Franco Taroni - Statistics and the Evaluation of Evidence for Forensic Scientists

The PoI may offer an alternative explanation. The jury can then assign a probability to the occurrence of the evidence, given that explanation. The two propositions to be considered would then be

: the blood was transferred during the commission of the crime;

: the explanation of the PoI is true,

and the jury could assess the evidence of the existence of transfer under these two propositions. Evaluation of the evidence of the DNA profile frequencies would be additional to this. The two parts could then be combined using the technique described in Section 5.3.2.

In the absence of an explanation from the PoI, the forensic scientist could conduct a survey of persons as similar as possible to the PoI in whatever are the key features of their behaviour or lifestyle. The survey would be conducted with respect to the PoI since it is of interest to learn about the transfer of bloodstains for people with their background. In a particular case, it may be that a survey of people of a violent background is needed. One example is that of Briggs (1978) in which 122 suspects who were largely vagrants, alcoholics, and violent sexual deviants were studied. The nature and lifestyle of the PoI determine the type of population to survey. Buckleton et al. (1991) reported also the work of Fong and Inami (1986) in which clothing items from persons of interest, predominantly in offences against the person, were searched exhaustively for fibres that were subsequently grouped and identified.

The idea of a relevant population is a very important one and is discussed further in Section 6.1.1 following the development proposed by Champod et al. (2004). Consider the example of offender profiling, one which is not strictly speaking forensic science but which is still pertinent during an investigation. Consider the application to rape cases. Suppose the profiler is asked to comment on the offender's lifestyle, such as age, marital status, existence and number of previous convictions, and so on, which the profiler may be able to do. However, it is important to know something about the distribution of these in some general population. The question arises here, as in Buckleton et al. (1991) described earlier, as to what is the relevant population. In rape cases, it may not necessarily be the entire male population of the local community. It could be argued that it might be the population of burglars, not so much because rapists are all burglars first but rather burglars are a larger group of people who commit crimes involving invasion of someone else's living space. Information from control groups is needed, regarding both the distribution of observed traits amongst the general non‐offending population and the distribution of similar offences amongst those without the observed traits. Discussions on relevant population have also been published in legal journals, see, for example, Lempert (1991).

The interpretation of scientific evidence may be thought of as the assessment of a comparison. The comparison is that between the recovered material (denote this by ) and the control material (denote this by ). Denote the combination by . As a first example, consider the bloodstains of Example 1.1. The crime stain is , the recovered evidential material (i.e. evidential material whose source is unknown), and is the genotype of biological material (e.g. blood, saliva swab) taken from the suspect under controlled conditions (i.e. so‐called control material whose source is known). From Example 1.2, suppose glass is broken during the commission of a crime. would be the fragments of glass (the control material) found at the crime scene, would be fragments of glass (the recovered material) found on the clothing of a suspect, and would be the two sets of fragments.

Qualities, such as genotypes, or measurements, such as the refractive indices of glass fragments, are taken from . Comparisons are made of the measurements made on recovered and control material. Denote these by and , respectively, and let denote the combined set. Comparison of and is to be made and the assessment of this comparison has to be quantified. The totality of the evidence is denoted and is such that .

Statistics has developed as a subject in which one of its main concerns is the quantification of the assessments of comparisons. The performance of a new treatment, drug, or fertiliser has to be compared with that of an old treatment, drug, or fertiliser, for example. Two sets of materials, control and recovered, are to be compared. It seems natural that statistics and forensic science should come together, and this has been happening over the last 40 years after strong criticisms from some outstanding quarters. Recall Kirk and Kingston (1964). They remarked that

When we claim that criminalistics is a science, we must be embarrassed, for no science is without some mathematical background, however meagre. This lack must be a matter of primary concern to the educator [ ]. Most, if not all, of the amateurish efforts of all of us to justify our own evidence interpretations have been deficient in mathematical exactness and philosophical understanding. (pp. 435–436)