The Federal Bureau of Investigation (FBI) has published its 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 FBI began use of STRmix in casework in December 2015. This publication details the extensive validation work done by the FBI to underpin that casework use. The findings show that STRmix™ – a sophisticated forensic software used by trained, experienced DNA experts to resolve mixed DNA profiles previously thought unresolvable – is sufficiently robust for implementation in forensic laboratories.
The FBI’s internal validation, published in FSI: Genetics, notes that STRmix™ offers numerous advantages over historical methods of DNA profile analysis and has greater statistical power for estimating evidentiary weight, all of which can be used reliably in human identification testing.
“The validation largely meets the requirements of the President’s Council of Advisors on Science and Technology (PCAST) for the foundational validity of STRmix™ for one-to-five person mixtures at a wide range of ratio and template,” explains Dr. John Buckleton, DSc, FRSNZ, Forensic Scientist at the New Zealand Institute of Environmental Science and Research (ESR).
Buckleton and fellow ESR scientist Jo-Anne Bright, 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.
Studies to assess and internally validate STRmix™ for casework use at the FBI were conducted using lab-specific parameters and more than 300 single-source and mixed contributor profiles. Simulated forensic specimens, including constructed mixtures that incorporated DNA from two-to-five donors across a broad range of template amounts and contributor proportions, were used to examine the sensitivity and specificity of the system via more than 60,000 tests that compared hundreds of known contributors and non-contributors to the specimens.
Conditioned analyses, concurrent interpretation of amplification replicates, and application of an incorrect contributor number were also performed to further investigate software performance and probe the limitations of the system. In addition, the results from manual and probabilistic interpretation of both prepared and evidentiary mixtures were compared.
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. In addition, STRmix™ is 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.