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brazil84 comments on Amanda Knox: post mortem - Less Wrong
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Let's make sure I understand your argument: You seem to be saying that the (apparent) lack of DNA evidence on the knife and bra clasp is convincing evidence that Knox and Sollecito were NOT involved in the murder.
Do I understand you correctly?
There was an abundance of physical evidence of Rudy Guede on the scene. If Knox and Sollecito had any physical involvement with the crime, they should have left biological evidence on a similar order, but in fact tests for evidence of their involvement did not return positive results except when corrupted by serious mishandling and poor testing procedures. The knife and bra clasp were major pieces of the prosecution's case because they alleged that they held genetic evidence implicating Knox and Sollecito, but in fact they did not, and even if they had carried evidence of Knox and Sollecito, it would have been a suspiciously small amount of evidence compared to what one would reasonably expect had they actually been involved.
Think of the allegation of Knox and Sollecito's involvement in the murder as a claim that there is an elephant in a room. Another party examines the room and finds...
X: Well, there does seem to be an elephantish sort of smell in the room, but it's otherwise devoid of large mammals.
Y: But you agree that it's a distinctly elephantish sort of smell?
X: Yes, it does smell much more like an elephant than anything else I can think of.
Y: In that case, the presence of an elephant makes it far more likely for you to observe this smell than the non-presence of an elephant, therefore it meets the bayesian definition of evidence, so you must revise your belief in the presence of an elephant upwards!
X: But hiding an elephant in a room is very difficult. If there were actually an elephant in this room, I would expect a lot more evidence than that. I can't think of any plausible way that you could get an elephant in here and hide every sign of its presence but its smell, only very implausible ones. Your claim was sufficient evidence to promote the hypothesis of an elephant in this room to my attention in the first place, so this is less evidence than I need to maintain my prior uncertainty. To maintain my prior uncertainty, I would need to observe evidence that gave significant reason for suspecting the presence of an elephant, such as this smell, while finding some sort of arrangement that could plausibly hide an elephant. This, on the other hand, is a small enough amount of evidence that I must revise my confidence to near certainty against.
It is reflective of a suspiciously poor case that the prosecution brought so little biological evidence against Knox and Sollecito compared to the one person we can be almost certain was actually involved. The pieces of evidence were mishandled, making them suspect to begin with. This provided an opportunity for us to make our beliefs pay rent. Given the state of the evidence against them, most of us on this site concluded that Knox and Sollecito were probably not involved in the murder, so the biological evidence brought forward by the police was probably not contingent on their presence at the crime, but due to subsequent corruption and mishandling of the evidence. Thus, we could predict that the independent experts' review of the evidence would find that when properly processed, it did not point to Knox and Sollecito's presence at the crime. If the evidence were contingent on their presence at the crime, we would not expect to observe this.
So do you concede that your high confidence in their involvement did not predict observations as well as our belief that they were probably not involved?
Well I'll consider this argument, but let's make sure we are on the same page about biological evidence. According to one web site I found, Guede's DNA was found (1) on a swab of Kercher's privates; (2) mixed with Kercher's blood on Kercher's handbag and the left cuff of her sweatshirt; and (3) on toilet paper in one of the bathrooms in the house.
Do you agree with that?
Not yet as I am skeptical of your argument that involvement by Knox or Sollecito would have most likely resulted in the same kind of biological evidence as there was implicating Guede. But I'm willing to consider your argument and I admit that I have not heard it before today.
Let's start by making sure we agree about the DNA evidence against Guede.
That reflects my understanding of the state of the DNA evidence against Guede.
Keep in mind that that does not imply that this is the sum of all DNA evidence Guede left on the scene; when you have already found this much biological evidence, there is no reason to continue searching in order to obtain a comprehensive inventory of every trace left by the suspect.
Well, I'm not an expert on DNA evidence but just based on common sense, it seems to me that if playing the primary role in a murder and sexual assault reasonably leaves 2 DNA traces in a person's blood stains a 1 trace in the person's privates, it's plausible that playing a secondary role might very well leave no DNA traces in the person's blood and none in their privates.
