Saturday, July 18, 2015

DNA match statistics overstated, news delayed scheduled execution

Reported AP's Michael Graczyk, "The Texas Department of Public Safety sent a notice June 30 that the FBI-developed population database used by the crime lab in Texas and other states had errors for calculating DNA match statistics in criminal investigations." The Texas Attorney General's Office informed attorneys for Clifton Lamar Williams of the issue on Wednesday and the Court of Criminal Appeals halted his execution scheduled for Thursday night and ordered the trial court to hold a hearing.

The math error overstated at trial: "Williams is black, and prosecutors said the probability of another black person with the same DNA profile found in [the victim's] missing car was one in 40 sextillion. Jurors in 2006 were told the probability was one in 43 sextillion. A sextillion is defined as a 1 followed by 21 zeros."  Of course, there are only 7 billion (nine zeroes) or so people on earth, so that statistic essentially told jurors it's nigh-on impossible it was anyone else.

Whether this issue will prevent rather than delay his execution is another matter. "Challenges to DNA evidence and arguments about Williams' guilt were not included in previous appeals from Williams, whose lawyers had argued unsuccessfully in earlier appeals that his legal help at his trial was deficient and that he was mentally impaired and therefore ineligible for the death penalty," reported Graczyk, and there was plenty of other evidence. "Williams led police to a pond where [the 93-year old victim's] purse that had contained about $40 was found, along with a knife from her kitchen that investigators believe was used to stab her."

Perhaps the racial frame cast upon the testimony explains the astronomical numbers. In previous coverage of the issue I've never seen overestimates range to the sextillions, which sounds like the forensic analyst and the DA in my hometown employed a particularly extreme version of this error at Mr. Williams' trial. Regardless, the DPS notification affects lots of cases besides his, even if delaying Williams' scheduled execution appears to have engendered the first public report of the DPS notice, that I've seen anyway.

The Hays County DA posted a copy of the notice from DPS on their website.


Anonymous said...

Several points: 1) The difference between original (1-in-43 sextillion) and corrected (1-in-40 sextillion) values is 7.5%, and is within the worst-case range of several percentage points reported in the FBI's paper. The absolute difference of 1-in-three sextillion doesn't have any real importance. It is the percentage difference that has meaning. 2) There isn't a racial frame here. DNA statistics are generally reported in Texas for the three principle Texas population groups: caucasians, african-americans, and southwest hispanics. Because gene flow is more limited between those population groups than within the groups, the statistics will differ between the groups. 3) There will be routine and regular rehearings on this issue for death penalty cases, either as part of the normal appellate process, or as in this case as a special circumstance hearing when other appeals have been exhausted.

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