It makes me wonder if in the future there might be false convictions based on DNA evidence, and not from chimeras or inaccurate testing. DNA found at a crime scene has basically come to mean that person is guilty in the public mind (thanks CSI). Even though it's unlikely, it's possible to get some DNA on someone you're near in public who then gets murdered. So you've got DNA on the person, your cell phone is in the area, and you don't have a particular alibi. In most cases that probably means that person is guilty, but I can imagine a number of cases where the person could just be in the wrong place at the wrong time.
From the article it sounds like from where the DNA was located and other corraborating evidence that this guy is guilty. But I'm glad the article ended by saying some of the geneticists weren't condemning the accused based on the DNA evidence alone. I hope that type of thinking expands into the wider public.
Sometimes I comb my hair, or bite my fingernails. What if part of it fell on the floor and some random person steps on it. Stuck on the bottom of their shoe. A bunch of coincidental stuff comes up and my DNA is tested against a fingernail/hair under victim's shoe... match. I'm doomed.
Indeed, I fear the mentality that DNA-match = Guilty.
How many people have been sent to jail/put to death based on false DNA results or even accurate but not via the crime they're accused of.
> Sometimes I comb my hair, or bite my fingernails. What if part of it fell on the floor and some random person steps on it. Stuck on the bottom of their shoe. A bunch of coincidental stuff comes up and my DNA is tested against a fingernail/hair under victim's shoe... match. I'm doomed.
This is a plot point in the film Gattaca. I'd recommend watching it if you haven't already:
From having sat on a criminal jury in the U.S., the opposite seems to be true: jurors wonder why the police didn't DNA test a small stolen object to "prove" it was held by the accused. There was no reason to run DNA (the object was found in the accused's possession, etc.) and it's not even clear that there would be any DNA to run (fingerprints would have been more likely), but it seems like jurors have come to expect CSI level stuff in every case. Instead, most cases seem to still be done on old fashioned eyewitness testimony and circumstance (the stolen goods were in your apartment, you were also around the place where they were taken...).
The problem with DNA evidence is that prosecutors like to jump to the end, the idea that a DNA match means that someone with DNA identical to that found at a crime scene has been found. In reality there is a long string of assumptions that must hold true in order for that supposition to also be true. The chain is only as strong as its weakest link.
>It makes me wonder if in the future there might be false convictions based on DNA evidence, and not from chimeras or inaccurate testing. DNA found at a crime scene has basically come to mean that person is guilty in the public mind (thanks CSI). Even though it's unlikely, it's possible to get some DNA on someone you're near in public who then gets murdered
Unlikely? It's very likely. We interact with tons of people each day. Also consider cases were the DNA is from a person close to the victim, but who isn't the murderer. Perhaps a friend came by the house and then left, and another came in and did the murder. Or a person sleeps with someone (perhaps an one-night-stand) and then is murdered, say by a jealous partner. The DNA could be from an interaction previous to the murder.
And of course, if the police is "sure" they have the suspect (like they often are, especially is he's say black or poor or something like that), they would also consider it "OK" to plant some DNA obtained for him to be "discovered" in the murder scene. Police has been found (in court) pulling such shit with falsified evidence) for decades.
I'm more concerned the Italian police were allowed to collect the DNA over 100 000 people in order to find the potential criminal.
People don't seem to realize how valuable and many times private DNA information really is otherwise a large number of them would have resisted this dragnet.
Am I the only one thinking this kind of large scale collection of DNA information should no be allowed without some very powerful circumstantial evidence.
Not just you happen to live in the same city a person was murdered in.
There was a series of murders in the north of England maybe 25 years ago that went unsolved. When DNA testing became available, the police essentially browbeat every male within plausible distance to be tested. They did solve the case. I'd have thought such questions would have been raised then, and perhaps they were.
Particulary interesting to me is the fact that the initial mass screening did not find the perpetrator, because he had paid somebody to give a sample for him. But he was eventually caught because the stand-in couldn't (or didn't want to) keep a secret.
Wikipedia mentions no controversy, although of course that doesn't mean there wasn't one.
I wouldn't mind submitting a DNA sample if it helps the police find a murderer. Maybe it was some distant cousin of mine, and my DNA will help them narrow their search.
One of my former professors has done a lot of work on the issue of how DNA evidence is presented to juries. Basically, when you quote the Random Match Probability, jurors (and apparently journalists like the author of the article) basically forget about everything else: http://bioforensics.com/conference/RMP/RMP%20-%20irrelevant.....
There's two different logical errors involved. First, jurors conflate proving the assertion "this is the defendant's blood" with "the defendant killed the victim." Second, the "one in a billion" theoretical random match odds turn into more like "one in several thousand" when accounting for laboratory error. In fact, laboratory error totally dominates the error rate of the overall process.
“The important fact is that this man’s DNA is the same as
the DNA found on Yara,” he said, noting that the odds of
anyone else sharing the same genetic profile was “one
person out of two billion of billions of billions” or
practically nonexistent.
