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DNA Mixture Study: Novel Metrics to Quantify the Intra- and Inter-Laboratory Variability in Forensic DNA Mixture Interpretation

NCJ Number
Emily Rogers; Roman Aranda IV; Philippa M. Spencer; Denise R. Myers
Date Published
August 2018
31 pages

Since variability in DNA interpretation occurs when the DNA sample is complex, has multiple contributors, or the DNA template is minimal, the current project developed a wide-scale quantitative assessment that uses novel metrics to measure the precision and accuracy of forensic DNA mixture interpretation.


The metrics measure the accuracy and precision of a DNA mixture interpretation for each contributor in a mixture. The results of applying these metrics to the DNA data show that 1) a significant amount of interpretation variability exists with and between laboratories; 2) accurate and precise interpretations are possible, with accuracy and precision being highly correlated; and 3) the quantitative results indicate the ongoing need for training and benchmarking within laboratories, as well as the need for dissemination of best practices among laboratories. The findings further suggest that two-person DNA mixtures with signal peaks above stochastic threshold are generally interpretable; however, three-person mixtures are currently beyond the scope or protocol limits for most participating examiners. The results highlight the impact of a reference profile and of strong peak heights in the interpretability of a mixture; however, there were laboratories and participants that were able to interpret the difficult three-person mixtures and resolve genotypes for each contributor with nearly equivalent contributor ratios, even under challenging conditions. 5 figures, 3 tables, and 40 references