Washington Monthly has used the notable "cold hit" case of John Puckett as a starting point for a remarkably significant, researched, extended piece on the use of DNA in court cases. An extract to show the quality:
Typically, law enforcement and prosecutors rely on FBI estimates for the rarity of a given DNA profile—a figure can be as remote as one in many trillions when investigators have all thirteen markers to work with. In Puckett’s case, where there were only five and a half markers available, the San Francisco crime lab put the figure at one in 1.1 million—still remote enough to erase any reasonable doubt of his guilt. The problem is that, according to most scientists, this statistic is only relevant when DNA material is used to link a crime directly to a suspect identified through eyewitness testimony or other evidence. In cases where a suspect is found by searching through large databases, the chances of accidentally hitting on the wrong person are orders of magnitude higher.
The reasons for this aren’t difficult to grasp: consider what happens when you take a DNA profile that has a rarity of one in a million and run it through a database that contains a million people; chances are you’ll get a coincidental match. Given this fact, the two leading scientific bodies that have studied the issue—the National Research Council and the FBI’s DNA advisory board—have recommended that law enforcement and prosecutors calculate the probability of a coincidental match differently in cold-hit cases. In particular, they recommend multiplying the FBI’s rarity statistic by the number of profiles in the database, to arrive at a figure known as the Database Match Probability. When this formula is applied to Puckett’s case (where a profile with a rarity of one in 1.1 million was run through a database of 338,000 offenders) the chances of a coincidental match climb to one in three.
Such coincidental matches are more than a theoretical possibility, as Chicago police can attest. In 2004, detectives investigating a string of robberies on the city’s North Side found some skin cells that the culprit had left behind at one crime scene, which contained six DNA markers. When they ran this profile against Illinois’s offender database, they found it matched a woman named Diane Myers. There was just one problem: when the burglaries in question were committed, Myers was already in jail, serving time on drug charges.
Indeed, the little information that has come to light about the actual rate of coincidental matches in offender databases suggests the chances of hitting on the wrong person may be even higher than the Database Match Probability suggests. In 2005… an Arizona state employee named Kathryn Troyer had run a series of tests on the state’s DNA database, which at the time included 65,000 profiles, and found multiple people with nine or more identical markers. If you believe the FBI’s rarity statistics, this was all but impossible—the chances of any two people in the general population sharing that many markers was supposed to be about one in 750 million, while the Database Match Probability for a nine-marker match in a system the size of Arizona’s is roughly one in 11,000.
It's technical stuff admittedly, but that's just what convicts people for things they may not have done. If you're interested in this subject, the article is really a "must read".
The piece is also covered by the security consultant blogging as Mass
Private I, who highlights the attempts of law enforcement to suppress
information about flaws in statistics rather than share it as they
should – which is significant, and terrible, as juries thinking the
numbers mean something very different to the truth can wind up
convicting the defendant.
This finds support in another useful piece about the Washington Monthly essay over at Rants
of a Public Defender; after pointing out the significant shortcomings of legal defence teams in relation to DNA evidence, she rightly concludes,
They don't want us poking around too much and investigating the truth of
their scientific claims. They don't want to do the hard work to get at
the right results; they just want the quick, dirty, and easy answers,
evidently without concern for whether those answers are the right ones.
Once again, when science and law enforcement come together, scientific
integrity inevitably takes the back seat…
We have got to fight against the criminal justice system's reluctance to
get at the best evidence. We have got to break the hold that DNA
evidence has over us. Otherwise, we'll never get to the truth of it.
A selection of related posts – the DNA myth, wrongful arrest due to DNA, the "return my DNA" campaign, DNA madness, less than 1% of crimes solved with DNA, removal of samples from innocent people is random, DNA / PNC, DNA testing and employability, arrests just to get us on the database, and an extended and updated piece from me on Con Home.
By Alex Deane