DNA is Not Infallible

DNA is portrayed in movies and real life as the gold standard of evidence. DNA found at a crime scene is incontrovertible proof that the persons whose DNA matches the sample was there. But, what if the odds for certain DNA matches were really only 1 in 3?

This was the case in a recent California rape conviction of a 70-year old man for a crime committed over 30 years ago. Without an eyewitness, without a medical examiners testimony, without the other likely suspect having his DNA tested, and without the victim herself identifying John Puckett as her rapist, he was convicted based solely on flimsy evidence acquired through DNA data mining.

How could that happen? Simply, the prosecution suppressed the real odds and the jury was told the odds of a false match were considerably smaller than 1 in 3. They were told it was 1 in 1.1 million.

The LA Times summaries the semantic tap dancing the prosecution had to go through to secure a conviction:

The chance that two unrelated people will share the same 13 markers can be as remote as 1 in a quadrillion -- a number with 15 zeros. Because the match in Puckett's case involved only 5 1/2 genetic locations, the chance it was coincidental was higher but still remote: 1 in 1.1 million.

But Barlow thought this figure vastly exaggerated the strength of the evidence. It did not take into account how Puckett had been identified: through a search of a large database.

The general-population figures used by prosecutors portray the odds of matching crime-scene DNA to a single, randomly selected person.

But because database searches involve hundreds of thousands or millions of comparisons, experts say using the general-population statistic can be misleading.

Think of a lottery. If you buy a single ticket, your chances of hitting the jackpot are remote. If you buy many tickets, your odds improve with each purchase. In Barlow's view, the prosecution in effect bought hundreds of thousands of lottery tickets to find the match with Puckett. She contended that this greatly increased the odds of a match to an innocent person.

Barlow argued during pretrial hearings that the jury should be told about the recommendation of two leading panels of scientific experts, one convened by the National Research Council and the other by the FBI. Both committees settled upon a statistical remedy to adjust for the many individual comparisons made during a database search. It has been widely but not universally embraced by scientists.

In every cold hit case, the panels advised, police and prosecutors should multiply the Random Match Probability (1 in 1.1 million in Puckett's case) by the number of profiles in the database (338,000). That's the same as dividing 1.1 million by 338,000.

…

Through dueling experts, the prosecution and defense offered jurors a dizzying array of numbers to consider in weighing the DNA match.

A "likelihood ratio" presented by a prosecution expert placed the chance of a coincidental match at 1 in 1.7 million. A "combined probability of inclusion" put it at 1 in 152 billion.

The numbers all pointed to the virtual certainty that the DNA at the crime scene was Puckett's.

In an interview after the trial, Ranajit Chakraborty, the prosecution's DNA expert, told The Times that he generally favors giving jurors the database adjustment. He did not present an adjustment in this case because the judge, like most others, would not allow it.

The full article is well worth reading since it shows the danger of making convictions solely on DNA.

High-tech methods of crime scene analysis will make it important to be increasingly skeptical of the proof presented. It’s only a matter of time before DNA will be taken from every crime scene and analyzed. Around that time it will be the MO of the smart criminal classes to leave someone else’s DNA at the scene to throw the investigators off. The odds may be several million to one that DNA matches, but that shouldn’t mean guilt is certain.