Does DNA testing work like that? If so, you'd just build more iterations into the test itself. If they refer to taking another sample, again, for such a critical test, why not take multiple samples? The coin analogy seems misleading as you can flip many times for free, and each flip is (usually) completely independent.<p>It'd seem a retest would only matter if there's a new test or the previous test was suspected to be improperly done. In any case, you'd need a reason to believe a different outcome is possible.<p>In this case, a quick search finds[1]. It says evidence and sample procedures were not properly followed, and there wasn't even evidence of blood on the knife.<p>I'm all for being corrected on statistics, but this doesn't seem like a case of bad math, does it?<p>1: <a href="http://www.westseattleherald.com/2011/06/29/news/update-dna-knife-jailed-amanda-knox-retested-foun" rel="nofollow">http://www.westseattleherald.com/2011/06/29/news/update-dna-...</a>
And yet the mathematics in the article seems bogus to me, too. Can anyone figure out what calculation Colmez is doing?<p>I'm not a statistician, but if you toss a <i>fair</i> coin 20 times, there is about 0.1% chance of getting 17 heads, but to figure out the probability <i>that</i> the coin is fair given this data, it seems you need Bayes' theorem, which requires a <i>prior</i> probability on the coin being fair.
Article on statistics, IDing, and trials from a former professor of mine: <a href="http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1432516" rel="nofollow">http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1432516</a> (free download).<p>Also very depressing to read his work on forensics. Long story short, everything in CSI besides DNA evidence is an unscientific sham. And even DNA evidence is dominated by lab error (1-2%). See: lst.law.asu.edu/fs09/pdfs/koehler4_3.pdf.
My understanding of the original scene is that the police first on the scene were not very good, so even if the DNA on the knife is hers, one has to think about contamination by the original (postal) police.<p><a href="http://en.wikipedia.org/wiki/Murder_of_Meredith_Kercher#Discovery_of_the_body" rel="nofollow">http://en.wikipedia.org/wiki/Murder_of_Meredith_Kercher#Disc...</a>
Math is great for winning court cases. I got pulled over 12 years ago, and went into the magistrate hearing with a drawn out model indicating how the cop was either dangerously speeding himself to pull me over or had inaccurately assessed my speed and bumped up the speed he indicated on my ticket. The lawyer took one look at my sheet full of equations and said "Well, just watch your speed next time." He let me off without any fine and dismissed the case. I would always advise high school students who say "I don't need math" to understand that the world is shifting such that people who know math are becoming very powerful. Especially here in America, where no one would ever admit to being "dumb" and will therefore do anything they can to avoid doing math and looking bad. This is a serious weakness, and can easily be exploited if you ever run into a crafty mathematician.
Here is a much better article on the application of probability to the Amanda Knox case: <a href="http://lesswrong.com/lw/1j7/the_amanda_knox_test_how_an_hour_on_the_internet/" rel="nofollow">http://lesswrong.com/lw/1j7/the_amanda_knox_test_how_an_hour...</a>
It will be fascinating to see how this discussion out in a mainly American forum. In very broad terms, the attitude that the Italians seem to have to Knox is similar to the attitude that the Americans had to Woodward, and very much vice versa.
A DNA Test doesn't work like that.<p>Probability tests do, but not things we label "probability" that aren't. "there is a 25% chance of rain tomorrow" isn't really a 25%. It is a confidence score.<p>Something people can grasp better than DNA: Let's say you have a partial thumb print in an imaginary murder. You could eliminate suspects who don't have that portion of the thumb print, but you couldn't confirm that the person or people who match did it. Only that the thumbprint is a "Pocked Loop", "Whorl", or "mixed" and so anyone with a "tent Arch" is not the killer.<p>You can be 100% confident that the print excludes the person with the "Tent Arch" and if you knew there were only 4 people in the room when the victim in our imaginary scenario died you could even go so far as that since only 2 of them have a potential match that you have a 50% confidence in the match. But Running the test 800 times will not get you to a number better than that.
I don't like that RFLP analysis is still so common. Practitioners of forensics should genotype a few hundred thousand markers and be done with it. I see that Illumina offers tools for this purpose, in fact: <a href="http://www.illumina.com/applications/forensics.ilmn?sciid=2013227IBN1" rel="nofollow">http://www.illumina.com/applications/forensics.ilmn?sciid=20...</a>