This fundamental misunderstanding of statistics really doesn't increase confidence in this investigation. When you go on a fishing expedition across 22,000 suspects, the odds of getting a false positive from one cause or another is nearly one.
Is this true? It feels wrong - that a false positive DNA test is not like picking the right number on a roulette table. Considering the number of bits of information that have to match to be a positive hit in DNA testing, even 22k tests shouldn't hit a false positive.
Statistics being what they are, what is the science behind false positives in DNA testing across a fairly large sample set?
I'm a bit rusty on my combinatorics, but borrowing from wikipedia[1] "if n(p;d) denotes the number of random integers drawn from [1,d] to obtain a probability p that at least two numbers are the same, then:"(...)
We have stated above that n=2, p=1/2(10^(39))=1/(2e27), so we have:
p ~ 1 - ( (d-1)/d )^(n(n-1)/2))
= 1 - ( (d-1)/d )^(2(2-1)/2)) # -> 2/2 -> 1
= 1 - ( (d-1)/d )
p-1=-(d-1)/d
(p-1)d=-d+1
pd-d=1-d
pd=1
d=1/p
d~2e27 #Yeah, I guess this should have been obvious, in
retrospect
For n=20k=2e4:
p(n=2e4,d=2e27) ~ 1-((d-1)/d)^(n(n-1)/2)
Which is so small that in Python I had to use the Decimal module to
compute p~1e-19 or so.
However, this assumes an even distribution across the possibilities --
and we've approximated the number of combinations based on the statement
of collision probability -- not based on actual number of base pairs, or
based on some model on how these are likely to be distributed... (eg: if
we test for sex, but only sample males -- does that mean we loose a
certain number of possibilities? How about caucasian vs other races? It
depends on what markers we are testing. Further we know that some
markers are likely to match, because there's bound to be some common
ansecstors among some members in the sample etc.)
Note that the math above might be entirely bogus -- I'm off to bed, so
I'd assume I've made some silly mistakes...
[edit: On actually reading the article, it could appear that the quote on "two billion billions of billions" are indeed meant to indicate he likelihood of someone sharing the same DNA, rather than the likelihood of two samples testing as similar.]
Keep in mind that doing 20k tests, there are likely to be mistakes made. In the case of a false positive (or a disputed positive), that shouldn't be too much of a problem -- just run the test again, perhaps using multiple labs to check the results.
For a false negative match -- you've just missed/"cleared" the probable perpetrator. Such an error is unlikely to be caught (I'd imagine) -- on the other hand if the error rate is low (say 0.5%?) -- the chance that an error is made when testing that (presumably) one swab that is from the suspect, is pretty low, even with 20k tests.
In addition, I suspect that once they had a match, they could now try to corroborate with other pieces of evidence.
I suspect that you could get a DNA match, by itself, thrown out. However, once you use the DNA to narrow down to "these three" you can probably build up the rest of the evidence you need for a solid case.
The suspect cell phone was attached to the tower where the girl was murdered in the same hours of the murdering, even if the suspect does not reside in the exact same area.
That's not extra evidence, because they were only testing the DNA of people whose cell phones were in the area. Thus the probability that the suspect's cell phone was in the area is always going to be exactly 1.
Actually this is not what happened, the suspect was tested because they found a partial match in the population, reached the dead father, reconstructed with the help of people the possible mother, and then identified the son and tested it. Eventually since the phone was among the ones in the area, they would have reached him, but it was a totally different line of investigation that allowed to reach this specific person, so the fact the cell phone was attached to the tower was an acknowledge.
The 22k tests where like screening tests, but for this suspect they were repeated and confirmed many times and using different traces from different parts of the victim's body. Thus, statistical errors in a single test repeated 22k do not count here.
You discount 'this is our man, I can feel it, make those inconclusive tests conclusive so the bad man doesnt get away with rape and murder'. This happens very often.
The need for prosecutors to be literate in technical fields such as statistics, probability, and perhaps basic forensics is now necessary. Over-zealous prosecutors are some of the most dangerous individuals that seem to go unchecked.
Criminals are actually fairly rare. Once everyone's genetic data is captured it's actually more likely that any given individual will be falsely accused of a crime by lazy cops and prosecutors. "You coughed on a window adjacent to the crime scene, some time in the week before the crime! Guilty!"
I absolutely agree, and I find this sort of total surveillance mentality completely abhorrent. My comment was mostly meant to illustrate the sort of reasoning that goes with people seriously suggesting that collecting newborn DNA is a good idea.
Blood test of all newborn are saved in Sweden since 1975 for research purposes. The police accessed this data to solve the murder of Anna Lindh (Vice Prime Minister) in 2003.
There is a similar case in France (albeit at a much smaller scale), where DNA tests were run on all male pupils and staff from a school in an effort to find the perpetrator of a rape. So far they haven't found a suspect.
From the article it sounds like from where the DNA was located and other corraborating evidence that this guy is guilty. But I'm glad the article ended by saying some of the geneticists weren't condemning the accused based on the DNA evidence alone. I hope that type of thinking expands into the wider public.