The Forensic Significance of Glass Composition and Refractive Index Measurements

The frequency distributions of refractive index and the concentrations of ten elements in 204 glass specimens received as evidence in casework were evaluated. These data were used to estimate the probability of randomly selecting, from a general population, a glass specimen that is indistinguishable in all measured characteristics from a given glass specimen. The probability of two unrelated glass specimens having indistinguishable elemental compositions and refractive indices is calculated and lies between the extremes of 10−5 and 10−13. For each of the 20,706 pairwise comparisons of the 204 specimens in this study, the two specimens are analytically distinguishable. The use of highly discriminating analytical methods for the comparison of trace evidence and the corresponding low probability of two unrelated glass specimens being indistinguishable eliminates the need to collect extensive databases for the purpose of making exact probability calculations. The approach used here can be applied to other highly discriminating parameters and trace evidence sample types.