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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.
If you are arrested, statistically YOU FUCKING DID THE CRIME !! Don't do the crime if you can't do the time: DON'T DO IT !!
And for all you fucking lip readers out there:
D.O.N.'.T. D.O. I.T. !.!.
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? The convicts who are now being released weren't convicted on DNA evidence. Since DNA evidence is now obviously a sham, it shouldn't be used to free someone who was justly convicted with other evidence.
You either believe your justice system is fair or else you scrap the entire thing. Your alternative would mean that we would have to release every murderer and rapist.
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.
just use the glove and If the glove doesn't fit, you must acquit!
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.
It seems to me that the simple defense to such a charge would be to go get the person who recently sucked the defendant's dick, test that person's DNA, and compare it to the other DNA samples. Presumably, however, even a bad lawyer would have thought of this, so I must not really understand the case.
Three cheers, for sure, for an opportunity to topically discuss fellatio on Slashdot.
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
uncertainty.
Each time the answer comes up the same, even in a test with much less accuracy, you improve the chances it's the right answer.
Throw some statistical analysis at it and come up with a way to combine the tests you have into one probability, which will be higher than all the probabilities you got from the measurements.
Unless the tests disagree, and then you're talking a homework problem.
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
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.
I always heard is was 90% of statistics are made up 50% of the time.
The study of statistics is the failed attempt to study sample paths in a stochastic dynamic system. Systems that is are so complex or chaotic that each sample path manifest very rich or seemingly random behavior. Unable to figure out why, statisticians resort to the study of its average, means, and variances by combining may sample paths and study aggregated behavior. However, this usually won't solve any problem, because the real mechanics behind of an observed event is usually hidden in the causal relationship within each sample path. That's why medical science is progressing so slowly and unpredictably.
Many cases are based on something like a few hairs, or a single cigarette butt.
Have you never had some random persons hair end up on you from a store, or the bus, or work, or what have you.
How about get a cig butt stuck to the sole of your shoe.. Or have you ever seen a bum grabbing old cig butts out of a ash tray and walk off?
Lots of ways for “DNA Evidence” of yours to end up some place you never were.
Now, if you somehow manage to get semen some place you never were
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
In reality it needs to be at no more than 1/18 to be sure, 1 defendant, 12 jury, 2 lawyers(minimum if not defending yourself), 1 judge, 1 bailiff, 1 of those people who records everything said, and usually some spectators. At 1/13 there must be at least a co-conspiritor in the room!
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
I'm being purely hypothetical, but let's work out a word problem.
So let's assume that there's some DNA in question, and that dna according to all 10 other calculations (assumed accurate) has a unique match number of 1 in 100,000.
Nobody ever asked if the defendant has a twin. (Again, assuming that an identical twin would have matching DNA which I don't actually know for certain).
Let's assume for simple math that in the real world twins occur 1/100 times.
Is the statistical uniqueness now 1/1000, 1/2, 1/50,000, or 1/100,000 or some other number.
60% of the time, it works every time.
No single raindrop believes it is to blame for the flood.
... 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.
DNA evidence is a far more useful tool for defense than prosecution. Showing that a DNA sample really came from a certain person is difficult, and can never be 100% accurate. But showing it didn't come from someone is easy. If it obviously doesn't match, that's that. There's no question at all in how to interpret it.
"I'm too busy to research this and form an educated opinion, but I do have time to tell everyone my uninformed opinion."
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.
And you know that the other evidence wasn't faulty, how? Police make mistakes, witnesses lie or remember things wrong, etc etc.
You can't be certain of anything.
But neither can you retry every case infinitely because there are some remaining doubts. There will always be doubts.
The appeals court is a court of law.
It's only job is to decide as a matter of law whether a conviction should stand.
You raise your objections to matters of fact or opinion in pre-trial proceedings. You raise them again at trial. But you must get your objections on record before your case goes to appeal.
It is a lot to ask an appellate judge to believe that matters which seemed inconsequential to you then should be given any weight now.
You can prove anything with statistics.
Also 99.9% of all statistics are made up.
