While it remains interesting, we don't seem to be getting any traction toward significantly changing each other's mind through this one-on-one debate, should we just cut our losses and end it? I'm open to other suggestions on how to proceed. You haven't yet presented arguments toward innocence, but if we just follow the same pattern we have been so far we're probably not going to get anywhere with those either.

I wouldn't have expected us to have made much progress on mind-changing by this point, since we've only had a couple of iterations of back-and-forth; and moreover we've been starting to succumb to the danger of going "shallow and wide" (i.e. listing disagreements on many different aspects of the case) rather than "narrow and deep" (i.e. trying to iron out a particular important point of disagreement). For instance, I think the contrasting opinions on the wounds, or inferences about how the knife was held, are areas of lesser importance, which we probably don't even need to get into at this point, since there are much more important issues to focus on (such as how likely contamination is, or the incompatibility of the stomach evidence with the prosecution's hypothesized time of death).

So making progress would probably require us to pick a small number of narrowly-defined issues to hash out, one at a time.

Here's an important question to assess whether we've said anything important yet: has anything I've said surprised you?

You haven't yet presented arguments toward innocence, but if we just follow the same pattern we have been so far we're probably not going to get anywhere with those either.

That may be right, so if we do continue further it will be best to try to avoid that pattern. In any case, I think it's probably worthwhile for me to at least state for the record what I believe to be the most important arguments for innocence, so I'll go ahead and do that. Here are what I would consider to be the three strongest arguments for innocence, from a starting prior of "Knox and Sollecito have been convicted":

(1) The hypothesized crime would be so singularly unusual as to make a miscarriage of justice a priori as likely as guilt, even without knowledge of defense arguments. In other words, the base rate of crimes in this reference class is less than or equal to the base rate of wrongful convictions.

(2) Guede is unquestionably guilty. Hence, Meredith Kercher's death itself does not require explanation. (There is furthermore little connection between Guede and either Knox or Sollecito.)

(3) The stomach evidence is incompatible with a time of death much beyond 9:00 - 9:30 pm, given the testimony from Meredith's friends about when her last meal took place. Yet there was activity on Sollecito's computer at 9:10 pm according to Massei-Cristiani, and 9:26 pm according to Sollecito's appeal.

(Note that although (1) is in principle screened off by detailed knowledge of prosecution and defense arguments, it is effectively preserved in the form of arguments about lack of motive, lack of criminal history, demographics, etc.)

Now here's what I predict about where this would lead us. Pursuing (1) would lead us to a discussion very similar to the one we've been having about the base rate of contamination. Before we started the discussion I would have been most curious about your response to (2), but given your numbers on the knife evidence it seems pretty clear that you think the evidence against Knox and Sollecito is strong enough to overcome my prior against Knox and Sollecito's guilt given Guede's (whatever yours is). That leaves (3). I'm not exactly sure what your reaction will be, because I don't know how aware you are of this line of argument already. I suspect I could probably get you to agree that it would be extremely unusual for no food to have passed into the duodenum 5 hours after a meal (as required by the prosecution theory), even conditioned on the already unusual fact of none having passed after 150 minutes. However, I can't predict how far you will lower your probability of guilt as a result.

If you want to proceed, I'll ask you whether you agree with my "one contamination in fifty homicides" estimate, and what your probability is that the independent report is basically correct, and whether you would agree with me that this puts an upper bound on the contamination probability.

I don't think I would have a problem positing that the expert report constitutes 50:1 evidence in favor of contamination, possibly much more.

I'll also...say that I feel you're giving too little weight to the statements of lab technicians, police officers, and trial judges, especially when those people were there and we weren't.

This is interesting, because it seems to me that you're giving too little weight to Conti and Vecchiotti. I would be curious to know specifically whose opinion I should be weighting more than I am, and what information you think they are likely to have that I don't. For example, I don't see why I should give particular weight to Massei and Cristiani's opinion, given that they made their reasoning plain in their 427-page report, and I can read it and decide what I think. And why should I trust Stefanoni, in light of what Conti and Vecchiotti say? Should I take seriously Edgardo Giobbi's assertion that his psychological profiling techniques are effective, when he has admitted to attaching significance to the fact that Knox and Sollecito were seen eating pizza? And Mignini, of course, has the Monster of Florence issue.

