Separately, I'm having trouble understanding how "suspect and victim are acquaintances" is screened off by "Guede killed Kercher". According to the wiki:
If A is a hypothesis and B and C are two pieces of evidence relating to A, then B is said to screen off C from A if P(A|B&C) = P(A|B). That is, if knowing C provides no additional information about A once B is known.
In the wiki, the example given is A="Knox killed Kercher", B="Guede killed Kercher", and C="Kercher was killed", so it's trivially true that P(A|B&C) = P(A|B) since B logically implies C. But if we replace C with D="suspect and victim are acquaintances" it's no longer trivially true that B screens off D. Actually I think it's false.
Consider the question, does P(A|B&~D) equal P(A|B&D)? Surely the probability that Knox is one of multiple attackers who killed Kercher, given that Guede killed Kercher (and no other evidence), would be smaller if Knox were just a random person with no relationship to Kercher? But B screens off D means that P(A|B&D) = P(A|B), which implies P(A|B) = P(A|B&~D).
What if we replace B with E="Guede killed Kercher and there is no strong evidence of another attacker"? Same thing, we still have P(A|E&D) > P(A|E&~D).
(Note that this is not an argument that Knox killed Kercher, but just that you seem to be using the concept of "screened off" incorrectly, and also wrongly claiming "everything else [besides bra clasp and knife] being near-negligible".)
You seem to be calling it "incorrect" if I say that "X = Y" when X is only approximately equal to Y. Obviously you're right in a literal sense, but it's an inappropriate criticism in this context.
"Suspect and victim are acquaintances" here is essentially the same event as "Knox's roommate was killed" -- something which significantly raises the prior probability that Knox committed murder. However, once we learn the details of the case, we find that the killing is entirely explained by the actions of Guede. ("En...
Recently, on the main section of the site, Raw_Power posted an article suggesting that we find "worthy opponents" to help us avoid mistakes.
As you may recall, Rolf Nelson disagrees with me about Amanda Knox -- rather sharply. Of course, the same can be said of lots of other people (if not so much here on Less Wrong). But Rolf isn't your average "guilter". Indeed, considering that he speaks fluent Bayesian, is one of the Singularity Institute's largest donors, and is also (as I understand it) signed up for cryonics, it's hard to imagine an "opponent" more "worthy". The Amanda Knox case may not be in the same category of importance as many other issues where Rolf and I probably agree; but my opinion on it is very confident, and it's the opposite of his. If we're both aspiring rationalists, at least one of us is doing something wrong.
As it turns out, Rolf is interested in having a debate with me on the subject, to see if one of us can help to change the other's mind. I'm setting this post up as an experiment, to see if LW can serve as a suitable venue for such an exercise. I hope it can: Less Wrong is almost unique in the extent to which the social norms governing discussion reflect and coincide with the requirements of personal epistemic rationality. (For example: "Do not believe you do others a favor if you accept their arguments; the favor is to you.") But I don't think we've yet tried an organized one-on-one debate -- so we'll see how it goes. If it proves too unwieldy or inappropriate for some other reason, we can always move to another venue.
Although the primary purpose of this post is a one-on-one debate between Rolf Nelson and myself, this is a LW Discussion post like any other, and it goes without saying that others are welcome and encouraged to comment. Just be aware that we, the main protagonists, will try to keep our discussion focused on each other's arguments. (Also, since our subject is an issue where there is already a strong LW consensus, one would prefer to avoid a sort of "gangup effect" where lots of people "pounce" on the person taking the contrarian position.)
With that, here we go...