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Two-Level Model for Evidence Evaluation in the Presence of Zeros

NCJ Number
Journal of Forensic Sciences Volume: 55 Issue: 2 Dated: March 2010 Pages: 371-384
Date Published
March 2010
14 pages

Likelihood ratios (LRs) provide a natural way of computing the value of evidence under competing propositions. This paper proposes LR models for classification and comparison that extend the ideas of Aitken, Zadora, and Lucy and Aitken and Lucy to include consideration of zeros.


Instead of substituting zeros by a small value, we view the presence of zeros as informative and model it using Bernoulli distributions. The proposed models are used for evaluation of forensic glass (comparison and classification problem) and paint data (comparison problem). Two hundred and sixty-four glass samples were analyzed by scanning electron microscopy, coupled with an energy dispersive X-ray spectrometer method and 36 acrylic topcoat paint samples by pyrolysis gas chromatography hyphened with mass spectrometer method. The proposed LR model gave very satisfactory results for the glass comparison problem and for most of the classification tasks for glass. Results of comparison of paints were also highly satisfactory, with only 3.0 percent false positive answers and 2.8 percent false negative answers. Tables, figures, references and appendix (Published Abstract)

Date Published: March 1, 2010