The only person who was actually "there" is Rudy Guede; is there any particular reason I should believe him?

Anyway, basically, my feeling on whether to proceed is the following: I don't consider it particularly costly to proceed, especially if done in a relaxed, low-key way. There's no particular urgency, beyond that automatically entailed by the quest for epistemic accuracy. It might be more effective to "drag it out" over a more extended period, and exchange fewer messages per unit time, than to try to accomplish mind-changing in a short intense session (which has more of a danger of feeling like a comptetition anyway).

If you don't want to proceed, then I thank you for humoring me as far as you did with this experiment, and I will express my condolences to all innocent people in jail, whether they include Amanda or not, and hope that justice prevails in this case.

I appreciate your saying this, however far we end up carrying the discussion, and I hope you still feel able to say the same at the end (if that turns out not to be now).

a smaller knife would have been compatible with all of the wounds as well as the imprint

(Massei, 170) seems to disagree (although I could be misreading it), but I welcome any counter-arguments.

The argument there is pretty weak: Massei and Cristiani simply find it hard to believe that a single knife could have caused such different-looking wounds. Not much more than an argument from personal incredulity; they don't support it with any detailed arguments or expert opinions indicating incompatibility of a smaller knife. In fact, they proceed to argue at length that Item 36 itself is compatible with all the wounds, by disputing the defense arguments that it is incompatible with some of them. (For counterarguments on this, see pp. 36-51 of Knox's appeal document, and pp. 20-23 of Sollecito's. Since you cited the original page number of Massei, I'm guessing you can read Italian. If not, I can provide translations. Briefly, the important things to note, besides the object-level defense arguments themselves, are that prosecution expert Bacci "disavowed" his earlier judgement of compatibility, and that civil-party expert Liviero wasn't even aware of the defense experts' counterarguments and hadn't considered them.)

The closest thing to evidence for (the knife being bleached) is that a police officer claimed to have smelled bleach upon opening the drawer that the knife was in. (That doesn't distinguish the knife in question from the others, needless to say.)

I disagree: "Let me state beforehand that it was extremely clean" (Massei, 99)

Such a statement strikes me as extremely weak evidence, likely tainted by hindsight among other things. Bleaching is the kind of thing you establish with chemical tests, not someone's judgement of "looking clean".

The innocent explanation is that (Raffaele) had been told about the knife DNA result, and believed it.

I give the odds that he would tell such a lie if guilty as .1, and if innocent as .001; what odds do you give? Why would Raffaele believe the DNA result? He's more likely to believe it's not the murder weapon if he's innocent, and also less likely to lie if innocent.

Firstly, since we don't have the transcript of the interrogation, we don't know that it was a "lie". It could, for example, have been an exchange like this:

-- We found Meredith's DNA on a kitchen knife in your house!

-- Well, I guess she must have come to my house and pricked herself, then!

But in any case my probability that he would make up some story similar to this (or otherwise say something that would likely be reported as him making up such a story), given (1) innocence, (2) that he had been told about the DNA result, and (3) had demonstrated confusion about his memories elsewhere (e.g. suggesting at one point that Knox had gone out on the evening of the crime), is in the region of 50%. People tend to trust authorities, and defying the data is an advanced rationalist skill that cannot be counted on, especially in the face of shouting policemen. From what I understand, it is relatively easy to get people to make things up in stressful interrogation situations.

I think if she's innocent, reacting to the knife like that is about .05 likely, and if she's guilty, about .005 likely. What odds do you give? You wouldn't be more likely to react strongly to an otherwise-irrelevant event if you knew it was going to practically end your life?

(I assume you meant to reverse those numbers.) Let's make sure we agree on what event we're referring to. My assumption was that you were talking about Amanda's distressed reaction when an officer opened up a drawer containing knives in her cottage (i.e. not Raffaele's apartment -- nothing to do with Item 36). I don't understand why this event would have any life-ending import. I'm not aware of any similar story regarding Item 36, the closest thing being an expression of concern about it to her parents at one point. Very plausible innocent explanations for both include, in the first case, distress at Meredith's demise primed by the sight of knives, and in the second, the knowledge that she had handled the knife while cooking.

