This article reports the findings and methodology of a project in which 17 laboratories participated in three interlaboratory exercises to assess the performance of refractive index, micro X-ray Fluorescence Spectroscopy (µXRF), and Laser Induced Breakdown Spectroscopy (LIBS) data for the forensic comparison of glass samples.
Glass fragments from automotive windshields were distributed to the participating labs as blind samples, and participants were asked to compare the glass samples (known vs. questioned) and report their findings as they would in casework. For samples that originated from the same source, the overall correct association rate was greater than 92 percent for each of the three techniques (refractive index, µXRF, and LIBS). For samples that originated from different vehicles, an overall correct exclusion rate of 82 percent, 96 percent, and 87 percent was observed for refractive index, µXRF, and LIBS, respectively. Special attention was given to the reporting language used by practitioners, as well as the use of verbal scales and/or databases to assign a significance to the evidence. There were wide variations in the reported conclusions between laboratories, demonstrating a need for the standardization of the reporting language used by practitioners. Moreover, few labs used a verbal scale and/or a database to provide a weight to the evidence. It is recommended that forensic practitioners strive to incorporate the use of a verbal scale and/or a background database, if available, to provide a measure of significance to glass forensic evidence (i.e., the strength of an association or exclusion). (publisher abstract modified)