Quantifying the strength of firearms comparisons based on error rate studies.

Area of Science:

• Forensic Science
• Firearms Examination
• Tool Mark Analysis

Background:

• Forensic firearms and tool mark examiners compare bullets and cartridge cases to determine source.
• Examiners use conclusion scales (e.g., Identification, Elimination) to communicate findings.
• Current scales lack direct calibration to evidence strength, relying on indirect error rate studies.

Purpose of the Study:

• To reanalyze firearms and cartridge case comparison data from error rate studies.
• To generate quantitative measures of evidence strength for examiner comparisons.
• To produce likelihood ratios for assessing the strength of forensic evidence.

Main Methods:

• Utilized an ordered probit model to analyze examiner responses in error rate studies.
• Aggregated data from firearms and cartridge case comparisons.
• Calculated likelihood ratios to quantify the strength of evidence.

Main Results:

• Likelihood ratios derived from the analysis can be as low as less than 10.
• This quantitative measure contrasts with current qualitative scales.
• The findings suggest current terminology may overstate evidence strength.

Conclusions:

• The study provides a quantitative method for evaluating the strength of firearms evidence.
• Current conclusion scales used by examiners may overstate the evidence strength by orders of magnitude.
• There is a need to calibrate examiner language with empirical data for accurate communication.