[IP] cold hits: innumerate DNA

[David Farber <dave@farber.net>]

________________________________________
From: Rod Van Meter [rdv@sfc.wide.ad.jp]
Sent: Wednesday, May 14, 2008 9:14 PM
To: David Farber
Subject: cold hits: innumerate DNA

Dave, for IP, if you wish...

>From the L.A. Times, via
http://www.kansascity.com/news/nation/story/614736.html

California Supreme Court weighs concerns of DNA matches
By JASON FELCH and MAURA DOLAN

<snip>
For more than three decades, Sylvester’s slaying went unsolved. Then, in
2004, a search of California’s DNA database of criminal offenders
yielded an apparent breakthrough: Badly deteriorated DNA from the
assailant’s sperm was linked to John Puckett, an obese, wheelchair-bound
70-year-old with a history of rape.

The DNA “match” was based on fewer than half of the genetic markers
typically used to connect someone to a crime, and there was no other
physical evidence.

Puckett insisted he was innocent, saying that although DNA at the crime
scene happened to match his, it belonged to someone else. At Puckett’s
trial earlier this year, the prosecutor told the jury that the chance of
such a coincidence was 1 in 1.1 million.

Jurors were not told, however, the statistic that leading scientists
consider the most significant: the probability that the database search
had hit upon an innocent person.

In Puckett’s case, it was 1 in 3.

The case is emblematic of a national problem, the Los Angeles Times has
found.

Prosecutors and crime labs across the country routinely use numbers that
exaggerate the significance of DNA matches in “cold hit” cases, in which
a suspect is identified through a database search.
</snip>

...and a related news article at

http://www.latimes.com/news/local/la-me-webdna9-2008may09,0,6637462.story
(same authors)
<snip>
In considering the issue over the last 12 years, the two leading
scientific bodies in the field -- the National Research Council and the
FBI's DNA Advisory Board -- have reached the same conclusion: In cold
hit cases, jurors should be given an adjusted calculation, called the
database statistic, that adjusts for the number of comparisons made
within a database.
</snip>

                --Rod



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