Of course the DNA traces in other parts of the house are far less interesting since Knox and Sollecito can be expected to have left DNA biological evidence there regardless of whether they played some role in the murder.
So I remain skeptical of your argument but I am happy to consider an authoritative source which says that even someone who plays a secondary role in a murder with a knife is very likely to leave biological traces behind. Do you have such an authority? Or are you just going by your own general knowledge and common sense?
I disagree. The authorities had every reason to continue searching for DNA evidence since they suspected others besides Guede.
The prosecution claimed that more than one person must have been involved in the murder because, they alleged, Kercher was stabbed more times than was likely by a single assailant, and the wounds were consistent with the use of more than one knife. However, the latter part has been challenged, and I am myself aware of murders known to be committed by a single person in which the victim was stabbed more times than Kercher was. The allegation was a particularly ignorant one in the first place. However, assuming that it was true that more than one person was involved in stabbing Kercher, then even if he was the only one who sexually assaulted her, then from my own knowledge of forensics I can say that it is extremely unlikely that only one person would have left biological traces on her unless everyone else was wearing extensive protection. Keep in mind that the contrast is not 3 traces versus zero traces, but three distinct samples which have considerably more than the threshold needed to be detectable by the forensic processes versus no traces above threshold level; if anyone else was involved in physically stabbing Ms. Kercher to death, as the prosecution claimed, they would have to have done so while leaving behind no traces within orders of magnitude of what Guede left.
I have studied enough of forensics and biological labwork to make more than a common sense guess in this matter, but if my say so isn't enough for you, I could email a forensics specialist, although you could do the same. I'm not aware of any website that explains this, although that doesn't mean that that there are none.
The traces of Guede's DNA were acquired by a simple procedure; find a spot likely to yield biological evidence, take a sample, test it. A swab from Kercher's genitals, for example, contained DNA from Guede. They did not test to see "let's see how many swabs we can take which register positive for Guede's DNA," because it's unnecessary.
You seem to be treating the distinct samples as if they were individual hairs, where a person could plausibly leave behind a few or none at all, but that simply isn't an accurate model.
None of that, of course, was the real reason. The real reason was that they had initially arrested Knox, Sollecito, and Lumumba on the basis of crackpot psychological theories, and had egg on their face after having to release Lumumba when he turned out to have an unimpeachable alibi; so they needed Knox and Sollecito to still be guilty in order not to look like the complete morons that they, in fact, were. The bottom line was already written, and it was motivated searching from that point on.
For what it may be worth, I don't think this is a completely accurate summary of the prosecution's argument. For example, it seems that the prosecution argued that the lack of defensive wounds suggests multiple attackers. Also, the fact that she was (allegedly) stabbed on both sides of her neck.
How much above the threshhold were each of the 3 samples which matched Guede?
Well what's your job; educational background, etc?
I'm not sure I understand your point. Do you agree with me that this sampling would have been done in batches? Do you agree that Knox and Sollecito were suspects almost from the very beginning? Do you agree that therefore the authorities would have had reason to continue collecting DNA evidence even after they found crime scene samples which clearly matched Guede?
Had she been physically restrained by anyone else, we should strongly expect that person to have left biological traces. Besides, it's easy to stab someone in both sides of the neck without switching hands. Like the claim that she was stabbed too many times for one person to account for, this sounds a great deal like reaching to rationalize a story.
I don't know, although from the fact that all sources, including the Massei-Cristiani report, agree that there was an "abundance" of genetic evidence, I can infer that the traces were well above the limit of quantification, whereas the alleged genetic material from Sollecito, when tested by the police, was amplified by a procedure which increases sensitivity by more than an order of magnitude above the base level of detection. This places a ceiling for the amount of genetic material attributed to Sollecito of about one thirtieth of what was contained in any of the samples from Guede, but that's assuming that Guede's samples were only slightly over the limit of quantification. To be described as an "abundance", there was probably many times more. And the independent experts' report determined that the DNA from Sollecito, along with three other people on the sample, was probably due to corruption of the evidence well after the crime was committed.