(I predict that if you took 100 young females and put them into a situation where they knew a friend or roommate of theirs had been killed with a knife by an intruder, and then showed them a drawer full of knives, the number of those who would react with visible emotion would be well into the double digits.)

My reasoning: suppose a chef grabs a knife in cooking position ten times and nose-picking position once. Suppose further DNA is only found in one spot. It's probably ten times more likely that the DNA is found in cooking position, and that his unhygienic habit will remain undetected (aside from the giant nasal scars).

This seems to be an argument against the proposition that Amanda used the knife for cooking more than for stabbing. But that seems unlikely to be sound, since even if it was used for stabbing at one point, it had presumably been used for cooking a fair amount before. It was a kitchen knife after all, and Amanda had spent a lot of time at Raffaele's apartment.

In any case, this kind of inference seems like it is subject to tremendous uncertainty. How much difference is there between cooking positions and stabbing positions anyway? Surely there are any number of ways to do both. Why 10:1 or 2:1, instead of 3:2 or 10:9? As a matter of fact, I'm curious in general: what sort of evidence of guilt would have a likelihood ratio below 2 for you?

But in this situation "knowing the knife isn't a murder weapon" doesn't come close to being necessary for innocence.

I disagree: not necessary, but probable, by a factor of >10 IMHO.

Can you explain this more? Why should an innocent person have knowledge of what the murder weapon was or wasn't? If she were innocent, why should she, under the circumstances she was in at the moment, with the information she had, doubt Raffaele's guilt any more strongly than she indicated in the diary excerpt (already fairly strong, IMO)?

I'm more concerned by: “This could have happened: Raffaele [killed Meredith] and then, having come back home, pressed my fingerprints — I was asleep — onto the knife", from London Times

That is almost certainly just a mistranslation of the passage I quoted above (in its original form). See here and here (#8). (Admittedly, I haven't found a "neutral" source explicitly recognizing this mistake, but it is extremely plausible a priori, not surprising at all that the media wouldn't bother to correct it, and moreover no one can produce photographic evidence of the second version, unlike the first. Even the guilters use the version I quoted.)

Could each side of the debate perhaps give a back of the envelope Bayesian calculation for P(guilty), just to summarize their current positions to bystanders who are joining in without having read the whole discussion up to this point?

Okay, so, obviously a lot of disagreement here.

Regarding the base rate of contamination, the first thing to remember is that in the case of the knife we're dealing with a Low Copy Number (LCN) sample, for which the risk of contamination is greatly increased unless extremely stringent protocols are followed -- protocols which were not followed here. So we're probably talking about an order of magnitude increase in the base rate: more like 1/5 instead of 1/50.

(Incidentally, there was never any "first independent report" prior to this one: the defense's request for an independent review during the first trial was rejected. What you are most likely referring to is the fact that Stefanoni's own bosses at the Polizia Scientifica signed off on her work, well before the trial. Based on what's in the Conti-Vecchiotti report I consider it likely that they didn't scrutinize it very carefully.)

Secondly, thanks to Conti and Vecchiotti, we have considerable Inside View information about the reliability of this sample in particular. And there's every reason to believe this result is completely bogus. Indeed, Conti and Vecchiotti come pretty close to accusing Stefanoni of outright scientific fraud. Here are some of their observations:

• The sample in question (Trace B) tested negative for blood, as did every other sample taken from the blade. (Samples from the handle were not tested for blood.) No attempt was made to scientifically determine the actual nature of the alleged biological material.

• When "quantification" (test to determine whether there was enough DNA to be analyzed) was performed, Traces B and C both yielded a result of "too low". Stefanoni reported Trace B as a positive result, and Trace C as a negative result, without any justification. There is no documentation in the lab data to support her statement in court that the Trace B sample was in the range of several hundred picograms. Stefanoni also claimed to have executed steps in the quantification procedure that are not documented.

• The "amplification" (chemical copying of the sample in order to produce a large enough amount for analysis) was performed only once, despite the fact (admitted by Stefanoni) that it should be repeated in order to be considered reliable.

• Stefanoni did not perform negative controls, which could have indicated the presence of contamination.

