In the latest edition of California Lawyer, Ed Humes has an article on that DNA controversy that the L.A. Times stirred up a few months back.
It’s a continuation of the L.A. Times distortions on this topic. Although I think Humes understands the issues better, I don’t think he makes them any clearer for the casual reader.
The article revolves around the case of John Puckett, a San Francisco man who was charged with murder due to a cold hit from a DNA database. This is the same man about whom the L.A. Times had written a misleading article, which expert David Kaye said had “mischaracterized” a key probability.
Humes is a clever writer who has a way with words. I have read and admired nearly all of his books — but I also recognize that he a) is an advocate for criminal defendants, and b) knows how to phrase damaging facts to downplay them. For example, when he tells us that criminal defendant Puckett has served prison terms for multiple rapes, Humes introduces the damaging facts with this sentence: “Still, Puckett was no Boy Scout.”
During Puckett’s San Francisco trial last January, the prosecution’s expert estimated that the chances of a coincidental match between the defendant’s DNA and the biological evidence found at the crime scene were 1 in 1.1 million. This, no doubt, gave the jurors a compelling reason to convict Puckett of the killing and send him to prison for at least seven years—if not the rest of his life.
What the jurors didn’t know, though, and what the judge didn’t think they needed to know, is that there’s another way to run the numbers. And according to that math, the odds of a coincidental match in Puckett’s case are a whopping 1 in 3.
This phrasing seems to imply that the same coincidental match could be 1 in a million, or 1 in 3. This is the same error that the L.A. Times made in its article on Puckett’s case. He’s talking about completely different concepts.
The thing is, Humes understands this stuff better than the L.A. Times‘s Maura Dolan and Jason Felch did. And in a very clever (and slippery) move, Humes goes on to explain that both numbers are correct, and then uses the example of the “birthday problem” to illustrate why the two sets of odds are so different. This is misleading in two ways.
1) First, Humes never tells readers, even once, that the “1 in 3” statistic makes sense only if jurors are told that the hit came from a database — because the only way to explain the meaning of the 1 in 3 number is by reference to a database. But generally, jurors are not told that the hit came from a database — to protect the defendant from being damaged by a jury’s inference that his DNA was in a database because of his criminal record.
If the jury isn’t told about the database, how can you make sense of the 1 in 3 number?
(Interested readers can read an analogy here.)
2) Second, Humes’s use of the birthday problem greatly magnifies the readers’ perception of the likelihood of a match, because the classic birthday problem doesn’t ask the likelihood that you will match someone in the room, but the likelihood that someone in the room will match someone else.
The chance that you will match someone in a room of 23 people is much smaller than the chance that there is a pair in the room that shares a birthday.
The bottom line is that using the birthday problem greatly exaggerates the effect of the database.
In my view, the birthday problem concept has little relevance to a trial like Puckett’s, where there is one man on trial who matches. The only relevance is whether the typical statistics given are undermined by recent data runs conducted by defense attorneys searching an Arizona database. The answer — misleading stories in the L.A. Times notwithstanding — is no.
David Kaye has promised a post on the Humes article on his blog. I will link it when it’s available.
UPDATE: Interestingly, Professor Kaye’s post and mine went up at almost the same time (a coincidence, I assure you). His post is here. I am on my way out the door and haven’t read it yet, but I will comment on it more later. I will note, however, that the first line confirms my own view: “This month’s issue of the California Lawyer perpetuates the confusion in the media about DNA database trawls.”
Indeed. Of course, the confusion all began thanks to the L.A. Times.