Likelihood ratio estimation of partial Y-STR profile matches using discrete Laplace models and marginalisation

Abstract

The discrete Laplace model can be used to estimate the evidential weight of a match between the Y-chromosome short tandem repeat (Y-STR) DNA profiles of the evidence material and a suspect. When the weight of evidence of a match between partial Y-STR profiles is assessed, a discrete Laplace model, restricted to the observed loci in the partial evidence profile, is constructed. However, fitting such a discrete Laplace model is time-consuming since it requires estimating the parameters of the discrete Laplace model and validating the model. We implemented the marginalisation method for discrete Laplace models to estimate the evidential weight of a match between a partial Y-STR evidence profile and a Y-STR reference profile. Since the marginalisation method is based on discrete Laplace models for complete Y-STR profiles, no additional models need to be constructed. We compared the likelihood ratios obtained with (1) the marginalisation method and (2) by fitting new discrete Laplace models restricted to the observed loci in the partial evidence profile. In most cases, the two methods yielded similar likelihood ratios.