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Variation in likelihood ratios for forensic evidence evaluation of XTC tablets comparison



In forensic comparison casework, where it is investigated whether items are from the same source, the likelihood ratio (LR) is a measure for evaluating the strength of evidence of the observed (dis)similarity between these items. The paper concentrates on evaluation of various LR models for comparison of XTC tablets from different production batches. Starting from a two-level random effect model, where means are considered random, the distribution of these means may be assumed normal or approximated by kernel density estimations (KDEs), and variation in the data may or may not be allowed to differ per batch. It is investigated what effect the corresponding model and estimation choices have on the distribution of LRs for same- and different-batch comparisons. Copyright © 2010 John Wiley & Sons, Ltd.