• The sample was analyzed in the same laboratory at the same time as numerous samples containing Meredith Kercher's DNA.

In short, everything smacks of a deliberate effort to obtain a certain result -- which was obtained easily enough given the lax procedural standards and absence of safeguards. I thus have very little hesitation in dismissing the knife DNA.

Now, regarding the "circumstantial evidence surrounding the knife" that you listed, I see two major general issues. One is that you are misinformed about a number of details, apparently as a result of uncritically accepting claims from pro-guilt sources (cf. the "first independent report" misconception above). You should be extremely skeptical of pro-guilt advocacy sites such as PMF or True Justice -- they are caught up in a death spiral of hate and constitute an absolute breeding ground of anti-epistemology the likes of which are usually only seen in political or religious conflicts. No, that doesn't mean everything they say is false, and yes, the pro-innocence community is vulnerable to the same disease, but in general I would advise against assuming any claim of theirs is true unless it is explicitly conceded by the other side. For example, this is wrong:

The knife was on top of the other knives, and matched one of the murder weapons

In actuality, the hypothesis of more than one murder weapon was invented specifically to accommodate the fact that this particular knife -- which investigators had already decided was the murder weapon based on "police intuition" followed by the bogus DNA result discussed above -- turned out not to match certain wounds on the victim, nor an imprint found in a bedsheet; a clear instance of motivated cognition and -- given that a smaller knife would have been compatible with all of the wounds as well as the imprint -- a patent violation of Occam's Razor. Another claim without any foundation is:

The same knife was bleached, and the other knives weren't

The closest thing to evidence for this is that a police officer claimed to have smelled bleach upon opening the drawer that the knife was in. (That doesn't distinguish the knife in question from the others, needless to say.)

The other major issue is that in my opinion you wildly overestimate the evidentiary strength of the observations that are true. This applies to all of items 3 through 6. For example, I see no reason why Raffaele's claim about Meredith visiting his house should even have a likelihood ratio as high as 2, let alone 100. The innocent explanation is that he had been told about the knife DNA result, and believed it. Given that he didn't have the Conti-Vecchiotti report available, why do you consider the likelihood of this to be only 1/100 that of a corresponding guilty scenario? Similarly, why is Amanda's reaction to the knife drawer even evidence of guilt at all, let alone 10 decibels' worth?

Item 5 seems to me to be another example of overconfidence:

Where Amanda's DNA was found on the handle suggests someone stabbing rather than cooking with it: meh, shift by 2.

Even if this is so (which I don't have any particular reason to accept), presumably all one has to do to to get one's DNA in this position is shift one's grip on the knife in some way -- as opposed to actually stabbing someone! Isn't the denominator of this ratio quite substantial?

Amanda wrote in her diary speculating whether Raffaele may have framed her by pressing the knife in her hand while she slept: Shift by 10, this is not something you'd write if you knew the knife isn't a murder weapon.

But in this situation "knowing the knife isn't a murder weapon" doesn't come close to being necessary for innocence. She knew that Raffaele was a suspect along with her, knew that Meredith had been killed with a knife, and (depending on the chronology) may have known that Raffaele's knife was considered the murder weapon by the police. Under those assumptions, it's an entirely natural speculation, it seems to me.

Also, here is Amanda's quote in context:

So unless Raffaele decided to get up after I fell asleep, grabbed said knife, went over to my house, used it to kill Meredith, came home, cleaned the blood off, rubbed my fingerprints all over it, put it away, then tucked himself back into bed, and then pretended really well the next couple of days, well, I just highly doubt all of that.

Okay, so, obviously a lot of disagreement here.

Regarding the base rate of contamination, the first thing to remember is that in the case of the knife we're dealing with a Low Copy Number (LCN) sample, for which the risk of contamination is greatly increased unless extremely stringent protocols are followed -- protocols which were not followed here. So we're probably talking about an order of magnitude increase in the base rate: more like 1/5 instead of 1/50.

(Incidentally, there was never any "first independent report" prior to this one: the defense's request for an independent review during the first trial was rejected. What you are most likely referring to is the fact that Stefanoni's own bosses at the Polizia Scientifica signed off on her work, well before the trial. Based on what's in the Conti-Vecchiotti report I consider it likely that they didn't scrutinize it very carefully.)

