Florida Murder Case Reaffirms Reliability of STRmix™

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A Florida murder case has reaffirmed the reliability of probabilistic genotyping generally, and the use of STRmix™ – a sophisticated forensic software used to resolve mixed DNA profiles previously thought unresolvable -- in particular.

A Florida murder case has reaffirmed the reliability of probabilistic genotyping generally, and the use of STRmix™ software in particular.

State of Florida v. Dwayne Cummings (12th Judicial Circuit in and for Manatee County, FL, Case No. 2016-CF-239) – in which the defendant was charged with two counts of first degree murder, as well as one count each of armed kidnapping and possession of a firearm by a convicted felon – also affirms that going forward, Florida will use the Daubert Standard to assess whether an expert’s scientific testimony is based on scientifically valid methods and can be properly applied to the facts at issue.

Unlike the previously used Frye Standard, which relies on pure opinion testimony, Daubert considers whether the scientific theory or technique in question has been tested; has been subjected to peer review and publication; and has attracted widespread acceptance within the scientific community. Daubert also considers the technique’s known or potential error rate, as well as the existence and maintenance of standards controlling its operation.

In the Cummings case, Florida’s 12th Judicial Circuit Court ruled that both probabilistic genotyping and STRmix™ – a sophisticated forensic software used by trained, experienced DNA experts to resolve mixed DNA profiles previously thought unresolvable – have been subjected to significant peer review and publication.

In its decision, the court noted, “The evidence before the court more than adequately convinces it that [probabilistic genotyping and STRmix™] have been accepted by peer review, as well as obtained general acceptance in the relevant community … Perhaps that is why nearly half of all forensic laboratories in the United States have purchased licenses to use the STRmix™ software….”

Relying on mixture samples taken from a vehicle involved in the Cummings case, the State’s expert witness, Rachel Oefelein of DNA Labs International, used STRmix™ to determine that there was “extremely strong support” to conclude that Cummings and three unknown persons had contributed to the mixed DNA profile.

The court concluded, “Probabilistic genotyping is testable. As Ms. Oefelein’s testimony made clear, testing the same sample can produce replicable results.” Conceding that retesting the samples would likely produce a slightly different result, the court found that the difference “will be minimal. The key here is that the results will be consistent and reproducible.”    

The Florida case comes on the heels of the FBI’s recent validation of STRmix™ for use on mixtures of up to five persons, as well as across a wide range of templates and mixture ratios.

The validation concluded that STRmix™ is sufficiently robust for implementation in forensic laboratories, offering numerous advantages over historical methods of DNA profile analysis and greater statistical power for estimating evidentiary weight, all of which can be used reliably in human identification testing.

John Buckleton and Jo-Anne Bright of the New Zealand Institute of Environmental Science and Research (ESR), in collaboration with Duncan Taylor from Forensic Science South Australia (FSSA), developed STRmix™. Using standard, well-established statistical methods, STRmix™ builds up a picture of the DNA genotypes that, when added together, best explains the observed mixed DNA profile. STRmix™ then enables users to compare the results against a person or persons of interest and calculate a statistic, or “likelihood ratio,” of the strength of the match.

Eighteen labs in the U.S. are now using STRmix™, while another 55 U.S. labs are at various stages of installation, validation, and training. STRmix™ is also being used by numerous local, state, and federal agencies, including the U.S. Army Criminal Investigation Laboratory (USACIL) and the California Department of Justice.

To date, there have been at least seven successful admissibility hearings for STRmix™ in the U.S., while DNA evidence interpreted with STRmix™ has been successfully used in more than 65 other court cases.

Internationally, STRmix™ has been used in casework since 2012, and has been used to interpret DNA evidence in thousands of cases. It is currently in use in labs in Australia, New Zealand, England, Scotland, Ireland, and Canada.

For more information about STRmix™ visit http://www.esr.cri.nz/ or http://strmix.esr.cri.nz/.

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