'Great Pause' Among Prosecutors As DNA Proves Fallible

As the saying goes, "uh oh".

'Great Pause' Among Prosecutors As DNA Proves Fallible

That shouldn't take too many years to straighten out. I'm sure you can all picture the district attorneys insisting that the convicted people are guilty regardless of the DNA test validity. Political animals hate to be found wrong.

If I may comment on the author of the article - he is making the same uneducated remark that the prosecutors constantly make:

Quote :

...a certainty of more than a million to one. That is, you'd have to go through more than a million people to find somebody else who'd match the sample.

That is not correct; that's not what it means. These people should take at least one basic statistics course to understand what they are talking about.

That's as bad as when the DA says the data show that only one person in eight billion has that DNA, and since there are only seven billion people on the planet, it is impossible for anyone else to have that DNA. Wrong. Wrong. Wrong.

Quote :

"When they retested it, the likelihood that it could be someone else was, I think, one in 30-something, one in 40. So it was a significant probability that it could be someone else," Torres says.

Statistics are hard.

Somebody reading this would not immediately think, "1-in-40? That's still a 97.5% chance that the person is guilty."

"One in forty" means that there's a twenty-five percent chance the guilty party is a member of the jury.

I've looked several times on the internet to find the margin of error in forensic DNA testing. All I have ever found is meaningless terms like "small" and "narrow." Never could find a number. Has anyone ever run across that?

I think that the time may come when DNA is used only for exclusion. Probably a good idea.

There is probably no published figure on the accuracy of DNA testing because it varies every time.  

Researchers of course don't look at the whole strand of DNA, which is trillions of base pairs long.  They look at slices, fragments, tiny runs of a couple hundred pairs only.  Such is the nature of DNA evidence collected at crime scenes.  

Using modern high speed computers they can sequence these fragments, and compare them to known sequences in the publicly-published DNA catalogs.  They can tell if the DNA is human, or canine, or beetle by the number of congruences with known DNA profiles.

The amount of usable DNA varies considerably, as does the success in finding matches.

No two DNA searches come up with the same result.

Only if a person has a known DNA anomaly, and only if DNA is collected that just happens to contain the fragment of DNA with that anomaly, can DNA testing be used to positively identify an individual.  Such an occurrence is so rare as to be vanishing, percentage wise.

Remember, humans share 96% of our DNA with chimpanzees. Within our species, we share something like 99.86%. Finding identifying markers is very very hard.