Secondly, thanks to Conti and Vecchiotti, we have considerable Inside View information about the reliability of this sample in particular. And there's every reason to believe this result is completely bogus. Indeed, Conti and Vecchiotti come pretty close to accusing Stefanoni of outright scientific fraud. Here are some of their observations:

• The sample in question (Trace B) tested negative for blood, as did every other sample taken from the blade. (Samples from the handle were not tested for blood.) No attempt was made to scientifically determine the actual nature of the alleged biological material.

• When "quantification" (test to determine whether there was enough DNA to be analyzed) was performed, Traces B and C both yielded a result of "too low". Stefanoni reported Trace B as a positive result, and Trace C as a negative result, without any justification. There is no documentation in the lab data to support her statement in court that the Trace B sample was in the range of several hundred picograms. Stefanoni also claimed to have executed steps in the quantification procedure that are not documented.

• The "amplification" (chemical copying of the sample in order to produce a large enough amount for analysis) was performed only once, despite the fact (admitted by Stefanoni) that it should be repeated in order to be considered reliable.

• Stefanoni did not perform negative controls, which could have indicated the presence of contamination.

• The sample was analyzed in the same laboratory at the same time as numerous samples containing Meredith Kercher's DNA.

In short, everything smacks of a deliberate effort to obtain a certain result -- which was obtained easily enough given the lax procedural standards and absence of safeguards. I thus have very little hesitation in dismissing the knife DNA.

Now, regarding the "circumstantial evidence surrounding the knife" that you listed, I see two major general issues. One is that you are misinformed about a number of details, apparently as a result of uncritically accepting claims from pro-guilt sources (cf. the "first independent report" misconception above). You should be extremely skeptical of pro-guilt advocacy sites such as PMF or True Justice -- they are caught up in a death spiral of hate and constitute an absolute breeding ground of anti-epistemology the likes of which are usually only seen in political or religious conflicts. No, that doesn't mean everything they say is false, and yes, the pro-innocence community is vulnerable to the same disease, but in general I would advise against assuming any claim of theirs is true unless it is explicitly conceded by the other side. For example, this is wrong:

The knife was on top of the other knives, and matched one of the murder weapons

In actuality, the hypothesis of more than one murder weapon was invented specifically to accommodate the fact that this particular knife -- which investigators had already decided was the murder weapon based on "police intuition" followed by the bogus DNA result discussed above -- turned out not to match certain wounds on the victim, nor an imprint found in a bedsheet; a clear instance of motivated cognition and -- given that a smaller knife would have been compatible with all of the wounds as well as the imprint -- a patent violation of Occam's Razor. Another claim without any foundation is:

The same knife was bleached, and the other knives weren't

The closest thing to evidence for this is that a police officer claimed to have smelled bleach upon opening the drawer that the knife was in. (That doesn't distinguish the knife in question from the others, needless to say.)

The other major issue is that in my opinion you wildly overestimate the evidentiary strength of the observations that are true. This applies to all of items 3 through 6. For example, I see no reason why Raffaele's claim about Meredith visiting his house should even have a likelihood ratio as high as 2, let alone 100. The innocent explanation is that he had been told about the knife DNA result, and believed it. Given that he didn't have the Conti-Vecchiotti report available, why do you consider the likelihood of this to be only 1/100 that of a corresponding guilty scenario? Similarly, why is Amanda's reaction to the knife drawer even evidence of guilt at all, let alone 10 decibels' worth?

Item 5 seems to me to be another example of overconfidence:

Where Amanda's DNA was found on the handle suggests someone stabbing rather than cooking with it: meh, shift by 2.

Even if this is so (which I don't have any particular reason to accept), presumably all one has to do to to get one's DNA in this position is shift one's grip on the knife in some way -- as opposed to actually stabbing someone! Isn't the denominator of this ratio quite substantial?

Amanda wrote in her diary speculating whether Raffaele may have framed her by pressing the knife in her hand while she slept: Shift by 10, this is not something you'd write if you knew the knife isn't a murder weapon.

