Читать книгу Statistics and the Evaluation of Evidence for Forensic Scientists - Franco Taroni - Страница 22

It is necessary to have clear definitions of certain terms. First note that the term evidence is generally used in the literature and in practice rather than terms such as finding, outcome, or material. The term evidence may be confusing. In some legal contexts it can refer to a judicial qualification of a finding. Forensic scientists are interested in the probative value of an observation before it qualifies as evidence in a trial. The European Networks of Forensic Science Institutes (ENFSI) Guideline for evaluative reporting (ENFSI 2015) provides definitions of both terms (pp. 19–20). Despite the risk of confusion, the term evidence will be used for the material examined by the scientist and for which the value is required.

The crime scene and suspect (associated with a PoI) materials have fundamentally different roles. The assignment of a probability for a correspondence between two randomly chosen sets of materials is not the important issue. One set of materials, crime scene or suspect, can be assumed to have a known source. It is then required to assess the probability of the corresponding material, suspect or crime scene, corresponding in some sense, to the known set of materials, under two competing hypotheses. Examples 1.1 and 1.2 serve to illustrate this.

Example 1.1. (continued) A crime is committed. A bloodstain is found at the scene of the crime. All innocent explanations for the presence of the stain are eliminated. A PoI is found. Their DNA profile is found to match that of the crime stain. The crime scene material is the crime stain. The suspect material is a swab with saliva. The finding to evaluate is the observed correspondence between the DNA profiles of the crime scene and suspect materials.

Example 1.2 (continued) As before, consider the investigation of a crime. A window has been broken during the commission of the crime. Several fragments are taken for investigation and measurements made of their refractive indices. These fragments, as their origin is known, are sometimes known as control fragments and the corresponding measurements are known as control measurements. A PoI is found. Fragments of glass are found on their person and measurements of the refractive indices of these fragments are made. These fragments are sometimes known as recovered fragments. Their origin is not known. They may have come from the window broken at the crime scene but need not necessarily have done so.

The crime scene material is the fragments of glass and the measurements of refractive index of these at the scene of the crime. The suspect material is the fragments of glass found on the PoI and their refractive index measurements.

Evidence where the source is known will be known as source evidence. These fragments of glass will be known as source fragments and the corresponding measurements will be known as source measurements, as their source is known. An alternative term for this type of evidence was bulk source evidence (Stoney 1991a) but this terminology appears to have fallen into disuse.

A PoI is identified. Fragments of glass are found on their person and measurements of the refractive indices of these fragments are made. Evidence such as this where the evidence has been received and is in particulate form will be known as transferred particle evidence. The fragments of glass in this example will be known as transferred particle fragments. Their origin is not known. They have been received from somewhere by the PoI. They are particles that have been transferred to the PoI from somewhere. They may have come from the window broken at the crime scene but need not necessarily have done so.

There will also be occasion to refer to the location at which, or the person on which, the evidence was found. Material found at the scene of the crime will be referred to as crime evidence. Material found on the suspect's clothing or in the suspect's natural environment, such as their home, will be referred to as suspect evidence. Note that this does not mean that the evidence itself is of a suspect nature!

Locard's principle (see Section 1.1) is that every contact leaves a trace. In the earlier examples the contact is that of the criminal with the crime scene. In Example 1.1, the trace is the bloodstain at the crime scene. In Example 1.2, the trace is the fragments of glass that would be removed from the crime scene by the criminal (and, later, hopefully, be found on their clothing).

The evidence in both examples is transfer evidence (see Section 1.1) or sometimes trace evidence. Material has been transferred between the criminal and the scene of the crime. In Example 1.1 blood has been transferred from the criminal to the scene of the crime. In Example 1.2 fragments of glass may have been transferred from the scene of the crime to the criminal. Note that the direction of transfer in these two examples is different. Also, in the first example the blood at the crime scene has been identified as coming from the criminal. Transfer is known to have taken place. In the second example it is not known that glass has been transferred from the scene of the crime to the criminal. The PoI has glass fragments on his clothing but these need not necessarily have come from the scene of the crime. Indeed if the PoI is innocent and has no connection with the crime, the fragments will not have come from the crime scene.

The term control evidence has been used to indicate the material whose origin is known. Similarly, the term recovered has been used to indicate the material whose origin is unknown.

Alternatively, questioned has been used for ‘recovered’. See, for example, Brown and Cropp (1987). Also Kind et al. (1979) used the word crime for material known to be associated with a crime and questioned for material thought to be associated with a crime. All these terms are ambiguous. The need to distinguish the various objects or persons associated with a crime was pointed out by Stoney (1984).

Definitions given in the particular context of fibre evidence are provided also by Champod and Taroni (1999). The object or person on which traces have been recovered is defined as the receptor, and the object or person that could be the source (or one of the sources) of the traces, and which is the origin of the material defined as known material, is defined as the known source.

Material will be referred to as control form where appropriate and to recovered or transferred particle form where appropriate. In the previous examples, there are two possibilities for the origin of the material which is taken to be known: the scene of the crime and the PoI. One or other is taken to be known, the other to be unknown. The two sets of material are compared. Two probabilities for what is assumed known are determined. One depends on an assumption of common source. The other depends on an assumption of different sources. The two possibilities for the origin of the material that is taken to be known are called scene‐anchored and suspect‐anchored, where the word ‘anchored’ refers to that which is assumed known (Stoney 1991a). The distinction between scene‐anchoring and suspect‐anchoring is important when determining relevant probabilities (Section 5.3.1.4); it is not so important in the determination of likelihood ratios or Bayes' factors (see Section 2.3.2). Reference to form (source or receptor) is a reference to one of the two parts of the evidence. Reference to anchoring (scene or suspect) is a reference to a perspective for the evaluation of the evidence.

It is sometimes useful to refer to material found at the scene of a crime as the crime scene item and to material found on or about a PoI as the suspect item. This terminology reflects the site at which the material was found. It does not indicate the kind of material (bulk or transferred particle form) or the perspective (scene – or suspect – anchored) by which the evidence will be evaluated.