An overview of log likelihood ratio cost in forensic science – Where is it used and what values can we expect?

Abstract

There is increasing support for reporting evidential strength as a likelihood ratio (LR) and increasing interest in (semi-)automated LR systems. The log-likelihood ratio cost (Cllr) is a popular metric for such systems, penalizing misleading LRs further from 1 more. Cllr = 0 indicates perfection while Cllr = 1 indicates an uninformative system. However, beyond this, what constitutes a “good” Cllr is unclear.Aiming to provide handles on when a Cllr is “good”, we studied 136 publications on (semi-)automated LR systems. Results show Cllr use heavily depends on the field, e.g., being absent in DNA analysis. Despite more publications on automated LR systems over time, the proportion reporting Cllr remains stable. Noticeably, Cllr values lack clear patterns and depend on the area, analysis and dataset.As LR systems become more prevalent, comparing them becomes crucial. This is hampered by different studies using different datasets. We advocate using public benchmark datasets to advance the field.

Keywords

• Cllr
• Log-likelihood ratio cost
• Automated likelihood ratio systems
• Benchmarking review
• Forensic evaluation
• Forensic datasets