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

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event that the area of Lithuania is greater than . Assign probabilities to each of these four events. Remember that these are four mutually exclusive events and that the four probabilities should add up to 1. Which do you consider the most probable and what probability do you assign to it? Which do you consider the least probable and what probability do you assign to it?

      2 Now, consider the information that Lithuania is the 25th largest country in Europe (excluding Russia). Use this information to reconsider your probabilities in part (a).

      3 Consider the information that Estonia, which is the 30th largest country in Europe, has an area of , and use it to reconsider your probabilities from the previous part.

      4 Consider the information that Austria, which is the 21st largest country in Europe has an area of , and use it to reconsider your probabilities from the previous part.

      The area of Lithuania is given at the end of the chapter.

       Example 1.5

      1 Imagine you are on a jury. The trial is about to begin but no evidence has been led. Consider the two events: the defendant is truly guilty; and , the defendant is innocent. What are your probabilities for these two events?

      2 The defendant is a tall Caucasian male. An eyewitness says he saw a tall Caucasian male running from the scene of the crime. What are your probabilities now for and ?

      3 A bloodstain at the scene of the crime was identified as coming from the criminal. A partial DNA profile has been obtained, with proportion 2% in the local Caucasian population. What are your probabilities now for and ?

      4 A window was broken during the commission of the crime. Fragments of glass were found on the defendant's clothing of a similar refractive index to that of the crime window. What are your probabilities now for and ?

      5 The defendant works as a demolition worker near to the crime scene. Windows on the demolition site have refractive indices similar to the crime window. What are your probabilities now for and ?

      This example is designed to mimic the presentation of evidence in a court case. Part (a) asks for a prior probability of guilt before the presentation of any evidence. It may be considered as a question concerning the understanding of the dictum ‘innocent until proven guilty’. See Section 2.7 for further discussion of this with particular reference to the logical problem created if a prior probability of zero is assigned to the event that the suspect is guilty.

      In part (c) it is necessary to check that the defendant has the same profile. It is not stated that he has but if he has not he should never have been a defendant. Secondly, is the local Caucasian population the relevant population? The evaluation of evidence of the form in (c) is discussed in Chapter 5.

      The evaluation of refractive index measurements mentioned in (d) is discussed in Chapter 7 . Variation both within and between windows has to be considered. Finally, how information about the defendant's lifestyle may be considered is discussed in Chapter 6 .

      It should be noted that the questions asked initially in Example 1.5 are questions that should be addressed by the judge and/or jury. The forensic scientist is concerned with the evaluation of their evidence, not with probabilities of guilt or innocence. These probabilities are the concern of the jury. The jury combines the evidence of the scientist with all other evidence and uses its judgement to reach a verdict. The theme of this book is the evaluation of evidence. Discussion of issues relating to guilt or otherwise of PoIs will not be very detailed.

      As a tail piece to this chapter, the area of Lithuania is images images.

      1 1 In earlier editions the term suspect was used in this context. It is now felt that the term person of interest is a more accurate description of the status of the person. The term suspect will still be used if use of ‘person of interest’ would be clumsy. The term defendant will only be used if the context is clearly that of a court.

      2 2 There is a recent critical discussion on the use of tests of significance. See, for example, Wasserstein et al. (2019), Amrhein et al. (2019), Ioannidis (2019), Haaf et al. (2019), and Johnson (2019). Note that such a discussion dates back to 1986 (Kaye 1986a) with a later update (Kaye 2017c).

      3 3 The use of the word trial here is a statistical one and is not to be confused with the legal use. In a statistical use, a trial is a particular event, such as the toss of a coin. More details are in the Appendix A.2.2.

      4 4 The term correspond is used here rather than the more commonly used match. The term ‘match’ suggests certainty or identity which is not always the case with trace evidence. Use of the term ‘correspond’ emphasises this and is a reminder to be careful with interpretation. See Section 2.5.11 and Friedman (1996) for further comment.

      5 5 The 2015 Guidelines for evaluative reporting can be found at http://enfsi.eu/wp-content/uploads/2016/09/m1_guideline.pdf.

      6 6 Note the use of the capitalised words ‘You’ and ‘Your’ in this quotation. This is a rhetorical device to help readers keep in mind that probabilities are their own degrees of belief based on the information they have at the time they make the judgement.

      7 7 Part of this sentence is also reported by Kadane and Schum (1996, p. 160).

      8 8 English version of the paper reprinted in Monari and Cocchi (1993).

      9 9 Note that the use of the term ‘sample’ in this context is one of a purely technical nature in statistics and has nothing to do with the widespread but inappropriate use of the same term for designating physical trace material recovered or collected in a forensic science context. In particular, the seizure (e.g. at crime scenes) and analysis of trace material has to deal with the material as it is, irrespective of its condition; there is no such thing as randomisation, for example.

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