Patterico's Pontifications

5/4/2008

Follow-Up on DNA and Cold Hits

Filed under: Crime,Dog Trainer — Patterico @ 6:43 pm

This is a follow-up to this morning’s post on DNA, cold hits, and statistics.

Prof. David Kaye, whom I cited in this morning’s post, has responded to my e-mail and given me permission to quote him.

Thanks for your inquiry. This is a surprisingly subtle statistical question. I have devoted two chapters to it in a forthcoming book to be published by Harvard University Press and have circulated a manuscript on the California cases and the general issue to law reviews. I served on the 1996 NRC committee that recommended adjustment, but I now find it difficult to defend that recommendation. Basically, there are two distinct questions:

Question 1. What is the chance that a database composed entirely of innocent people (with respect to crime being investigated) will show a match? For databases that are small relative to the number of people who could have committed the crime, the NRC adjustment makes sense. The British experience mentioned in article shows that this chance is much larger than the random match probability. But why is this “innocent database” probability important when considering what the evidence of a match to a named individual proves?

Question 2. How much does fact that the defendant identified by a trawl through the database matches – and no one else in the database does — change the odds that he is the source of the DNA at the crime-scene? This is the question that is of interest to a jury trying to weigh the evidence. It is the one that Peter Donnelly and other statisticians have addressed. The answer is that the single match in the database raises the odds even more (but only slightly more) than does testing a single person at random and finding that he matches. As you point out, in the limit of a database that includes every person on earth, the evidence of a single match in the database becomes conclusive. How can the value of the evidence possibly decline as small databases get slightly bigger, then somehow switch direction and get immensely stronger as they get bigger still?

The discussion of the issue in the news and the courts is oversimplified and misleading (but entertaining). The manuscript of the law review article is attached. Feel free to quote from it as “submitted for publication.”

With best wishes,

DHK

I quoted from Prof. Kaye’s article in comments to the previous post. Let me quote one of those passages here, because I think it sheds light on the issue:

We can approach this question in two steps. First, we consider what the import of the DNA evidence would be if it consisted only of the one match between the defendant’s DNA and the crime-scene sample (because he was the only person tested). Then, we compare the impact of the match when the data from the trawl are added to give the full picture. . . . In the database trawl case . . . [i]f anything, the omitted evidence makes it more probable that the defendant is the source. On reflection, this result is entirely natural. When there is a trawl, the DNA evidence is more complete. It includes not only the fact that the defendant matches, but also the fact that other people were tested and did not match. The more people who are excluded, the more probable it is that any one of the remaining individuals — including the defendant — is the source. Compared to testing only the defendant, trawling therefore increases the probability that the defendant is the source. A database search is more probative than a single-suspect search.

Interesting.

I should note that Prof. Kaye’s exposition of the two relevant questions is similar to, but somewhat different from the questions that I posed in my original post. In an attempt to illustrate what I believed to be the questions addressed by the two competing camps, I posited two similar questions: 1. What are the chances that a search of this database will turn up a match with the DNA profile? and 2. What are the chances that any one person whose DNA matches a DNA profile is indeed the person who left the DNA from which the profile is taken?

Prof. Kaye’s questions state the issue in a more refined and, I believe, more accurate manner. As to my questions, he says in a follow-up e-mail:

The answer to your question #1 depends on the chance that the database contains the source (and, if “a match” means exactly one match, no one else with the matching type). That is not the question that the statisticians who favor an adjustment to the random-match probability are considering. The proposed statistical adjustment relates to the following modified version of your #1:

1′. What is the chance that a search of a database will turn up exactly one match when the source of the crime-scene DNA is someone who is unrelated to everyone in the database?

Likewise, the statisticians who argue that the database search is better evidence than the single-suspect search (and they are the majority of those writing on the topic) focus on a variation of your second question:

2.’ What is the chance that the named individual whose DNA matches is the source?

I confess that I did not read the coin example you provided too closely. I suspect that it is correct. I have an example along these lines in my article (inspired by an example in the Donnelly-Friedman article).

Thus, I think the thrust of your remarks are on target, but some of the details of your analysis could be refined.

I thank Prof. Kaye for his correspondence. And yes, the coin example was rather long.

Incidentally, I have an e-mail in to Prof. Peter Donnelly, the Oxford statistician whom I cited in my earlier post. He is out until May 13.

I also received a nice e-mail from Jason Felch, one of the authors of the L.A. Times article, in a response to an e-mail I sent him. I have asked him for permission to quote from the e-mail and am awaiting his reply.

