Logical evaluation of evidence when a person is suspected of committing two separate offences.

We consider the problem of drawing inferences within a legal framework when a person is a suspect for two separate offences. Although we are primarily concerned with scientific evidence the issue inevitably arises as to how that evidence interacts with other, non-scientific evidence. We show that, in this particular context, the evidence can be conveniently classified into three categories that concern, respectively: the first crime only; the second crime only; and evidence that relates to similarities between the two crimes. Two case examples are considered and we consider DNA, fibres and eyewitness evidence. These are viewed from the perspective of a prosecutor who has to decide whether or not to charge a suspect with one or both crimes. Graphical sensivity analyses are presented which have features that are not intuitively obvious.     

An efficient statistical procedure for interpreting DNA single locus profiling data in crime cases.

This paper describes a simplified approach to calculating the Bayesian likelihood ratio for the case where two DNA single locus profiles are to be compared. It explains how the calculation allows for band shift and also for the variation in precision with molecular weight. A simple basic theory section explains the principles of the analysis but more detailed explanations are given in an advanced theory section. Experiments on substantial data collections are described which demonstrate the robustness of the method.     

An investigation of the feasibility of inferring ethnic origin from DNA profiles.

A small experiment is described which uses databases of Afro-Caribbeans and Caucasians to establish the quality of information that might be given to an investigator about the ethnic group of the donor of an unknown crime stain. The results show that the technique can provide qualified indications, but categoric conclusions are not, in general, feasible. An appraisal is given of the technique within the operational context.     

Quantitative assessment of evidential weight for a fingerprint comparison I. Generalisation to the comparison of a mark with set of ten prints from a suspect

The authors have published elsewhere a quantitative method for assessing weight of evidence in the case where a finger mark from a crime scene is compared with a control print taken from a single finger of a suspect. The approach is based on the notion of calculating a likelihood ratio (LR) that addresses a pair of propositions relating to the single finger that was the origin of the crime mark. In practice, things are rather different because the crime mark will not just be compared with a single finger from a suspect but with a set of prints from all of his/her fingers; likewise, when the mark is compared with a database, this will consist of ten print records from random individuals. It is clear that "finger propositions" are not realistic in this situation and we show how our approach may be generalised to address a pair of propositions that relate to the person that made the crime mark. It often is the case that information is present at the crime scene that enables some inference to be drawn relating to which of the offender's ten fingers left a particular mark of interest. This kind of inference may profitably be drawn into the formal analysis. We illustrate our approach with an example.     

Inferring ethnic origin by means of an STR profile

We report the use of DNA profiles from six STR loci for inferring the ethnic origin of a crime stain and discuss how such inference may be used as intelligence information to reduce the expected number of interviews to resolve a case. To enable this work, databases have been created for five British ethnic groups (Caucasians, Afro-Caribbeans, Indian sub-continentals, Southeast Asians and Middle Easterners) that together comprise 99.7% of the UK population.     

Establishing the most appropriate databases for addressing source level propositions.

Previous papers in Science & Justice have described the work of the Case Assessment and Interpretation (CAI) project that has been running for several years within the Forensic Science Service (FSS). The principles of the CAI model, which have developed through casework, are the foundation of a balanced, robust and logical approach to interpretation. The question arises frequently as to what is the most appropriate database that should be available to assist in assigning a value to a given probability. In this paper we present a set of guidelines in the form of flowcharts and explore them within the context of a range of case examples.     

Paternity calculations from DNA multilocus profiles

DNA profiles prepared using multilocus probes can provide valuable evidence in cases of disputed parentage. This paper describes a simple approach to the analysis of the profiles from a mother, child and putative father in a case of disputed paternity. Although the treatment has not yet been extended to deal with cases in which mutant bands may have appeared, it has already proved useful in casework. The analysis is illustrated by a numerical example calculated on a computer spreadsheet.