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WASHINGTON, DC (3/14/18) – A U.S. District Court judge in New Mexico has denied a defense motion to exclude expert testimony provided by an FBI DNA analyst in an aggravated sexual abuse case.


Ruling in U.S. v. Melvin Russell (No. CR-14-2563 MCA), Chief U.S. District Judge M. Christina Armijo found that DNA Analyst Tiffany Smith’s “background, education, experience, skill, and training qualify her to testify as an expert witness” in the case.


Moreover, the Court determined that the government had met its burden to demonstrate that STRmix™ – a software package that performs probabilistic genotyping, used by the FBI to analyze the significance of DNA profiles obtained from samples in the case – “has been tested for the purpose relevant here, that such tests have been peer-reviewed and published in scientific journals, and that its analyses are based on calculations recognized as reliable in the field.”


The FBI published its validation of STRmix™ – a sophisticated forensic software used to resolve mixed DNA profiles previously thought to be too complex to interpret – in mid-2017for use on mixtures of up to five persons, as well as across a wide range of templates and mixture ratios.


The FBI’s internal validation 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 findings also show that STRmix™ is sufficiently robust for implementation in forensic laboratories.


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.


Likelihood ratios also factored into Judge Armijo’s decision in U.S. v. Melvin Russell. Rejecting defense arguments that the government’s expert witness misunderstood the nature of Y-STR analysis (short tandem repeats found on the male-specific Y chromosome, used by forensic laboratories to examine sexual assault evidence), the court concluded that Ms. Smith’s testimony was admissible.


Similarly, Judge Armijo ruled that the government had sufficiently demonstrated that the formula used to calculate the confidence interval in the case was reliable, and that the margin of error calculations employed in the case were based on reliable methodologies.

Thirty U.S. labs now routinely use STRmix™ in resolving DNA profiles. STRmix™, which recently celebrated its fifth anniversary of use in live casework, is also in various stages of installation, validation, and training in 51 other U.S. labs.



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