The 'factor of two' issue in mixed DNA profiles

Area of Science:

• Forensic Science
• Genetics
• Statistics

Background:

• The hierarchy of propositions is a key concept in forensic analysis.
• DNA analysts frequently report findings at the sub-source level, focusing on contributors to a DNA profile.
• Reporting can occur at an even lower 'sub-sub-source' level, comparing individuals to specific mixture components.

Purpose of the Study:

• To explore the accurate construction of likelihood ratios (LR) for DNA mixture analysis.
• To address the distinction between sub-source and sub-sub-source reporting levels.
• To provide clarity on calculating LR for complex DNA evidence.

Main Methods:

• Investigated the calculation of likelihood ratios (LR) in DNA mixture analysis.
• Examined the impact of contributor order and genotype set order on LR.
• Utilized expert systems capable of calculating LR for any mixture.

Main Results:

• Demonstrated how to construct LR to report correctly at the sub-source level.
• Highlighted the importance of considering contributor and genotype orders for accurate reporting.
• Provided worked examples of LR calculations for DNA mixtures.

Conclusions:

• Accurate LR calculation is crucial for correct reporting in DNA analysis.
• Understanding the hierarchy of propositions, particularly sub-source vs. sub-sub-source levels, is vital.
• This work clarifies complex issues in DNA mixture interpretation using likelihood ratios.