FBI DNA Calculation Errors: What’s the Big Deal?

“This past spring, the FBI notified crime labs there were errors in the data used to calculate the chances that DNA found at a crime scene matched an individual. But it downplayed the impact of the errors on court cases, setting off a low but distinct rumble throughout the criminal justice system nationwide.”

Source: New Protocol Leads to Reviews of “Mixed DNA” Evidence | The Texas Tribune

The low rumble started with the announcement. That rumble was met with assertions from crime labs that “this really won’t make much difference.” However, as months go by, we are now starting the see the ripple grow into more of a wave.

The Problem

The FBI notified labs that there were errors in the population data which they published in 1999 and 2001 (you can view and download the FBI memo along with Texas DPS response here). In essence, they revealed discrepancies due to human error and technological limitations of the time which made DNA comparisons seem more accurate and unique; this data caused DNA labs across the country to report statistics that overinflated the likelihood that a suspect or defendant was the person who’s DNA was analyzed. This is not to say that defendants became excluded as a possible source of the DNA, but the likelihood went from something like 1 in a billion to less than 1 in 100. And that’s a significant difference when we are talking about taking someone’s life or liberty based on the result of the DNA test.

The problem, statistically, became even greater and more significant when labs started looking at the likelihood of particular DNA belonging to the defendant where the “sample” of DNA found at a crime scene was a “mixture.” A mixture refers to an evidence sample that contains the DNA of more than one person. (The classic example being a rape kit where there is more than one person’s DNA within the swabs collected during the exam.)

In addition to some very different calculations of probability and statistics, there were some mixed samples that were either too low to report but were included or were high enough to report but were not. This means some samples might have actually contained more people “mixed” into the sample than was previously reported. It could also mean there were less people in the sample than previously reported.

In light of all this statistical reporting being change, the DPS Crime Laboratory Service recently acknowledged DNA analysis is a developing science and as standards change, their approach does as well. The lab announced on September 10 more stringent and more conservative standards for analyzing “mixtures.”

The Effect

These newly discovered problems with DNA statistics means that for the past 15 years, DNA analysis that has been used wholly or in part to lead to the conviction of so many must be reanalyzed to determine whether or not the investigation, prosecution, or defense might have been different.

This is a major undertaking and is only just beginning. We will be discussing this and more on this week’s Reasonable Doubt.

Who are the Harris County Criminal Lawyers?

We are the largest local criminal defenses bar in the U.S. with more than 700 active members engaged in the defense of citizens accused.

Our Mission Is to Assist, Support, and Protect the Criminal Defense Practitioner in the Zealous Defense of Individuals and Their Constitutional Rights. It Is Further Our Mission to Educate and Inform the General Public Regarding the Administration of Criminal Justice and the Need for an Independent, Ethical, and Professional Criminal Defense Bar.

Collectively, our voice is strong and loud and helping to shape the criminal justice system in Harris County and surrounding counties.

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For years, HCCLA has hosted a weekly call-in show through Houston Media Source: live Thursdays from 8-9pm. Currently, our hosts are Jimmy Ardoin and Julio “JV” Vela.

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