Amanda Knox's DNA was found in easily measurable levels on a knife that, independent report determined, had no traces of blood on it and could not have been used in the murder at all; it was improperly flagged as a piece of evidence in the first place.
I have a Bachelor's in Environmental Science, currently pursuing credentials to become a high school chemistry teacher. In the meantime, I tutor.
Yes, yes, no. The sampling was done in batches, but not in a comprehensive "let's test everything in the room at once" sweep. Knox and Sollecito were suspects from near the beginning, but it was on the basis of a scenario that was quickly proven to be wrong. Once it was proven that the person who they concluded that Knox's testimony had implicated could not possibly have been involved, they had practically no case for suspecting Knox and Sollecito at all. What they did have was a great deal of motivation to find evidence implicating Knox and Sollecito, because otherwise they would look and feel like idiots.
When they did the initial sampling and found multiple traces of evidence from Guede, they had already reached a state where it was unlikely that if someone else had been physically involved, they would have missed the evidence of them. They should have revised downwards from the prior likelihood that any randomly selected murder would be committed by multiple people. They had obtained evidence such that the hypothesis of Knox and Sollecito's involvement should have been discarded, given that under interrogation they had failed to implicate the one person who was definitely involved, the other person "implicated" by their testimony had been conclusively proven not to be there, and they had not found biological evidence of their involvement under an initial search that would probably have found it if there were any. At this point, their initial scenario was effectively disproven beyond reasonable doubt, but still they insisted on continuing to search for evidence implicating two people they now had every reason to believe were innocent.
So you agree then, that you mischaracterized the prosecution's argument?
I'm not sure I understand your point. You seemed to be arguing that the amount of DNA traces left by Guede were "orders of magnitude" above the detection threshhold, i.e. at least a hundred times higher. And that if Knox or Sollecito had been in physical contact with Kercher, they would have left at least a hundredth of the DNA left by Guede -- enough to have been unambiguously detected.
Did I misunderstand your argument?
Ok, and assuming that's true, they would have had a strong motive to do more swabs right? And if they in fact did so, then under your hypothesis, they should have found additional Guede DNA material, right?
And what classes did you take which gave you your expertise in forensics?
Not in any way that makes their case seem weaker than it actually was.
The traces left by Guede were all above the limit of quantification. The only piece of evidence on the scene that contained traces above the limit of detection for anyone other than Kercher or Guede was the bra clasp, which contained DNA from Kercher as the major contributor, and DNA above the limit of detection but below the limit of quantification for several other people, according to the independent experts' report, DNA which they claim was most likely due to mishandling and contamination. That leaves no traces above the limit of detection from any uncontaminated source for any potential perpetrator other than Guede, so if we are to posit that anyone else was involved, we must suppose that they did it while shedding orders of magnitude less biological material.
By the time they went to retrieve further evidence, Meredith Kercher had already been buried. Her body, along with the objects on which they had found traces of Guede's DNA, had already been removed. They did not re-swab her body to see if they could get more of Guede's DNA off of it, nor did they attempt to see if they could get more traces from any of the other articles on which they had already found his DNA. Had they scoured the scene for more traces of Guede, I would be surprised if they did not find any, but they weren't looking for more traces of Guede, they were looking for traces of Knox and Sollecito. Further traces of Guede found on the scene 47 days later might not even have been kept due to their lesser value as evidence since there was already plenty of opportunity for corruption and the case against him was rock solid; if they found, say, small traces of his DNA on the floor on the reexamination, they'd have no particular reason to make note of it.
Two semesters of biology labs covered forensics. My labs in atmospheric and environmental chemistry also dealt with collection and analysis of trace chemicals from a scene.
Err, that should be "very likely to leave biological traces behind in sufficient amounts that investigators are likely to find and match them."