Inter-laboratory Variation in Assessments of Number of Contributors for DNA Mixtures

The purpose of the presentation is to discuss the results collected and variation observed in assessments of suitability and number of contributors reported by forensic laboratories for a total of 29 DNA mixture profiles. These results were reported as part of the last two phases of the NIJ-funded (Grant 2020-R2-CX- 0049) study entitled Inter-laboratory Variation in Interpretation of DNA Mixtures (hereafter referred to as “DNAmix 2021”).

DNAmix 2021 is a rigorous, large-scale study conducted to evaluate the extent of consistency and variation among forensic laboratories in interpretations and statistical analyses of DNA mixtures, and to assess the effects of various potential sources of variability. To this end, this study utilized a four-phased approach:

1. Policies and Procedures (P&P) Questionnaire — Online questionnaire to assess laboratory policies and procedures relevant to DNA mixture interpretation (notably systems, types of statistics reported, and parameter settings used).
2. Casework Scenario Questionnaire — Assess analysis procedures or decisions that may vary depending upon the case scenario and the nature of mixture casework conducted by the
3. Number of Contributors (NoC) Subtest — Assessment of suitability and number of contributors, given electropherogram data for 12 DNA mixtures.
4. Interpretation, Comparison, and Statistical Analysis (ICSA) Subtest — Interpretations, comparisons, and statistical analyses for 8 DNA mixtures provided with DNA profiles of potential

In total, each participating laboratory that completed the NoC and ICSA Subtests submitted responses on 20 distinct mixtures (provided to participants as electropherograms); partial participation was accepted. These mixtures were created under controlled conditions to allow for ground truth attribution, and were selected using a “bracketing approach” (as detailed by Butler, et. al. 2021¹) to cover as much of the factor space as practical given the limited number of samples completed by each participating laboratory. Mixtures were created to be broadly representative of the range of attributes encountered in casework. All DNA profiles used in this study (mixtures and references) were from real people; no profiles were simulated or contrived.

This presentation will specifically focus on a portion of the results from the overall study: over 1,200 assessments of suitability and number of contributors (if applicable) reported by participating labs in Phases 3 and 4 of DNAmix 2021. Participants submitted responses regarding whether an assigned mixture was suitable, any factors that indicated the mixture was not suitable, the estimated number of contributors in a mixture, and any factors that affected this estimate. During this presentation, we will report on the observed level of variation in these responses as well as discuss the amount of variation that can reasonably be explained by differences in laboratory standard operating procedures.

1 J. M. Butler, H. Iyer, R. Press, M. K. Taylor, P. M. Vallone, and S. Willis, “DNA Mixture Interpretation: A NIST Scientific Foundation Review (NISTIR 8351-DRAFT),” 2021. [Online]. Available: https://doi.org/10.6028/NIST.IR.8351-draft.