Hicklin et al. 2023

Variability in DNA Mixture Assessments

Overview

• Focus on interpreting DNA mixtures, vital for forensic analysis.

• Study conducted with 134 participants across 67 labs, evaluating 29 DNA mixtures.

• Key aspects assessed: suitability for comparison and number of contributors (NoC).

Findings

Suitability Assessments

• Variability exists in how labs determine if a mixture is suitable for analysis.

• Agreement rate on suitability between labs: 66%.

• High variability stems from differing lab policies and SOPs.

• Importance of initial suitability assessment affects overall interpretation results.

Number of Contributors (NoC)

• Inaccurate NoC assessments can affect statistical analyses.

• Correct NoC assessment rate for labs following SOPs: 79%.

• When two labs assess NoC, they agreed on correctness 63% of the time.

• High rates of incorrect assessments were typically overestimations.

Implications

• Discrepancies in suitability and NoC have direct impacts on case outcomes in forensic contexts.

• Standardization in procedures and training may improve consistency and reliability in results.

Background

• DNA analysis is a gold standard for forensic science but is complex with mixtures involving multiple contributors.

• Initial assessments of DNA mixtures influence further analysis, comparison, and interpretation.

Assessment Characteristics

• Suitability depends on:

  • Total DNA quantity.

  • Number of contributors and their ratios.

  • Observed peaks and overall DNA quality.

• Laboratories exhibit variability in evaluating:

  • Maximum allowed number of contributors.

  • Minimum DNA threshold for analysis.

Methodology

Study Structure

1. Policies and Procedures (P&P) Questionnaire: Assessment of lab practices regarding DNA mixture interpretation.

2. Casework Scenario Questionnaire: Evaluate decisions regarding interpretations and outcomes in real cases.

3. NoC Subtest: Participants assessed suitability and contributor numbers based on electropherogram (EPG) data.

4. Interpretation, Comparison, and Statistical Analysis (ICSA) Subtest: Judgement based on mixtures provided with DNA profiles.

Data Collection

• Feedback gathered from various labs across the globe, including 57 U.S. laboratories and 10 international labs.

Results

Suitability and NoC Results

• Suitability assessment outcomes revealed:

  • 40.4% of assessed mixtures deemed unsuitable.

  • 50.1% agreed a mixture was suitable based on overall quality.

• NoC response accuracy:

  • Corrections ranged significantly, particularly for complex mixtures (≥5 contributors).

• Notable differences observed among various laboratories and SOP implementations.

Conclusion

• Study underscores variability in suitability assessments and NoC estimations among forensic laboratories.

• Highlights need for improved consistency through standard operating procedures and training initiatives in DNA analysis protocols.