - Less Wrong

30 Post author: komponisto 24 November 2010 03:40AM

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Comment author: jsteinhardt 24 November 2010 03:47:17PM 1 point [-]

But, of course, the mathematics of probability theory don't work that way. A hypothesis, such as that the apparent burglary in Filomena Romanelli's room was staged -- doesn't get points for its ability to explain the data unless it does so better than its negation. And, in the absence of the assumption that Knox and Sollecito are guilty -- if we're presuming them to be innocent, as the law requires, or assigning a tiny prior probability to their guilt, as epistemic rationality requires -- this contest is rigged. The standards for "explaining well" that the fake-burglary hypothesis has to meet in order to be taken seriously are much higher than those that its negation has to meet, because of the dependence relation that exists between the fake-burglary question and the murder question.

This isn't quite true. If the prior probability of being a murderer is 1 in 10^6, and I can find 30 things that are explained twice as well by the murder hypothesis as the non-murder hypothesis, then the posterior probability of being a murderer is 99.9%, in the absence of mitigating factors (since 2^30/10^6 is about 1000.) So, many pieces of weak evidence for an unlikely proposition can still establish that proposition.

Comment author: Nisan 24 November 2010 06:10:05PM 4 points [-]

You'd also need those 30 things to be independent.

Comment author: NihilCredo 24 November 2010 10:55:10PM 1 point [-]

Probably superfluous nitpicking: you can build a strong case even with partially-interdependent pieces of evidence, you will just need more of them since you have to work off their conditional probabilities (which is mathematically equivalent to "splitting" them into independent pieces of evidence).