The calcuLatoR—software for evaluation of evidence value of continuous type data

Abstract

The role of the forensic chemist can be to evaluate physicochemical data (evidence— E) in the context of the prosecution proposition H p and the defence proposition H d . From a forensic point of view, the most suitable form of evaluating the evidence value of physicochemical data is by calculation of the likelihood ratio (LR). There are many LR models which could be used for measuring the evidence value of results of analysis of glass samples. A disadvantage of these models is that there is a lack of commercially available software that is suitable for use by forensic experts, who may lack experience in programming. Therefore such users are required to write their own case specific routines, e.g. in R software ( www.r-project.org). The Graphical User Interface software named “calcuLatoR” for the calculation of the LR value for univariate data (such as refractive index data) has been developed. The LR model assumes two sources of variation, i.e. between replicates within the same object (within object variability) and between replicates between various objects (between object variability). It was assumed that the within object distribution is normal with constant variance. The between object distribution was modelled by a univariate kernel density estimator, i.e. using Gaussian kernels. The calcuLatoR could also be used for evaluating the evidential value of multivariate data but it must be assumed that the considered variables are independent. In this situation, LR should be calculated for each variable separately and a final value of LR is equal to their product. Validation of the calculations performed by the calcuLatoR was carried out by comparing results obtained by routines written in R.