But in this situation "knowing the knife isn't a murder weapon" doesn't come close to being necessary for innocence. She knew that Raffaele was a suspect along with her, knew that Meredith had been killed with a knife, and (depending on the chronology) may have known that Raffaele's knife was considered the murder weapon by the police. Under those assumptions, it's an entirely natural speculation, it seems to me.

Also, here is Amanda's quote in context:

So unless Raffaele decided to get up after I fell asleep, grabbed said knife, went over to my house, used it to kill Meredith, came home, cleaned the blood off, rubbed my fingerprints all over it, put it away, then tucked himself back into bed, and then pretended really well the next couple of days, well, I just highly doubt all of that.

My guess is it's probably figured by assuming there are ~500 objects being examined by the lab. However, I would point out that contaminations are unlikely to be independent of each other, and are instead probably correlated via the general level of standards in the laboratory. In other words, if there is a single contamination, there are likely to be others as well (reported or not).

So, I'll start things by picking up our conversation from the other thread. Rolf says he believes that the knife and bra clasp are among the strongest pieces of evidence against Knox and Sollecito; I responded by pointing to the recent independent review that strongly critiqued that evidence.

If [the independent experts'] main point is that the evidence doesn't meet the standard of scientific rigor, then I might not disagree with them on anything factual. Very little evidence, either way, does meet the standard of scientific rigor. Fingerprints never reach the standard of scientific rigor. DNA testing, as practiced, probably rarely if ever meets the standard of scientific rigor. Eyewitness testimony obviously can never come close to meeting the standard of scientific rigor. Heck, most science doesn't meet the standard of scientific rigor. We still need to evaluate evidence on its full merits.

Absolutely true -- but of course the reason scientific rigor exists is because errors happen when it isn't applied, and so we need to take this into account and scrupulously avoid overconfidence when evaluating such evidence. And, even more to the point here, Conti and Vecchiotti (the independent experts) don't merely say that it doesn't meet the standard of scientific rigor; they actually say specifically that it is "not reliable". This is important, because they were asked by the court to assess "the degree of reliability"; hence they could presumably have answered anywhere along a continuum -- and the answer they came back with was "pretty much zero".

So, to confirm that our initial analysis here diverges, around how much are you currently shifting based on the test results for the knife and for the bra clasp? For you, did either one shift P(guilt) by a factor of 100? 10? Not at all?

Roughly speaking, I would say that together, all the evidence against Knox and Sollecito, of which the bra clasp and knife are for me the overwhelming majority (everything else being near-negligible), shifted P(guilt) upward by about an order of magnitude -- a factor of 10, from a prior somewhere between 0.0001 and 0.001 to a posterior somewhere between 0.001 and 0.01. The Conti-Vecchiotti report cuts that down a bit, though not necessarily hugely, since their findings were pretty much what I already expected them to be.

You can guess my hypothesis for why the DNA tests came out the way they did. Do you have a specific alternative hypothesis or hypotheses you want me to consider? Is your main claim here that you believe the knife was accidentally contaminated in the laboratory, and the bra strap was accidentally contaminated in Kercher's room? Or is there a different alternative hypothesis I should consider first?

Yes, I think those would be my best explanations for those results, though there is significant uncertainty even about the attribution of the DNA in the first place. It's also possible that the knife was accidentally contaminated by investigators at Sollecito's apartment, that the clasp was contaminated in the lab, or that there was deliberate malfeasance applied to either of the items at some point; but these are less likely secondary possibilities, in my view.

I assume we're not going to quash any evidence, since we're a court of Bayes and not a court of law? That is, I'm proposing that whenever we want to exclude or diminish evidence, we should have a Bayesian reason for why the evidence doesn't really alter P(guilt). The proposal is partly because it's the correct Bayesian thing to do, and partly because trying to divine and mimic Italian criminal procedure and admissability rules would add (IMHO unnecessary) additional complexity.

Agree entirely. I would only add that a court of Bayes should always be more accurate than a court of law and never less.

So, I'll start things by picking up our conversation from the other thread. Rolf says he believes that the knife and bra clasp are among the strongest pieces of evidence against Knox and Sollecito; I responded by pointing to the recent independent review that strongly critiqued that evidence.

