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How Statistics Can Foul the Meaning of DNA Evidence
azoblue writes with a piece in New Scientist that might make you rethink the concept of "statistical certainty." As the article puts it, "even when analysts agree that someone could be a match for a piece of DNA evidence, the statistical weight assigned to that match can vary enormously, even by orders of magnitude." Azoblue writes: "For instance, in one man's trial the DNA evidence statistic ranged from 1/95,000 to 1/13, depending on the different weighing methods used by the defense and the prosecution."
Isn't it the case that we are more often in the way of our own discovery and explana-tative power
You can prove anything with statistics.
Also 99.9% of all statistics are made up.
"You want to know how to help your kids? Leave them the fuck alone." -George Carlin
"Members of the jury, there's only a 1 in 13 chance that the defendant is actually the killer based on the DNA evidence. If the defendant were sitting in the jury with you, then there's an equal chance that it was any one of you. And since we can eliminate all 12 of you, that leaves only the defendant left over. So you must find the defendant guilty of all charges since he's the only one left out of 13 people. The prosecution rests."
Want to improve your Karma? Instead of "Post Anonymously", try the "Post Humously" option.
But liars love to use numbers!
Your post sounds like a good reason for you to shut the hell up.
Since when in the hell do you count common matches as proof that it comes from one person? Some of these labs are doing something very wrong, and I hate to think of both the false positives, and negatives, that came from their "expert" opinions.
Absolute power corrupts absolutely. indymedia
And add to the fact if you have served on a jury before, many times this information is highly technical and is very easily miss-represented by the lawyers to jury members from all walks of life.
Why don’t you just suggest that anyone who’s arrested is “statistically” guilty and we should just skip the trial...
Alexander Peter Kristopeit bought his basement from his mommy for one dollar.
it shouldn't be used to free someone who was justly convicted with other evidence.
And you know that the other evidence wasn't faulty, how? Police make mistakes, witnesses lie or remember things wrong, etc etc.
You either believe your justice system is fair or else you scrap the entire thing.
Or you ditch that false dichotomy and realize that within every system mistakes will be made. There is nothing in fixing past errors that means you throw out the whole system.
Your alternative would mean that we would have to release every murderer and rapist.
No, actually it wouldn't.
Genetics means "out of your control" and touches on some raw nerve issues, so there's a lot of throwing around of "statistical" information and unrealistic mental models.
For example of statistical confusion:
New research shows that at least 10 percent of genes in the human population can vary in the number of copies of DNA sequences they contain--a finding that alters current thinking that the DNA of any two humans is 99.9 percent similar in content and identity.
http://www.hhmi.org/news/scherer20061123.html
And broken mental models:
http://en.wikipedia.org/wiki/Lewontin's_Fallacy
Until our knowledge improves, you're going to see more "politicization" of DNA-related science.
Futurist Traditionalism
I speak from personal experience. I use them al the time and still don't really understand them. Not how they apply in criminal investigations anyway.
Let's say you have evidence that matches 1 in a thousand people. You search through your database of all 1000 suspects and you get a single match. Did he do it? Logically you'd expect this to mean you can be 99.9% sure. You then search through the database of a million random people. You get 1000 matches. Does this mean there's only a 0.1% chance that your original suspect was guilty? Well, maybe there's some other compelling evidence that makes it most likely that one of those 1000 people were the culprits. But you have 10000 outliers. They're each a tenth as likely to have committed the crime. You get 10 matches. So, once again we're at the 50% probability of guilt, or something in that ballpark.
I'm sure this is a somewhat different example than that given in the article but that's not the point. The point is that is there a 99.9% probability, a 0.1% probability, a 50% probability or some other probability of guilt? Or am I just trying to confuse you by throwing numbers at you?
Ladies and gentlemen of this supposed jury...
This... is Chewbacca.
Ice Cream has no bones.
