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1 UCL. 14 Mar 05 UCL Monday 14 Mar 2005 Forensic inference – is the law a ass?  The Forensic Science Service 2004 Ian Evett Forensic Science Service.

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Presentation on theme: "1 UCL. 14 Mar 05 UCL Monday 14 Mar 2005 Forensic inference – is the law a ass?  The Forensic Science Service 2004 Ian Evett Forensic Science Service."— Presentation transcript:

1 1 UCL. 14 Mar 05 UCL Monday 14 Mar 2005 Forensic inference – is the law a ass?  The Forensic Science Service 2004 Ian Evett Forensic Science Service

2 2 UCL. 14 Mar 05 “If the law supposes that,” said Mr. Bumble... “the law is a ass – a idiot.” Charles Dickens: Oliver Twist ch 51.

3 3 UCL. 14 Mar 05 Match What is the probability that the crime profile would match the defendant if some unknown person had left it? Either: the defendant left the crime stain Or: some unknown person left the crime stain What is the probability that the crime profile would match the defendant if he had left it?

4 4 UCL. 14 Mar 05 What is the probability that the crime profile would match the defendant if he had left it? If the profile is not a mixture, then this probability is, effectively, one It is important to bear in mind that in more complex cases - such as paternity and mixture cases - this probability is substantially less than one and need careful consideration

5 5 UCL. 14 Mar 05 What is the probability that the crime profile would match the defendant if some unknown person had left it? Statisticians agree that this is correctly termed a match probability

6 6 UCL. 14 Mar 05 DNA profiling is a model for the evolution for all scientific evidence Ideally, we would like to see our opinions supported by statistics in all cases. However, it is clear that the courts are poorly equipped to ensure that the statistics are presented to the jury in a method that best informs their decision making.

7 7 UCL. 14 Mar 05 Match DNA The probability of finding that profile if the crime stain had come from someone else is 1 in a billion But the court is interested in the probability that the crime stain was left by the defendant

8 8 UCL. 14 Mar 05 That probability depends not just on the scientific evidence......but also on whatever other evidence there is in the case But the court is interested in the probability that the crime stain was left by the defendant

9 9 UCL. 14 Mar 05 There are various ways of setting the scientific evidence in the context of a trial The prosecutor’s fallacy Doheny and Adams judgment Probability of another person Prior/posterior illustrations The probability of finding that profile if the crime stain had come from someone else is 1 in a billion

10 10 UCL. 14 Mar 05 Match So it follows that there is a 1 in a billion probability that the crime stain had come from someone else... The prosecutor’s fallacy Hence it is almost certain that the crime stain came from the defendant This second statement does not follow from the first... This mistake was made in the Deen, Doheny and G. Adams trials The probability of finding that profile if the crime stain had come from someone else is 1 in a billion

11 11 UCL. 14 Mar 05 Match The likelihood of it being anybody other than Alan Doheny? R v. Doheny...I calculated the chance of finding all of those bands...to be about 1 in 40 million Is about 1 in 40 million.

12 12 UCL. 14 Mar 05 Is it possible that the semen could have come from a different person from the person who provided the blood sample? I can estimate the chances of this semen having come from a man other than the provider of the blood sample. R v. G Adams Match It is possible but it is so unlikely as to really not be credible. I can work out the chances as being less than 1 in 27 million

13 13 UCL. 14 Mar 05 Match Doheny and Adams judgment When the scientist gives evidence it is important that he should not overstep the line which separates his province from that of the jury He will properly explain to the Jury the nature of the match... He will...give the Jury the random occurrence ratio – the frequency with which the matching DNA characteristics are likely to be found in the population at large

14 14 UCL. 14 Mar 05 Match Doheny and Adams judgment Note that it is difficult to imagine a case in which it is reasonable to consider every man, woman and child in the UK to be a possible perpretrator......nevertheless, let us see where this leads to Provided he has the necessary data...it may be appropriate for him then to say how many people with matching characteristics...are likely to be found in the UK

15 15 UCL. 14 Mar 05 Match How many other people in the UK are likely to have the same profile? Approximately one twentieth of a person It is difficult to believe that statements about fractions of people are helpful to the jury Doheny and Adams judgment The probability of finding that profile if the crime stain had come from someone else is 1 in a billion

16 16 UCL. 14 Mar 05 Doheny and Adams judgment Problems with the Doheny and Adams prescription Conflicting evidence: eyewitness, alibi, geography, other scientific evidence Concept of pool of equiprobable suspects Fractional people Relations of the defendant

17 17 UCL. 14 Mar 05 Match Doheny and Adams judgment When the scientist gives evidence it is important that he should not overstep the line which separates his province from that of the jury The scientist should not be asked his opinion on the likelihood that it was the Defendant who left the crime stain, nor when giving evidence should he use terminology which may lead the jury to believe that he is expressing such an opinion

18 18 UCL. 14 Mar 05 Match The evidence is a million times more probable if the first proposition is true Either: the defendant left the crime stain Or: some unknown person left the crime stain This is extremely strong support for the first proposition That statement is counter to the Doheny/Adams judgment.

