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Wei_Dai comments on Experiment: Knox case debate with Rolf Nelson - Less Wrong Discussion

18 Post author: komponisto 08 July 2011 08:22AM

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Comment author: Wei_Dai 09 July 2011 05:49:04PM *  2 points [-]

The important kind of confusion would be where you think my prior is too low, rather than too high.

I was trying to understand what evidence has been taken into account in your prior (i.e., is there some other information that might be considered Bayesian evidence against Knox, but which is already in your prior), so that I can understand what other evidence you consider "negligible". I think at this point that confusion has been resolved.

I still wonder why the two sides don't each post a more detailed Bayesian calculation. Let's say A="Knox killed Kercher", B="Kercher has been killed and Knoxed lived in Italy and is an intelligent 20-year-old female college student with no criminal history", C="evidence against Guede", D="Knox and Kercher were roommates", E="evidence of a staged burglary", F="bra and clasp", G="all other information about the case". What are

  • P(A|B)
  • P(A|B&C)
  • P(A|B&C&D)
  • P(A|B&C&D&E)
  • P(A|B&C&D&E&F)
  • P(A|B&C&D&E&F&G)

(Or some other set of evidence and order of evaluation that might be more appropriate.) Wouldn't that help to quickly pinpoint where your disagreements are?

Comment author: komponisto 09 July 2011 06:42:12PM 3 points [-]

Let's say A="Knox killed Kercher", B="Kercher has been killed and Knoxed lived in Italy and is an intelligent 20-year-old female college student with no criminal history", C="evidence against Guede", D="Knox and Kercher were roommates", E="evidence of a staged burglary", F="bra and clasp", G="all other information about the case".

I'll redefine slightly:

  • A := "Knox killed Kercher, given background info about both, but not the fact of their acquaintance". P(A) = tiny.

  • B := "Kercher killed". P(A|B) = approximately P(A). (We are not yet given that they were roommates.)

  • C := "evidence against Guede". P(A|B&C) = approximately P(A). (No significant connection between Guede and Knox.)

  • D := "Knox and Kercher were roommates". P(A|B&C&D) = slightly higher than P(A), but still well below the threshold of consideration.

  • E := "Facts cited as evidence of staged burglary". P(A|B&C&D&E) = approximately P(A|B&C&D). (Likelihood ratios involved are close to unity; certainly small relative to P(~A)/P(A).)

  • F := "bra clasp and knife". P(A|B&C&D&E&F) = possibly as much as an order of magnitude higher than P(A|B&C&D). (Explaining results is a minor puzzle.)

  • G := "all other information". P(A|B&C&D&E&F&G) = approximately P(A|B&C&D&E&F). (Other evidence weak; slightly inculpatory facts canceled out by slightly exculpatory facts.)

Comment author: Wei_Dai 09 July 2011 06:55:45PM 1 point [-]

Thanks, that's very helpful. Perhaps you could copy this to the main debate branch, so Rolf would see it and possibly respond in a similar fashion? Also, to seek a bit more clarification, what is your estimate of P(A|B&C&D) / P(A|B&C)?