This document contains the slides of a virtual presentation given by Xiaoyu Alan Zheng at the 2024 Annual Meeting of the Expert Working Group on Firearms and GSR of the European Network of Forensic Science Institutes.
There has been a major paradigm shift in firearm and toolmark analysis towards the use of 3D topography measurements. The new approach bolsters objectivity through SI traceable measurements. Several manufacturers are now offering specialized 3D microscopes for toolmark measurement which rely on differing measurement principles that each has advantages and challenges. These instruments are currently being used in laboratories for Virtual Comparison Microscopy (VCM). The future goal for these instruments is to generate measurements used to report on the statistical weight of evidence of comparison in casework. For impressed toolmarks, there is no comprehensive study that characterizes the resulting differences in 3D data obtained using different instrument types and their effect on objective similarity scores. This is an important gap in the quest for objective comparison results and quantitative weight of evidence reporting. This gap needs to be addressed to ensure consistency in results among labs and provide associated foundational data for future Daubert hearings.
The research evaluated the effect of measurement source variations on similarity metrics. This was accomplished through a round-robin study where each lab/instrument measures the same set of 120 cartridge cases fired from four sets of consecutively manufactured firearms. Twelve laboratories participated in this study to generate a total of 18 datasets across 9 different 3D instruments. To quantify the differences between labs and technologies, each lab’s measurements were analyzed using two well-established similarity scores: the normalized Areal Cross Correlation Function (ACCFMAX) and the number of Congruent Matching Cells (CMC). The results were used to generate Known Matching (KM) and Known Non-Matching (KNM) score distributions, which were used to statistically analyze for differences between labs and systems. Results will facilitate improvements in the consistency of measurement results while providing the foundational research data required to defend the future use and interoperability of 3D measurements in casework.
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