19 19 UCL. 14 Mar 05 Either: the defendant wrote the questioned handwriting Or: some unknown person wrote it. In my opinion, there is extremely strong support for the first proposition Thank you, Dr Bloggs, that is very helpful.

20 20 UCL. 14 Mar 05 In my opinion, the crime mark and control print were made by the same person. Thank you, Mr Holmes - that is amazingly helpful.

21 21 UCL. 14 Mar 05 R v. Dennis John Adams Rape: Hemel Hempstead DNA match between profile from vaginal swab and Mr Adams Match probability 1 in 200 million Victim said that Mr Adams did not look like the rapist Mr Adams had an alibi

22 22 UCL. 14 Mar 05 R v. Dennis John Adams The central problem in this case was that of combining statistically based scientific evidence in support of the prosecution......with non scientific evidence in support of the defence

23 23 UCL. 14 Mar 05 There were two trials and two appeals. R v. Dennis John Adams To cut a long story short......defence argued that the problem should be approached by inviting the jury to quantify their assessment of the non- scientific evidence...then combining all of the various elements by means of Bayes’ theorem

24 24 UCL. 14 Mar 05 R v. Dennis John Adams Judgment: First appeal It seems to us that the difficulties which arise in the present case stem from the fact that, at trial, the defence were permitted to lead before the jury evidence of the Bayes theorem. But we have very grave doubt as to whether that evidence was properly admissible, because it trespasses on an area peculiarly and exclusively within the province of the jury, namely the way in which they evaluate the relationship between one piece of evidence and another.

25 25 UCL. 14 Mar 05 R v. Dennis John Adams Judgment: First appeal...the attempt to determine guilt or innocence on the basis of a mathematical formula, applied to each separate piece of evidence, is simply inappropriate to the jury’s task. Jurors evaluate evidence... by the joint application of their individual common sense and knowledge of the world...

26 26 UCL. 14 Mar 05 R. v. D.J. Adams Retrial Defence argued that the non-DNA evidence should be evaluated numerically. All evidence should be combined using Bayes’ theorem Questionnaire prepared and given to the jury

27 27 UCL. 14 Mar 05 R. v. D.J. Adams Retrial An example from the questionnaire: Bearing in mind that Ms Marley said that Mr Adams appeared to be in his early forties what is the probability that she would say that her assailant was in his early twenties if Mr Adams were indeed the assailant?

28 28 UCL. 14 Mar 05 R. v. D.J. Adams Retrial Mr Adams was convicted. Mr Adams appealed

29 29 UCL. 14 Mar 05 R v. Dennis John Adams Judgment: Second appeal...the jury...would have to ask themselves whether they were satisfied that only X white European men in the UK would have a DNA profile matching that of the rapist who left the crime stain It would be a matter for the jury, having heard the evidence, to give a value of X.

30 30 UCL. 14 Mar 05 Match The probability of finding that profile if the crime stain had come from someone else is 1 in 200 million How many adult white males in the UK would have that profile? Dennis Adams judgment Approximately one tenth of a man

31 31 UCL. 14 Mar 05 R v. Dennis John Adams X is approximately one tenth of a man It would be a matter for the jury, having heard the evidence, to give a value of X. They would then have to ask themselves whether they were satisfied that the defendant was one of those men. Well, that’s pretty straightforward, isn’t it? Well, that’s pretty straightforward, isn’t it?

32 32 UCL. 14 Mar 05 R v. Dennis John Adams...consideration of the case along the lines indicated would, in our judgment, reflect a normal course for a properly instructed jury to adopt.... It is the sort of task which juries perform every day. This appears to be a deliberate instruction for juries to reason irrationally

33 33 UCL. 14 Mar 05 The present position If an expert is unable to carry out a statistical assessment of the weight of the evidence* then courts allow him to express an opinion of identity of source * e.g. fingerprints, handwriting, toolmarks, footwear If an expert is able to carry out a statistical assessment of the weight of the evidence** then courts will not allow him to express an opinion of identity of source - the statistic is put to the jury ** DNA profiling

34 34 UCL. 14 Mar 05 It is our hope, as forensic scientists, that expert opinions will increasingly be based on databases and sound statistical methods The DNA experience suggests that if the weight of evidence can be measured by a numerical statistic then it is the statistic that should be put to the jury, rather than an expert opinion of identity of source

35 35 UCL. 14 Mar 05 Most forensic scientists have no problem with this view... Nevertheless, it would seem desirable that we should explore with the judiciary and the legal profession ways of presenting statistical evidence......then it is the statistic that should be put to the jury, rather than an expert opinion of identity of source...in a manner that is balanced, robust and logical

36 36 UCL. 14 Mar 05 And present illustrations based on a range of priors agreed by the court The most rational compromise would be to explain the meaning of the likelihood ratio

37 37 UCL. 14 Mar 05 That’s all folks!


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