How Statistics Can Foul the Meaning of DNA Evidence 215
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."
Numbers don't lie. (Score:3, Insightful)
Re:Hmm... Good (Score:2, Insightful)
Your post sounds like a good reason for you to shut the hell up.
Whaa? (Score:5, Insightful)
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.
Re:Damn Lies and Statistics! (Score:4, Insightful)
JE (always wanted to use that example. May have the justification a bit!)
Re:1/13 (Score:4, Insightful)
Here’s a tip: Go fuck yourself. (Score:4, Insightful)
Why don’t you just suggest that anyone who’s arrested is “statistically” guilty and we should just skip the trial...
Re:Hmm... Good (Score:5, Insightful)
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.
Re:Lies, Damn Lies, and Statistics! (Score:5, Insightful)
You can prove anything with statistics.
No. You can prove anything with BAD statistics. Unfortunately, most statistics are bad.
-Scientist Statistician (enough to know that I don't know statistics)
Everyone knows 9/5 of statistics are made up (Score:3, Insightful)
Re:Damn Lies and Statistics! (Score:1, Insightful)
I'm a phd student in physics, and I am very grateful that my advisor insists on doing statistics from first principles, i.e. understanding everything from the principle of maximum likelihood. There are lots of subtle statistical errors you can make if you don't completely understand what you're doing.
Re:It's fine for saying "it's somebody else". (Score:5, Insightful)
Simply, DNA evidence is by nature exclusionary. The scientifically correct result of a DNA test is excluded or not-excluded.
Re:Here's a tip: DON'T GET ARRESTED !! (Score:1, Insightful)
Re:Whaa? (Score:4, Insightful)
Since when you're a prosecutor that doesn't really care if the defendant is guilty or not as long as he gets locked up and you get credit.
The way the "justice system" currently works, if crime magically stopped right now, prosecutions and convictions would continue unabated.
Re:People don't understand statistics (Score:5, Insightful)
DNA testing probabilities go something like this ....
we found say 5 markers that match the defendant and the sample. (I picked a small number to make the example shorter)
each of those has the following probabilities of occuring in a random person :
1) 1 in 1000
2) 1 in 10
3) 1 in 10000
4) 1 in 7
5) 1 in 100
so we multiply all those together and get a probability of mismatch of : 1 in 7,000,000,000
I even told a guy at the state crimelab that was stupid - not that he cared.
Re:It's fine for saying "it's somebody else". (Score:3, Insightful)
"Of course, I'm sure the fact that we end up paying people who get imprisoned because the government fucks up had no bearing at all on the government's desire to convince everyone they didn't fuck up."
Actually, probably not. The DA may just not want to admit they made a mistake. It's uncomfortable to process those facts so the DA doesn't. They probably even believe that the person is guilty. Cognitive dissonance and the like is pretty powerful.
Innocence not guilt (Score:3, Insightful)
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.
Re:People don't understand statistics (Score:5, Insightful)
If the only thing that pointed you at him was that search of the database then it tells you almost nothing about how likely he is to be guilty on it's own.
If you find a suspect be searching through a database of a million people with a test that has a 1 in a million chance of making a false positive and no other evidence exists then the chances of that match should not be used in any way to establish guilt in court.
But then lawyers don't care about using stats correctly.
If however you find someone, they have a knife with the victims blood on it and they have a motive and you compare their DNA to the DNA found at the scene then that same test with a 1 in a million chance of a false positive is a perfectly valid piece of data to submit in court.
Re:It's fine for saying "it's somebody else". (Score:3, Insightful)
DAs are rewarded for getting guilty verdicts and sending people to prison, not for finding the guilty party and punishing them. There's a very subtle difference there, and it means that a DA with so-so evidence against a defendant who's easily portrayed as scum (with a PD for a lawyer) versus rock-solid evidence against an upstanding citizen (who can afford their own attorney) will prosecute the former over the latter. It's an easy win, who cares if the guy is really guilty?
MADD mothers do it all the time (Score:4, Insightful)
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.
Re:People don't understand statistics (Score:4, Insightful)
Re:Here's a tip: DON'T GET ARRESTED !! (Score:2, Insightful)
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.
Re:It's not over until the fat lady sings. (Score:3, Insightful)
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.
Re:Here's a tip: DON'T GET ARRESTED !! (Score:1, Insightful)
Why is this rant rated insightful?
It's just an opinion. No less no more, and it gives no facts and / or examples.
Just feeding the trolls
H