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identified. Then the numbers of stains for each of the different profiles are examples of discrete, quantitative data. Other examples of discrete quantitative data are the number of glass fragments found on a PoI, or the number of gunshot residue particles on hands.

      The refractive indices and elemental concentrations of glass fragments are examples of continuous measurements. In practice, variables are rarely truly continuous because of the limits imposed by the sensitivity of the measuring instruments. For example, refractive indices may be measured only to a certain number of decimal places.

      Observations, or data, may thus be classified as qualitative or quantitative. Qualitative data may be classified further as nominal or ordinal, and quantitative data may be classified further as discrete or continuous.

1.6 Populations

‘Who is “random man”?’ This is the title of a paper by Buckleton et al. (1991). In order to evaluate evidence, it is necessary to have some idea of the variability or distribution of the evidence under consideration within some population. This population will be called the relevant population (and a more formal definition will be given later in Section 6.1.1) because it is the population that is deemed relevant to the evaluation of the evidence. Variability is important because if the PoI did not commit the crime and is, therefore, assumed innocent it is necessary to be able to determine the probability of associating the evidence with them when they are innocent. Surveys of populations are required in order to obtain this information. Surveys for reference data are regularly published in forensic science or legal medicine journals (e.g. Forensic Science International, Science & Justice, Journal of Forensic Sciences, International Journal of Legal Medicine); they are widely available to the scientific community.

      Care has to be taken when deciding how to choose the relevant population. Buckleton et al. (1991) describe two situations and explain how the relevant population is different for each.

The first situation is one in which there is transfer from the criminal to the crime scene as in Example 1.1 and discussed in greater detail in Section 5.3.1.4. In this situation, the details of any PoI are irrelevant under , the proposition that the PoI was not present at the scene of the crime. Consider a bloodstain at the crime scene that, from the background information , it is possible to assume is of blood from the criminal. If the PoI was not present, then clearly some other person must have left the stain. There is no reason to confine attention to any one group of people. In particular, attention should not be confined only to any group (e.g. ethnic group) to which the PoI belongs. However, if there is some information that might cause one to reconsider the choice of population, then that choice may be modified. Such information may come from an eyewitness, for example, who is able to provide information about the offender's ethnicity. This would then be part of the background information . Further comments on the role of background information may be found in Section 2.4.4. In general, though, information about DNA profile frequencies would be required from a survey, which is representative of all possible offenders. For evidence of DNA profiles, it is known that age is not a factor affecting a person's profile but that ethnicity is. It is necessary to consider the racial composition of the population of possible offenders (not persons of interest). Normally, it is necessary to study a general population since there will be no information available to restrict the population of possible criminals to any particular ethnic group or groups.

The second situation considered by Buckleton et al. (1991) is possible transfer from the crime scene to the PoI or criminal and discussed further in Section 5.3.2.4. The details of the PoI are now relevant even assuming they were not present at the crime scene. Consider the situation where there is a deceased victim who has been stabbed numerous times. A PoI, with a history of violence, has been apprehended with a heavy bloodstain on their jacket that is not of their own blood. What is the evidential value in itself, and not considering possible DNA evidence, of the existence of such a heavy bloodstain, not of the blood of the PoI? The probability of such an event (the existence of a heavy bloodstain) if the PoI did not commit the crime needs to be considered.

      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).

1.7 Probability

      

      1.7.1 Introduction

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 Скачать книгу