

Amanda Knox and bad maths in court - mitmads
http://www.bbc.co.uk/news/magazine-22310186

======
MichaelGG
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.

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.

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.

I'm all for being corrected on statistics, but this doesn't seem like a case
of bad math, does it?

1: [http://www.westseattleherald.com/2011/06/29/news/update-
dna-...](http://www.westseattleherald.com/2011/06/29/news/update-dna-knife-
jailed-amanda-knox-retested-foun)

~~~
Symmetry
More to the point, even if her blood was on the knife would that necessarily
be conclusive evidence? I know that I've left a bit of blood on various
cooking knives of people I've lived with.

~~~
DigitalTurk
No offense (maybe a giggle) but that seems highly unusual

~~~
kevingadd
Anyone that isn't a professional chef is probably going to leave a little
blood on a knife by cutting themselves with it.

There's a difference between a knife drenched in blood and a tiny bit of blood
from a cut, certainly, but as far as forensics goes that may not matter - they
may want to find any evidence of blood, like some blood caught in the space
between the handle and the blade, on the assumption that a killer may have
tried to clean blood off a weapon.

~~~
DigitalTurk
> Anyone that isn't a professional chef is probably going to leave a little
> blood on a knife by cutting themselves with it.

That's a strong claim.

I'm not a professional chef but I like to use a chef's knife, and I cut rather
fast. Yet I don't remember ever cutting myself to the point of bleeding. I
only remember scraping off nails and skin.

> like some blood caught in the space between the handle and the blade

I reckon that if it gets stuck between the handle and the blade then we're
talking about more than a little blood.

~~~
WalterBright
I've cut my fingers deeply with a kitchen knife more than once, and bled
profusely.

I rarely cook, but I work on cars a lot. Cutting my hands and fingers is part
of the routine. Oddly, such cuts have never gotten infected, even when the
dirt gets embedded below the skin. I think car grease is an effective
bacteriacide!

(I find wearing gloves while working on a car uncomfortable, though I'll do it
when cracking a rusted nut loose - when it finally gives way you'll always
bang your hand against something.)

~~~
lostlogin
That same grease make my cuts all red and inflamed. What gets me is that its
rarely jobs that you know will be hard that result in cuts. It's the small
simple ones. Switch a ram stick on this beige box, change a set of windscreen
wipers. Adjust the temp on the hot water cylinder. Do anything maintenance
related to a washing machine.

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taejo
And yet the mathematics in the article seems bogus to me, too. Can anyone
figure out what calculation Colmez is doing?

I'm not a statistician, but if you toss a _fair_ coin 20 times, there is about
0.1% chance of getting 17 heads, but to figure out the probability _that_ the
coin is fair given this data, it seems you need Bayes' theorem, which requires
a _prior_ probability on the coin being fair.

~~~
gburt
This is how frequentist statistics works. You ask the wrong question ("the
chance of the data occurring given the assumption") and use clean, rigorous,
impeccable math to get an answer. Bayesian statistics is (usually) the
opposite - you ask the correct question ("the chance of the assumption being
correct") but find that there is no way without making some big assumptions to
get to the answer.

Here's some good (possibly more "fair" than I've been above) discussion if
anyone wants to read/think about this more:

[http://stats.stackexchange.com/questions/22/bayesian-and-
fre...](http://stats.stackexchange.com/questions/22/bayesian-and-frequentist-
reasoning-in-plain-english)

[http://www.quora.com/What-is-the-difference-between-
Bayesian...](http://www.quora.com/What-is-the-difference-between-Bayesian-and-
frequentist-statisticians)

~~~
WildUtah
A concise explanation of the difference between Bayesian and Frequentist
techniques in statistics:

<http://xkcd.com/1132/>

~~~
gburt
You know, I didn't understand that comic when it was posted (despite feeling
like I have an understanding of Bayesian vs. frequentist statistics) and I
still don't. So, I looked it up and apparently I'm not the only one.

It seems to me, and the commenters on stats.stackexchange [1] that this comic
both misinterprets frequentist statistics and misrepresents Bayesian
statistics. I realize that XKCD is a nerdy comic meant to be entertaining - I
just wanted to leave this discussion here in case anyone else is confused; I
think this is an important distinction and one most people interested in
statistics should spend some time thinking about.

[1] [http://stats.stackexchange.com/questions/43339/whats-
wrong-w...](http://stats.stackexchange.com/questions/43339/whats-wrong-with-
xkcds-frequentists-vs-bayesians-comic)

Edit: I can't edit my first comment now, but gweinberg's post (sibling to the
grandparent of this) words the problem perfectly.

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rayiner
Article on statistics, IDing, and trials from a former professor of mine:
<http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1432516> (free download).

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.

~~~
gburt
If you know anything about it, how would you/he propose fixing the problem
from an institutional perspective?

Section V of that paper suggests that being more precise in language choice
(an important problem in statistics) is his main advice there, but perhaps
making the data and conclusions open in some "anonymized" form would support
this endeavour?

------
e40
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.

[http://en.wikipedia.org/wiki/Murder_of_Meredith_Kercher#Disc...](http://en.wikipedia.org/wiki/Murder_of_Meredith_Kercher#Discovery_of_the_body)

------
xrd
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.

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viveutvivas
Here is a much better article on the application of probability to the Amanda
Knox case:
[http://lesswrong.com/lw/1j7/the_amanda_knox_test_how_an_hour...](http://lesswrong.com/lw/1j7/the_amanda_knox_test_how_an_hour_on_the_internet/)

------
waterlion
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.

~~~
stephencanon
For anyone who doesn't know what "Woodward" refers to:
<http://en.wikipedia.org/wiki/Louise_Woodward_case>

~~~
mikeash
Stuff like this terrifies me:

"In the days following the verdict it emerged that the jury had been split
about the murder charge, but those who had favoured an acquittal were
persuaded to accept a conviction."

I just don't understand it. How can a person serving on a jury, with another
person's life in their hands, be "persuaded to accept a conviction" when they
don't actually believe the defendant is guilty?

~~~
Evbn
Watch 12 Angry Men backwards.

~~~
gwern
Funny you bring it up, because from a Bayesian perspective, the main character
makes some pretty specious arguments and the final choice to not convict is
probably wrong; see [http://rationallyspeaking.blogspot.com/2012/11/odds-
again-ba...](http://rationallyspeaking.blogspot.com/2012/11/odds-again-bayes-
made-usable.html) and the excerpts from
[http://scholarship.law.duke.edu/cgi/viewcontent.cgi?article=...](http://scholarship.law.duke.edu/cgi/viewcontent.cgi?article=2472&context=faculty_scholarship)
("Was He Guilty as Charged? An alternative narrative based on the
circumstantial evidence from '12 Angry Men'", Vidmar et al) in
<http://studiolo.cortediurbino.org/how-useful-is-bayesianism/>

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drakaal
A DNA Test doesn't work like that.

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.

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.

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.

~~~
hashmymustache
Exactly, that's why it helps to have multiple tests with different
sensitivities and specificities for ruling people out and in, respectively.
Whorls and a grip circumference of 14cm, say

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carbocation
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:
[http://www.illumina.com/applications/forensics.ilmn?sciid=20...](http://www.illumina.com/applications/forensics.ilmn?sciid=2013227IBN1)

