Implementation and assessment of a likelihood ratio approach for the evaluation of LA-ICP-MS evidence in forensic glass analysis

Andrew van Es; Wim Wiarda; Maarten Hordijk; Ivo Alberink; Peter Vergeer

doi:10.1016/j.scijus.2017.03.002

Implementation and assessment of a likelihood ratio approach for the evaluation of LA-ICP-MS evidence in forensic glass analysis

Sci Justice. 2017 May;57(3):181-192. doi: 10.1016/j.scijus.2017.03.002. Epub 2017 Mar 20.

Authors

Andrew van Es¹, Wim Wiarda², Maarten Hordijk³, Ivo Alberink⁴, Peter Vergeer⁵

Affiliations

¹ The Netherlands Forensic Institute, P.O. Box 24044, 2490 AA The Hague, The Netherlands. Electronic address: a.van.es@nfi.minvenj.nl.
² The Netherlands Forensic Institute, P.O. Box 24044, 2490 AA The Hague, The Netherlands. Electronic address: w.wiarda@nfi.minvenj.nl.
³ The Netherlands Forensic Institute, P.O. Box 24044, 2490 AA The Hague, The Netherlands.
⁴ The Netherlands Forensic Institute, P.O. Box 24044, 2490 AA The Hague, The Netherlands. Electronic address: i.alberink@nfi.minvenj.nl.
⁵ The Netherlands Forensic Institute, P.O. Box 24044, 2490 AA The Hague, The Netherlands. Electronic address: p.vergeer@nfi.minvenj.nl.

PMID: 28454627
DOI: 10.1016/j.scijus.2017.03.002

Abstract

For the comparative analysis of glass fragments, a method using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is in use at the NFI, giving measurements of the concentration of 18 elements. An important question is how to evaluate the results as evidence that a glass sample originates from a known glass source or from an arbitrary different glass source. One approach is the use of matching criteria e.g. based on a t-test or overlap of confidence intervals. An important drawback of this method is the fact that the rarity of the glass composition is not taken into account. A similar match can have widely different evidential values. In addition the use of fixed matching criteria can give rise to a "fall off the cliff" effect. Small differences may result in a match or a non-match. In this work a likelihood ratio system is presented, largely based on the two-level model as proposed by Aitken and Lucy [1], and Aitken, Zadora and Lucy [2]. Results show that the output from the two-level model gives good discrimination between same and different source hypotheses, but a post-hoc calibration step is necessary to improve the accuracy of the likelihood ratios. Subsequently, the robustness and performance of the LR system are studied. Results indicate that the output of the LR system is robust to the sample properties of the dataset used for calibration. Furthermore, the empirical upper and lower bound method [3], designed to deal with extrapolation errors in the density models, results in minimum and maximum values of the LR outputted by the system of 3.1×10^-3 and 3.4×10⁴. Calibration of the system, as measured by empirical cross-entropy, shows good behavior over the complete prior range. Rates of misleading evidence are small: for same-source comparisons, 0.3% of LRs support a different-source hypothesis; for different-source comparisons, 0.2% supports a same-source hypothesis. The authors use the LR system in reporting of glass cases to support expert opinion in the interpretation of glass evidence for origin of source questions.

Keywords: Calibration; Glass; LA-ICP-MS; Likelihood ratio; Validation.