Tongue-in-cheek aside, the real problem with statistics is that it only deals in probabilities. As such, It can never describe something conclusive. It can only tell you something is probable. (or very, very probable) It can never identify an answer/solution/event as certain. Thus, it can never tell you the absolute truth. It can only give you the most probable occurrence.
Therefore, somewhere in the evaluation of court evidence, one must decide what probability constitutes "beyond a reasonable doubt". That's a very fuzzy line, and should be drawn by the jurors, nobody else. Additionally, if I were ever on a jury, I'd want to know exactly how those numbers were reached.
From the article:
"There are some labs that are just reporting that they think it's a match- in their opinion," says Butler. "That's a problem, because the jury says: 'Oh, it's DNA? It matches? Guilty.'"
I won't join Slashcott. OTOH, If Beta goes live, I just won't be back until it's fixed. Sorry Dice.
The right way to use DNA is this. First, you find some suspects using traditional methods. When you have a set of suspects, you then do a DNA test against DNA from the crime scene that is known not to be from the victims or witnesses. It likely only matches one of your suspects. That's very likely the criminal.
The wrong way to use DNA is this. First, you find some DNA at the crime scene that doesn't belong to anyone there who is known not to be the criminal (e.g., it doesn't belong to the victims or witnesses). Then you check that DNA against some database of DNA samples. One match is spit out. You arrest that guy and try to convict him solely on the DNA match, telling jurors the odds of a match are 1 in some-large-number.
The problem with the second approach is that they don't test the whole sequence. They just test part of it--not enough to uniquely identify a single person. There may be a dozen people who match the criminal's DNA, depending on the test they use. If only one of those people happens to have done something to get himself into a DNA database that is available to law enforcement, he's screwed if any of the others who match decide to commit a crime.
The first approach (pick the suspects first, then use DNA to single out one) does not suffer from that problem.
I need a car analogy or STFU.
Dick, Dick Cheney, is that you?
Harry
Hey, it's Slashdot. Somebody had to notice.
Contrary to what the average lay-person imagines about DNA testing, it does not involve actually matching all of the DNA present or even most of it. Only a small bit of a person's entire genome is used. And then only statistically in comparison to some presumed statistical population reference.
To put it in other terms, DNA testing is like identifying Dickens as the author of a suspect text by comparing the first word of each chapter to a reference that is the average first words of chapters of some, but not even most, of his known works which is combined with some other unknown and undetermined books written by other authors.
Once you understand this, you quickly realize the margin for error is far bigger than typically presumed.
You don't get a license to drive without saying as much. Don't drink-drive BOY or I surely will throw you to the wolves, all within the law I am sworn to uphold. I believe you would prefer your next flight's aircrew WERE REQUIRED to pass a basic sobriety test before taking the controls. Too bad they aren't, but by golly, I think you'd be all for it, and if it weren't for their union, they would. When you get rear-ended by a drunk in Denver, think back about whose rights were violated again. Then go PISS OFF and die in your burning wreck!!
For an alternative insight into some of the issues here, you could do worse than to read the report "Probability and Statistics in the Law", by Philip Dawid.
The abstract is
"The field of legal reasoning is full of logical subtleties and probabilistic pitfalls. I survey a number of these, pointing out some of the problems and ambiguities, and various attempts to deal with them. Some celebrated court cases are used for illustration".
(PDF warning) http://www.gatsby.ucl.ac.uk/aistats/fullpapers/123.pdf
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. http://www.worldpixelmile.com/
Whether it's 1 in 95,000 or 1 in 13, reasonable doubt exists.
1 in 95,000 is still 3,157 possible people just in the US.
I don't understand exactly what's going on here, but then again, I'm smart enough to know that I don't know what's going on.
I listen to both RIAA and non-RIAA stuff if I like the music, tangential business/politics nonwithstanding.
Richard Dawkins devotes a whole chapter to exactly this problem in his "Unweaving the Rainbow". It comes down to understanding the statistical probability of a chance match for the particular test method - of which there are many and some are just bad. Until lawyers can articulate this in court, juries will have problems...