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Reporting likelihood ratio for casework in firearm evidence identification.

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Summary

This study introduces a new method for firearm evidence identification using congruent matching cells (CMC) scores to calculate likelihood ratios (LR). A CMC threshold of 6 or more provides extremely strong support for same-firearm conclusions.

Keywords:
congruent matching cellsfirearm evidence identificationfirearm toolmarklikelihood ratio

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Area of Science:

  • Forensic Science
  • Ballistics
  • Statistical Modeling

Background:

  • Likelihood ratio (LR) is crucial for estimating evidence weight in firearm identifications.
  • Current LR methods use known-matching (KM) and known-nonmatching (KNM) scores but face theoretical and practical limitations.
  • Existing procedures may hinder unbiased LR estimation in casework.

Purpose of the Study:

  • To develop and validate a novel approach for LR estimation in firearm evidence identification.
  • To address limitations of current LR procedures using a new reference dataset and method.
  • To assess the performance of the congruent matching cells (CMC) method for LR calculation.

Main Methods:

  • Established a reference dataset using test fires from 130 firearms (260 cartridge cases).
  • Employed the congruent matching cells (CMC) method to generate scores for statistical distributions.
  • Utilized 130 known-matching (KM) and 8385 known-nonmatching (KNM) pairwise breech face images.

Main Results:

  • The KM and KNM distributions intersected at CMC = 2, corresponding to LR = 1.
  • Increasing the CMC threshold to 6 or higher yielded LR values exceeding one million.
  • High CMC thresholds demonstrated extremely strong support for same-firearm identification.

Conclusions:

  • The CMC method provides a robust framework for LR estimation in firearm evidence.
  • A CMC threshold of 6 or more offers substantial statistical support for identifying the same firearm.
  • This approach enhances the reliability and objectivity of firearm evidence analysis in forensic casework.