This should only suprise people who think court cases are about facts and justice. It is well known that facts just get in the way of what's true and real.
This sounds like a good reason to stop releasing all of those convicted murderers and rapists who were freed on DNA evidence.
Not at all.
There is no problem determining that the DNA is from somebody else than the accused. All it takes is a single marker that's different. That's easy.
The problem is going from some bunch of markers that match to saying "This IS the bum! (Well, except for a one-in-[some number] chance it really isn't.) That requires a lot of information about prevalence of genetic markers, whether there is a correlation between their distribution. That information isn't well researched and the different estimates are based on different wild guesses by different experts. Further, the whole independent-probability thing gets knocked into a cocked hat with FAR lower numbers if the police found the accused by searching a DNA database for matches. And what if he had an evil identical twin? Or somebody with access to PCR gene-amplification materials, a DNA sample, and an atomizer decided to frame him?
IMHO DNA evidence is decisive for the defense. But pending a lot more research it's still voodoo for the prosecution.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
I would imagine the lip readers you're addressing are too interested in the sex to pay attention to your letters.
"When you have eliminated the unacceptable, whatever is left, however improbable, must be the truthiness" - Holmes
When expert A says he's certain of a match to "1 in a billion" he's really saying he's certain to 0.999999999. When expert A says he's certain to 1 in a million that's certain to 0.999999.
Compare this to the "not so far apart" difference between expert A saying "he's 1 in 10" and expert B saying "he's 1 in 5." The difference between 0.9 and 0.8 certainty is a lot greater than the difference in certainty in the first example.
By the way, if I'm on a jury, I'm interested in "who else could've done it" not raw numbers. If two people leave the crime scene and blood is a "certain to 1 in 5" match to the defendant, that is, there's a 20% chance of a mistake, and the only other person who was at the crime scene has been ruled out, the only way I'll acquit is if the defendant either makes a very very strong case he didn't do it or provides some explanation for the evidence that doesn't require either of the initial suspects to be guilty.
In more practical terms, if you can raise the odds of certainty high enough that it's implausible that two people within 100 miles of the crime scene at the time of the crime are a match, and you make a very strong claim that the DNA sample is a result of the criminal being there at the time of the crime, the defense is going to have to work very hard to get me to acquit.
Knowledge is how to play a game, intelligence is how to win, wisdom is knowing what game to play.
In this case, however, there were many people present at the discovery of the object from which the DNA was taken for analysis. As it happens, several of these people were relatives (brother, mother) of the person the prosecution were trying to persuade us was the person that possessed (in legal terms) the object.
The question that I kept hoping the defense attorney would ask was "what are the probabilities of an erroneous match if the people are relatives, not just two random people off the street"? Unfortunately, he didn't.
As it happened, there were so many other peculiarities in this case as well as some pretty bizarre testimony from prosecution witnesses that we voted to acquit without making much of the DNA evidence.
udin
DNA, like a fingerprint, should not be enough to convict. Many articles have been written on the faulty statistics that are used by prosecutors to posit faulty odds like 1 in a million, when in fact the odds are more like there are many possible people who could have done this, and we have randomly chosen one. The job is then to prove that this is not just a random choice from a database, but, based on other evidence, this is person who actually committed the crime.
This is going to become more of an issue as we get more DNA in databases and solve crimes by matching DNA to the database. In this case, the match will be a random choice between several people, and it will be a mistake to convict based primarily on DNA evidence.
"She's a scientist and a lesbian. She's not going to let it slide." Orphan Black
... in a case I was on the jury for. (Sorry for the bait-and-switch title, couldn't resist.)
This was a case of armed home invasion. The victim was a big bruiser of a man, a multiple convicted drug addict. The defendant was a scrawny young Cape Verdean guy. (Cape Verdean drug gangs are common in the area: this is important later.) The victim testified that, after buying drugs from the defendant, he got a series of enraged voicemails demanding the return of the defendant's cell phone. A few hours later, the defendant allegedly shows up at the victim's house with a gun and barges in yelling. A struggle ensued, a shot was fired into the floor, and the guy with the gun fled.
