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Variation in assessments of suitability and number of contributors for DNA mixtures

NCJ Number
Forensic Science International: Genetics Volume: 65 Dated: July 2023
Date Published
July 2023

This study found notable variability in laboratories’ assessments of DNA mixtures.


This study evaluated laboratories’ assessments of DNA mixtures and found notable variation in the results. In the study, 134 participants from 67 forensic laboratories provided a total of 2272 assessments of 29 DNA mixtures (provided as electropherograms). The laboratories’ responses were evaluated in terms of the variability of suitability assessments, and the accuracy and variability of NoC assessments. Policies and procedures related to suitability and NoC varied notably among labs. The authors observed notable variation in whether labs would assess a given mixture as suitable or not, predominantly due to differences in lab policies: if two labs following their standard operating procedures (SOPs) were given the same mixture, they agreed on whether the mixture was suitable for comparison 66% of the time. The interpretation of a DNA mixture (a sample that contains DNA from two or more people) depends on a laboratory/analyst’s assessment of the suitability of the sample for comparison/analysis, and an assessment of the number of contributors (NoC) present in the sample. Differences in suitability assessments have a direct effect on variability in interpretations among labs, since mixtures assessed as not suitable would not result in reported interpretations. For labs following their SOPs, 79% of assessments of NoC were correct. When two different labs provided NoC responses, 63% of the time both labs were correct, and 7% of the time both labs were incorrect. Incorrect NoC assessments have been shown to affect statistical analyses in some cases, but do not necessarily imply inaccurate interpretations or conclusions. Most incorrect NoC estimates were overestimates, which previous research has shown have less of an effect on likelihood ratios than underestimates. (Published Abstract Provided)

Date Published: July 1, 2023