Friday, July 3, 2015

DNA Evidence Causes Wrongful Convictions 15% of the Time!?

Today's Forensic Magazine reports that
Recent estimates indicate that as many as 15 of every 100 incarcerated offenders where DNA was an element in their trial may be wrongfully convicted because of misused DNA evidence matching techniques. One common reason for this error is scientifically invalid testimony on forensic evidence. [FULL STORY]
Did I read this correctly? Fifteen percent -- one out of every six or seven -- of the men and women in prison are innocent? That is almost four times the 4% rate estimated in Gross et al. (2013). But that study, the leading one in the field, was limited to defendants with death sentences. Their cases would have been scrutinized especially carefully, so the 4% figure is probably on the low side. Maybe 15% for the broader prison population is realistic.

But wait. Forensic Magazine was not referring to the entire prison population, but only to those cases that went to trial. And within that group, 15% is the figure for defendants for whom "DNA was an element in their trial[s]." Because almost no defendants introduce DNA evidence at trial, these must be cases in which the prosecution has linked the defendant to the crime by DNA testing and the state "misused DNA ... techniques." Can it be that 15% of inmates convicted because of DNA tests are falsely convicted? That is an intolerable error rate for what is supposed to be the gold standard in forensic science. Is it time to halt DNA testing until we can find out why it generates so much "scientifically invalid testimony"?

Or are the editors of Forensic Magazine seeking sensational news rather than reading what they are reporting? Let's look at the "recent estimates." To get to them requires a few steps backwards. Forensic Magazine lists the National Institute of Justice (NIJ) -- a part of the U.S. Department of Justice -- as the source of its story. NIJ funded the Rand Corporation to do a study on improving expert performance in computing a posterior probability (which is not what forensic experts routinely do). The words are Rand's rather than NIJ's. The study cautions that "[o]pinions or points of view expressed are those of the author(s) and do not necessarily reflect the official position or policies of the U.S. Department of Justice." Rand investigated two questions: "Is bias reduced when experts do not know whether the prosecution or defense is the hiring party?," and "Is bias reduced by expert consensus feedback, wherein expertise is culled from multiple sources and those sources examine the majority view to move toward a group consensus?"

The summary at the start of the Rand research report begins with the very two sentences that Forensic Magazine broadcast. The researchers gave no references to indicate where these "recent estimates" came from, but page one of the report supplies an apparent answer. It reads as follows:
As many as 15 of every 100 incarcerated offenders may be wrongfully convicted, according to DNA evidence–matching techniques (Roman et al., 2012). One reason for this is scientifically invalid testimony on forensic evidence (e.g., Gould et al., 2012; Innocence Project, 2013).
This passage reveals that Rand simply misrepresented the "recent estimates" in the first instance. Roman et al. (2012) is an NIJ-funded study from the Urban Institute that involved no cases of wrongful convictions because of misused DNA evidence. Instead, that study "analyzed the results of new DNA testing of old physical evidence from 634 sexual assault and homicide cases that took place in Virginia between 1973 and 1987." In other words, it looked at postconviction exonerations in cases in which the trials took place before DNA evidence was even available. This study of wrongful convictions for reasons other than faulty DNA evidence "found that in five percent of homicide and sexual assault cases DNA testing eliminated the convicted offender as the source of incriminating physical evidence. When sexual assault convictions were isolated, DNA testing eliminated between 8 and 15 percent of convicted offenders and supported exoneration."

Are there any cases of false convictions caused by DNA evidence? Certainly. Does the rate of such convictions approach 15%? Neither Forensic Magazine nor the Rand Corporation offers any reason to believe it.


  1. Courtesy of Jill Spriggs, I received a copy of a letter dated July 23 from the California Association of Crime Laboratory Directors to the Rand Corporation pointing out the problems discussed here and concluding: "The statement in the first sentence of the RAND Corporation report's executive summary is demonstrably unsupported, and is contradicted in a later portion of the report. Given this, and given the statement's potentially misleading and inflammatory nature, we strongly urge the authors of the study to re-examine their work, and retract or amend it as appropriate."

    1. The expurgated version of the Rand study now available at omits the offending sentence.

  2. I don't disagree with your central thesis in this entry. However, I would like to point out briefly some ways in which DNA evidence can be oversold. One, sub-source DNA (DNA from an unknown body fluid or tissue) can be treated as if it were from a known source. This is sometimes done by making an inference based on peak heights. Two, cold cases can produce unreliable results, as is almost surely what happened with respect to Gary Letterman, John Ruelas, and the Jane Mixer murder. Three, a defendant can be falsely included in a mixture, a point explored by Dror and Hampikian.

  3. post script, I see that the Dror-Hampikian study is well known to you already.