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komponisto comments on The Amanda Knox Test: How an Hour on the Internet Beats a Year in the Courtroom - Less Wrong

42 Post author: komponisto 13 December 2009 04:16AM

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Comment author: komponisto 25 March 2010 05:53:30AM *  7 points [-]

Thanks for the kind words!

Actually, looking back, I now think I could have done better. In particular, I wish I had been more explicit about the central probability-theoretic point: the fact that the evidence against Guede screens off Kercher's death as evidence against Knox and Sollecito. This point was missed by a number of commenters; if you read the discussion you'll find various people saying that the prior probability "should" take into account the fact that a murder occurred in Knox's house. In actuality, of course, it doesn't matter where you start, so long as you eventually incorporate all of the relevant information; but what must be understood is that if you start with probability mass assigned to Knox and Sollecito because of Kercher's death, then you have to take (most of) that probability mass away upon learning of the evidence against Guede. In other words, under this setup, evidence of Guede's guilt becomes evidence of Knox's and Sollecito's innocence -- something which is counterintuitive and very easy to forget (with tragic consequences).

This issue of "choosing the prior" and other Bayesian subtleties encountered in these discussions may be worth revisiting at some point.

Comment author: redlizard 17 August 2013 12:32:40AM *  1 point [-]

In particular, I wish I had been more explicit about the central probability-theoretic point: the fact that the evidence against Guede screens off Kercher's death as evidence against Knox and Sollecito.

I think this insight warrants a great amount of emphasis. The fact that Kercher's death is screened off by some factor unrelated to Knox and Sollecito means that the question of whether the given evidence against Knox and Sollecito is sufficient to judge them co-conspirators is equivalent to the question of whether the given evidence against them would have been sufficient to judge them murder-conspirators in the absence of a body. And I don't think anyone believes THAT is the case.