Colorado Court Rules STRmix™ Is “Relevant and Reliable” Practice for Interpreting Likelihood Ratios

The Colorado District Court for Eagle County has ruled that STRmix™ – sophisticated forensic software used to resolve mixed DNA profiles previously thought to be too complex to interpret – “is a relevant and reasonably reliable practice for interpreting likelihood ratios under 1,000 with low quality samples.”

Likelihood ratios (LRs) quantify the probability that a suspect or person of interest is a DNA contributor to a crime scene sample. Typically, the higher the LR is, the stronger the suggestion that the suspect is a DNA contributor.

For the case in question, Colorado v. Ackerson (Case No. 2018CR85), the defendant was accused of first degree murder, first degree burglary, and aggravated robbery. Defense argued that analysis of DNA samples conducted by the Colorado Bureau of Investigation (CBI) using STRmix™ was unreliable because the samples were of low-quality and had likelihood ratios under 1,000.

In denying the defendant’s Motion to Exclude Expert Testimony regarding evidence generated by STRmix™, Judge Paul R. Dunkelman ruled, “By using STRmix™, CBI could analyze a sample that it previously could not. It was reanalyzed and analyst opined that the DNA mixture found on swabs from a knife handle found in … [the victim’s] … bathroom is ‘at least 150 times more likely to be observed if it originated from [defendant] Ms. Ackerson and three unknown, unrelated individuals than if it originated from four unknown, unrelated individuals.’ ”

The court went on to state, “STRmix™ analysis provides a level of support that Defendant Ackerson had physical contact with the knife handle [used in the murder] and has the tendency to make it more probable that Defendant Ackerson participated in the assault and killing of [the victim] … than it would without the evidence.”  

Since it was introduced eight years ago, STRmix™ has been used to interpret DNA evidence in more than 120,000 cases around the world. It has also been used successfully in numerous U.S. court cases, including at least 35 successful admissibility hearings.

“Increasingly, forensic labs are turning to STRmix™ because it greatly improves the usability of DNA to produce evidence in a wide range of criminal cases,” explains John Buckleton, DSc, FRSNZ, one of the developers of STRmix™.

According to Buckleton, organizations using STRmix™ are reporting an increase of interpretable DNA in gun cases from about 40% to more than 70%. Similarly, STRmix™ is delivering a significantly higher rate of interpretable results in sexual assault cases.

At present, STRmix™ is being used by forensic labs at 55 U.S. agencies, including the FBI and the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF). It is also in various stages of installation, validation, and training in more than 60 other U.S. organizations.