If [the independent experts'] main point is that the evidence doesn't meet the standard of scientific rigor, then I might not disagree with them on anything factual. Very little evidence, either way, does meet the standard of scientific rigor. Fingerprints never reach the standard of scientific rigor. DNA testing, as practiced, probably rarely if ever meets the standard of scientific rigor. Eyewitness testimony obviously can never come close to meeting the standard of scientific rigor. Heck, most science doesn't meet the standard of scientific rigor. We still need to evaluate evidence on its full merits.

Absolutely true -- but of course the reason scientific rigor exists is because errors happen when it isn't applied, and so we need to take this into account and scrupulously avoid overconfidence when evaluating such evidence. And, even more to the point here, Conti and Vecchiotti (the independent experts) don't merely say that it doesn't meet the standard of scientific rigor; they actually say specifically that it is "not reliable". This is important, because they were asked by the court to assess "the degree of reliability"; hence they could presumably have answered anywhere along a continuum -- and the answer they came back with was "pretty much zero".

So, to confirm that our initial analysis here diverges, around how much are you currently shifting based on the test results for the knife and for the bra clasp? For you, did either one shift P(guilt) by a factor of 100? 10? Not at all?

Roughly speaking, I would say that together, all the evidence against Knox and Sollecito, of which the bra clasp and knife are for me the overwhelming majority (everything else being near-negligible), shifted P(guilt) upward by about an order of magnitude -- a factor of 10, from a prior somewhere between 0.0001 and 0.001 to a posterior somewhere between 0.001 and 0.01. The Conti-Vecchiotti report cuts that down a bit, though not necessarily hugely, since their findings were pretty much what I already expected them to be.

You can guess my hypothesis for why the DNA tests came out the way they did. Do you have a specific alternative hypothesis or hypotheses you want me to consider? Is your main claim here that you believe the knife was accidentally contaminated in the laboratory, and the bra strap was accidentally contaminated in Kercher's room? Or is there a different alternative hypothesis I should consider first?

Yes, I think those would be my best explanations for those results, though there is significant uncertainty even about the attribution of the DNA in the first place. It's also possible that the knife was accidentally contaminated by investigators at Sollecito's apartment, that the clasp was contaminated in the lab, or that there was deliberate malfeasance applied to either of the items at some point; but these are less likely secondary possibilities, in my view.

I assume we're not going to quash any evidence, since we're a court of Bayes and not a court of law? That is, I'm proposing that whenever we want to exclude or diminish evidence, we should have a Bayesian reason for why the evidence doesn't really alter P(guilt). The proposal is partly because it's the correct Bayesian thing to do, and partly because trying to divine and mimic Italian criminal procedure and admissability rules would add (IMHO unnecessary) additional complexity.

Agree entirely. I would only add that a court of Bayes should always be more accurate than a court of law and never less.

That'd be great! What forum should we use, should we start a Google Group? We can do it here if you like, but I'm concerned about getting distracted by other comments; the original threads got too bulky to hold a coherent conversation.

That's a problem that would presumably exist to some extent on any public forum. I'm not too bothered by it myself; my feeling is that in such a situation one is not necessarily obliged to reply to all comments individually.

Here's my suggestion: why don't we try it here first, and see how it works? I'd be interested to see if this kind of thing can work on LW. There probably won't be as many comments now as before, since it isn't a new topic.

If it does get unwieldy, we can always move somewhere else.

For my part, I'm happy to walk through the Micheli report or one of the many lists of evidence against Knox and Sollecito on the Internet, but I would pick as my top three pieces of evidence:

1. The double-DNA knife

2. Sollecito's DNA on Kercher's bra strap

3. The mixed-DNA blood

Rather than the Micheli report (which was the first-level ruling in the Guede case), you'd presumably be better off with the Massei-Cristiani report, which was actually about the Knox-Sollecito case (and is even available in translation by the PMF people).

Interestingly, I agree with you that the knife and bra clasp are the strongest pieces of prosecution evidence (though I would have put them in the reverse order). However, they've been pretty severely punctured by Conti and Vecchiotti in their report. You can read the conclusions of that report here.

Anyway, I'll create a new Discussion thread upon confirmation that that's acceptable to you.

Do you and Rolf want to have that debate now?

I've been ready for over a year!