Accuracy and reproducibility of forensic tire examination decisions

Tire impression evidence can be a valuable tool during a crime scene investigation-it can link vehicles to scenes or secondary locations, and reveal information about the series of events surrounding a crime. The interpretation of tire impression evidence relies on the expertise of forensic tire examiners. To date, there have not been any studies published that empirically evaluate the accuracy and reproducibility of decisions made by tire impression examiners. This paper presents the results of a study in which 17 tire impression examiners and trainees conducted 238 comparisons on 77 distinct questioned impression-known tire comparison sets (QKsets). This study was conducted digitally and addressed examinations based solely upon the characteristics of the tire impression images provided. The quality and characteristics of the impressions were selected to be broadly representative of those encountered in casework. Participants reported their decisions using a multi-level conclusion scale: 68% of responses were class associations (Association of Class Characteristics or Limited Association of Class), 21% were definitive decisions (ID or Exclusion), 8% were probable decisions (High Degree of Association or Indications of Non-Association), and 3% were neutral responses (Not Suitable or Inconclusive). Although class associations were the most reported response type, when definitive decisions were reported, they were often correct: 96% of IDs and 89% of Exclusions were consistent with ground truth regarding the source of the known tire in the QKset. Overall, we observed 4 erroneous definitive decisions (3 Exclusions on mated QKsets; 1 ID on a nonmated QKset) and 1 incorrect probable decision (Indications of Non-Association on a mated QKset). Decision rates were notably associated with both quality (lower quality questioned impressions were more likely to result in class associations) and dimensionality (2D questioned impressions were more likely to result in definitive decisions), which were correlated factors. Although the study size limits the precision of the measured rates, the results of this study remain valuable to the forensic science and legal communities and provide empirical data regarding examiner performance for a discipline that previously did not have any such estimates.