Salon’s Joan Walsh: Most Media Hate Hillary

Filed under: 2008 Election,Media Bias — DRJ @ 5:53 pm

[Guest post by DRJ]

Newsbusters’ Noel Sheppard was impressed with Salon Editor-in-chief Joan Walsh’s candor and impartiality in today’s CNN interview:

I was struck when I got to Iowa and New Hampshire in January by how our media colleagues were just swooning over Barack Obama. That is not too strong a word. They were swooning. I was at a speech, I remember it, I will write about it some day, in Manchester, and every, the biggest names in our business were there, and they were, they could repeat some of his speech lines to one another. It was like a Bruce Springsteen concert where the fans sing along. And, you know, I respected it to some extent. He’s a towering political figure. Of our generation, he’s probably the best politician, he’s inspiring. And, reporters, white reporters, black reporters, reporters of every race, we want to get beyond racism in America. So, he was, he was inspiring, I understood it, they’re humans, they responded. The downside though is that they hate, hate Hillary Clinton, most of them. Hate is not too strong a word.”

Walsh also criticized McCain for accepting Hagee’s endorsement but said that since Hagee wasn’t McCain’s pastor, it wasn’t the same as the Obama-Wright relationship.

It’s refreshing to hear a liberal say these things. It will be more refreshing if she’s still saying them in October.

— DRJ

Michelle Obama on Why Barack Can’t Put Hillary Away

Filed under: 2008 Election — DRJ @ 2:43 pm

[Guest post by DRJ]

Don Surber notes an interview with Michelle Obama in The Telegraph in which she says Barack is frustrated, angry, and “sick of the battle against Hillary:”

“Part of the problem may be this quote: Mrs. Obama said: “We were taught that you don’t rip your opponents to pieces, you don’t leave them on the roadside.”

In other words, Obama has had Enough. Give him what he wants.

Michelle Obama’s interview with The Telegraph is here.

— DRJ

America’s Ethanol Mistake

Filed under: Economics,Politics — DRJ @ 1:24 pm

[Guest post by DRJ]

Texas Senator Kay Bailey Hutchison plans to introduce legislation that will freeze US biofuel mandates at current levels. She claims artificially inflating ethanol production was an energy and farming mistake that has resulted in rising food prices and starvation in some parts of the world.

— DRJ

Is Obama in Trouble? (Updated)

Filed under: 2008 Election — DRJ @ 1:11 pm

[Guest post by DRJ]

First Jeremiah Wright, then William Ayers, and lingering in the background is Tony Rezko. Barack Obama has some skeletons in his closet and when you add in recent polls, Tom Elia at The New Editor wonders if Obama is in bigger trouble than anyone thinks:

“The latest CBS News/New York Times poll contains an interesting tidbit (pdf file) about a potential Obama-McCain match-up, particularly considering the tilt of the poll.

According to the poll, “McCain now holds a three-point lead over Obama among women; last month, women gave Obama a 13-point margin.”

As a point of comparison, the CNN exit poll after the 2004 presidential election showed Democrat John Kerry winning the women’s vote by a three-point margin.

November’s a long way off, but if that six-point swing among female voters holds up for McCain, it’s a big, big development if Obama is the Dems’ nominee.”

Elia also notes that Gallup has McCain over Obama by 6 points.

We all know polls can be wrong which is why I have another test: A worried politician often puts his children on display to bolster the campaign. Chelsea Clinton went public on behalf of her mother after Obama became a factor in Iowa. Now it looks like Obama’s girls are more involved, and that suggests to me Obama is feeling the heat.

UPDATE: Don Surber says Obama is so deep in trouble he’s “Kerry Deep.”

— DRJ

Obama and Clinton Disagree on Iran

Filed under: 2008 Election — DRJ @ 12:48 pm

[Guest post by DRJ]

Barack Obama and Hillary Clinton disagree on what the U.S. policy should be toward Iran:

“Barack Obama scolded Democratic rival Hillary Rodham Clinton on Sunday for saying that U.S. would “totally obliterate” Iran if it attacks Israel, and likened her to President Bush. Ms. Clinton stood by her comment.”

I think Hillary Clinton is right and Obama is wrong when it comes to Iran, which means Hillary’s position will hurt her with liberals. Maybe she’s aiming for moderate and crossover voters, or maybe she thinks most Americans have had enough of Iran.

— DRJ


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