The present paper discusses the methodology and findings from a research study that had two main goals: to evaluate the strength of evidence provided by both the ASTM E2927 match criterion and likelihood ratio calculation comparing vehicle glass fragments of known origin using an interlaboratory study with 13 participants; and to utilize five research and casework background databases from different countries and their combinations to understand if the size, origin, and nature of the background database affected the overall decision when using LRs for blind vehicle casework samples.
Glass samples were analyzed by 13 laboratories participating in an interlaboratory study that used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with a standard test method (ASTM E2927–23) for the forensic analysis and comparison of vehicle glass. The aim of this study was to explore the performance of the application of a match criterion described in the standard test method and from likelihood ratio (LR) calculations when reporting the significance of glass evidence comparisons. Five databases populated in different countries and combinations of the databases were used as background data to calculate LRs for two casework scenarios involving vehicle glass comparisons. When the ASTM E2927–23 was used to compare vehicle glass samples that originated from the same source, all laboratories (except one) correctly reported the samples to be indistinguishable thus concluding that the possibility that the glass originated from the same source could not be eliminated. The paper provides in-depth discussion of LR calculations and rates of misleading evidence. The laboratories reported approximately 20 percent false support for same-source proposition (or “false inclusion”) and 7 percent false support for different-source proposition (or “false exclusion”) when using the ASTM match criterion in the first scenario. All “false inclusions” were derived from the comparison of chemically similar samples, such as inner and outer panes from the same windshield, thus “error rates” on this dataset should not be generalized outside of the context of this study. A database composed of about 2000 background samples originating from different countries and analyzed in different laboratories, produced consistent results. The rate of misleading evidence of LR for same-source comparisons for the databases and their combinations was below two percent and the rate of misleading evidence for different-source comparisons was below two percent. An empirical cross entropy (ECE) plot was used to evaluate the calibration of all the databases and their combinations, which resulted in the log-likelihood ratio cost (Cllr) of less than 0.02. (Published Abstract Provided)