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       1Uncertainty in Forensic Science

1.1 Introduction

The purpose of this book is to discuss the statistical and probabilistic evaluation of scientific evidence for forensic scientists. A formal definition of evidence is given in Section 1.4. For the most part the evidence to be evaluated will be the so‐called transfer or (physical) trace evidence, but the general logic also applies to other types of evidence, such as digital, pattern, or testimonial evidence.

      There is a well‐known principle in forensic science known as Locard's principle, which states that every contact leaves a trace (Locard 1920).

       [

This is translated as (Inman and Rudin 2001)

       [

      The principle was reiterated using different words in Locard (1929). This has been translated by the same author in 1930. Locard (1930) wrote

       For the microscopic debris that covers our clothes and bodies are the mute witnesses, sure and faithful, of all our movements and of all our encounters. (p. 276)

Transfer evidence and Locard's principle may be illustrated as follows. Suppose a person gains entry to a house by breaking a window and assaults the man of the house, during which assault blood is spilt by both victim and assailant. The criminal may leave traces of their presence at the crime scene in the form of bloodstains from the assault and fibres from his clothing. This evidence is said to be transferred from the criminal to the scene of the crime. The criminal may also take traces of the crime scene away with them. These could include bloodstains from the assault victim, fibres of their clothes, and fragments of glass from the broken window. Such evidence is said to be transferred to the criminal from the crime scene. A person of interest¹ (PoI) is soon identified, at a time at which they will not have had the opportunity to change their clothing. The forensic scientists examining the PoI's clothing find similarities amongst all the different types of evidence: blood, fibres, and glass fragments. They wish to evaluate this evidence. It is hoped that this book will enable them so to do.

      However, for evaluation, it is not only similarity that is important but also the rarity of the characteristics of interest. Hence, quantitative issues relating to the distribution of these characteristics will be discussed. However, there will also be discussion of qualitative issues such as the choice of a suitable population against which variability in the measurements of the characteristics of interest may be compared. Also, a brief history of statistical aspects of the evaluation of evidence is given in Chapter 3 .

1.2 Statistics and the Law

The book does not focus on the use of statistics and probabilistic thinking for legal decision making, other than by occasional reference. Also, neither the role of statistical experts as expert witnesses presenting statistical assessments of data nor their role as consultants preparing analyses for counsel is discussed. There is a distinction between these two issues (Fienberg 1989, Tribe 1971). The main focus of this book is on the assessment of evidence for forensic scientists, in particular for identification purposes. The process of addressing the issue of whether or not a particular item came from a particular source is most properly termed individualization. ‘Criminalistics is the science of individualization’ (at p. 236) as defined by Kirk (1963) but established forensic and judicial practices have led to it being termed identification. The latter terminology will be used throughout this book. An identification, however, is more correctly defined as ‘the determination of some set to which an object belongs or the determination as to whether an object belongs to a given set’ (Kingston 1965a). Further discussion is given by Kwan (1977), Evett et al. (1998), and Champod et al. (2016b). For a critical discussion of individualisation as a decision, see Cole (2014), Biedermann et al. (2008a), and Saks and Koehler (2008). More details are given in Section 2.5.9.

      For example, in a case involving a broken window, similarities may be found between the refractive indices of fragments of glass found on the clothing of a PoI and the refractive indices of fragments of glass from the broken window. The assessment of this evidence, in consideration of the association or otherwise of the PoI with the scene of the crime, is part of the focus of this book.

      For those interested in the issues of statistics and the law beyond those of forensic science, in the sense used in this book, there are several books available and some of these are discussed briefly.

‘The evolving role of statistical assessments as evidence

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