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Evaluation of Forensic DNA Mixture Evidence: Guidelines for Evaluation, Interpretation, and Statistical Calculations using the Combined Probability of Inclusion

Published

Author(s)

Frederick R. Bieber, John Buckleton, Bruce Budowle, John M. Butler, Michael D. Coble

Abstract

The evaluation and interpretation of forensic DNA mixture evidence faces greater interpretational challenges due to increasingly complex mixture evidence. Such challenges include: casework involving low quantity or degraded evidence leading to allele and locus dropout; allele sharing of contributors leading to allele stacking; and differentiation of PCR stutter artifacts from true alleles. There is variation in statistical approaches used to evaluate the strength of the evidence when inclusion of a specific known individual(s) is determined, and the approach(es) used must be supportable. There are concerns that methods utilized for interpretation of complex forensic DNA mixtures may not be implemented properly in some casework. Similar questions are being raised in a number of U.S. jurisdictions, leading to some confusion about mixture interpretation for current and previous casework. To address these issues, herein the key elements necessary for the interpretation and statistical evaluation of forensic DNA mixtures are described. Given the most common method for statistical evaluation of DNA mixtures is the Combined Probability of Inclusion/Exclusion (CPI/CPE), this method is the focus of this article. Formulae and other supporting materials are provided. The intent is to provide detailed guidance on application of the CPI/CPE method in the analysis of forensic DNA mixtures. This description, in turn, should help reduce the variability of interpretation with application of this methodology and thereby improve the quality of DNA mixture interpretation throughout the forensic community.
Citation
Forensic Science International: Genetics
Volume
17
Issue
125

Keywords

Forensic DNA mixtures, combined probability of inclusion, CPI, allele drop-out, stochastic threshold

Citation

Bieber, F. , Buckleton, J. , Budowle, B. , Butler, J. and Coble, M. (2016), Evaluation of Forensic DNA Mixture Evidence: Guidelines for Evaluation, Interpretation, and Statistical Calculations using the Combined Probability of Inclusion, Forensic Science International: Genetics, [online], https://doi.org/10.1186/s12863-016-0429-7 (Accessed September 17, 2021)
Created August 30, 2016, Updated June 24, 2021