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.
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.
I disagree. The authorities had every reason to continue searching for DNA evidence since they suspected others besides Guede.
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.
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.
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.
...but three d
Continuing my interest in tracking real-world predictions, I notice that the recent acquittal of Knox & Sollecito offers an interesting opportunity - specifically, many LessWrongers gave probabilities for guilt back in 2009 in komponisto’s 2 articles:
Both were interesting exercises, and it’s time to do a followup. Specifically, there are at least 3 new pieces of evidence to consider:
Point 2 particularly struck me (the press attributes much of the acquittal to the expert report, an acquittal I had not expected to succeed), but other people may find the other 2 points or unmentioned news more weighty.
2 Probabilities
I was curious how the consensus has changed, and so, in some spare time, I summoned all the Conscientiousness I could and compiled the following list of 54 entries based on those 2 articles’ comments (sometimes inferring specific probabilities and possibly missing probabilities given in hidden subthreads), where people listed probabilities for Knox’s guilt, Sollecito’s guilt, and Guede’s guilt:
It’s interesting how many people assign a high-probability to Knox being guilty; I had remembered LW as being a hive of Amanda fans, but either I’m succumbing to hindsight bias or people updated significantly after those articles. (For example, Eliezer says .15 is too high, but doesn’t seem otherwise especially convinced; and later one reads in Methods of Rationality that "[Hagrid] is the most blatantly innocent bystander to be convicted by the magical British legal system since Grindelwald's Confunding of Neville Chamberlain was pinned on Amanda Knox.")
EDIT: Jack graphed the probability against karma:
2.1 Outliers
If we look just at >41% (chosen to keep contacts manageable), we find 12 entries out of 54:
I have messaged each of them, asking them to comment here, describing if and how they have since updated, and any other thoughts they might have. (I have also messaged the first 12 commenters or so, chronologically, with <41% confidence in Knox’s guilt, with the same message.) The commenters:
AngryParsley / Cyan / Daniel_Burfoot / Eliezer_Yudkowsky / GreenRoot / John_Maxwell_IV / LauraABJ / Mario / Matt_Simpson / Morendil / Psychohistorian / Shalmanese / Threads / Unknowns / badger / bentarm / bgrah449 / bigjeff5 / brazil84 / dilaudid / jimmy / kodos96 / lordweiner27 / mattnewport / nerzhin / tut
I look forward to seeing their retrospectives, or indeed, anyone's retrospectives on the matter.