Evidence against the defendant included eyewitness testimony from the defendant, matching ammunition found at the defendant's house, and crucially a do-rag found at the scene of the scuffle. DNA tests matched the do-rag to a mixture of at least 3 people, including the defendant. The DNA mixing was probably due to really awful police work: a paper bag borrowed from the defendant's cupboard is not a proper evidence collection container.
As in TFA, mixed DNA dramatically affected the "probability of exclusion" statistics: the state's expert testified there was a 1 in 50 chance that a random man on the street would match the DNA on the do-rag. The odds that a random *black* man on the street would match were much higher, like 1 in 20; the defense pointed out that the odds that a random *Cape Verdean* would match would be much higher.
We've grown used DNA evidence saying things like, "not one other person on the planet could match this DNA", but in this case, the odds were good that the DNA evidence would match at least one other person sitting in the *courtroom*. The defense also took the unusual tactic of introducing the defendant's sister, who testified that her *other* brother looks very much like the defendant, and she said it was *his* voice on the enraged voicemails. What are the odds that the DNA matches the *brother* instead? Damned good.
Between the fact that the eye witness seemed shifty and unreliable and was probably on crack at the time of the incident, and the fact that all the physical evidence could just as well implicate the brother as the defendant, we couldn't rule out the possibility that the cops got the wrong guy, so we found him not guilty. If I had to take a bet, I'd say he did it, but I wouldn't bet his life on it.
Anyway. Moral of the story is: on cop shows and in the public awareness, DNA evidence is rock solid and incontrovertible. But in the real world, the statistics of DNA mixtures make things a whole lot less cut-and-dried.
That's being done routinely all over the world today.
People who drink are statistically more likely to commit traffic accidents, so they are convicted without the need to actually do any harm to anyone.
Temperature effectsm, the same properties of the nucleotides (even if from distinct DNA), the same crime scene contamination effects, the same statistical laws (if it turns out not to be as unique as we think, all tests necessarily fail) and at least some of the chemical reactions are in common.
In one documented case, a surprise common factor was a quality control person who didn't know the non-sterile swabs still couldn't be touched before packaging. (The CSI:NY episode borrowed that from real life).
Wall street fits in because they too believed that a series of high risk propositions (this test is accurate or this sub-prime loan won't default) could be somehow bundled together to make a low risk proposition (AAA bonds in the Wall street case, beyond reasonable doubt in the DNA case). In both cases, the unexamined dependencies are the downfall.
That's why medical science is progressing so slowly and unpredictably.
On average anyway.
that would be true if cops took their jobs seriously in objective manners.. they statistically do not. the kind of person that gravitates to law enforcement is one with deep seated insecurities and thus the desire to make others conform to his expectations. it's no wonder that fallacies like appeal to authority and appeal to popularity are among cops' favorite justifications.
But neither can you retry every case infinitely because there are some remaining doubts. There will always be doubts.
You can and you should retry cases if there are doubts; you should acquit immediately if there are any reasonable doubts.
The rule of law in this country is founded on the idea that people are innocent until proven guilty, beyond a reasonable doubt. Not "pretty sure," not "it's too much trouble to give you a fair trial, so we'll just convict you anyway." Beyond a reasonable doubt. That standard of proof will inevitably mean that people who actually committed crimes will be let free, and some will indeed go on to commit more crimes, and that is unfortunate. This country, however, is supposed to be based on the love of freedom, and the notion that everyone deserves not to be railroaded by a kangaroo court bent on throwing as many people as they can into a life of permanent second-class citizenship (convicted criminals have few rights in this country, and remain persecuted even long after they've "paid their debt to society.") You can't have that if you are willing to sacrifice freedom for temporary, largely illusory, safety.