The Likelihood-Ratio Framework and Forensic Evidence in Court: A Response to R v T
| Author | Geoffrey Stewart Morrison |
| Date | 01 January 2012 |
| Published date | 01 January 2012 |
| DOI | 10.1350/ijep.2012.16.1.390 |
| Subject Matter | Article |
THE LIKELIHOOD-RATIO FRAMEWORK AND FORENSIC EVIDENCE IN COURT
The likelihood-ratio
framework and forensic
evidence in court: a
response to RvT
Geoffrey Stewart Morrison*
Director, Forensic Voice Comparison Laboratory, School of Electrical
Engineering & Telecommunications, University of New South
Wales, Sydney, Australia
Abstract In RvTthe Court of Appeal concluded that the likelihood-ratio
framework should not be used for the evaluation of evidence except ‘where
there is a firm statistical base’. The present article argues that the court’s
opinion is based on misunderstandings of statistics and of the likelihood-ratio
framework for the evaluation of evidence. The likelihood-ratio framework is a
logical framework and not itself dependent on the use of objective
measurements, databases and statistical models. The ruling is analysed from
the perspective of the new paradigm for forensic-comparison science: the use of
the likelihood-ratio framework for the evaluation of evidence; a strong
preference for the use of objective measurements, databases representative of
the relevant population, and statistical models; and empirical testing of the
validity and reliability of the forensic-comparison system under conditions
reflecting those of the case at trial.
Keywords Bayesian; Bayes; Admissibility; Validity; Reliability
doi:10.1350/ijep.2012.16.1.390
THE INTERNATIONAL JOURNAL OF EVIDENCE & PROOF (2012) 16 E&P 1–29 1
* Email: geoff-morrison@forensic-voice-comparison.net. Parts of this article were part of an invited
presentation, ‘The New Paradigm in Forensic Science’, at the National Judicial College of
Australia’s Expert Evidence Conference, 12–13 February 2011. This research was supported by the
Australian Research Council, the Australian Federal Police, New South Wales Police, Queensland
Police, the National Institute of Forensic Science, the Australasian Speech Science and Technology
Association, and the Guardia Civil viaLinkage ProjectLP100200142. Thanks toCharles E.H. Berger,
David Hamer, Hugh Selby, Cuiling Zhang, two anonymous reviewers, and the editor for comments
on earlier drafts of this article. Unless otherwise explicitly attributed, the opinions expressed
herein are those of the author and do not necessarily represent the policies or opinions of any of
the above-mentioned organisations or individuals.
‘Numeracy is not favoured by British justice.’1
he board of directors of a large corporation realise that there is a major
problem in part of their operations. The board members themselves are
not experts in this particular area so they hire consultants to analyse the
problem and make recommendations for fixing it. They hire some of the top
people in the country, internationally recognised as experts in this particular
area. They have extensive meetings with the consultants where the consultants
not only give their recommendations but carefully explain the rationale behind
them. The board of directors then decides that the consultants are wrong and do
not implement the recommendations. They publish this in the annual report to
shareholders. One would imagine that the board of directors would be called to
account at the next shareholders’ meeting.
In RvT2the Court of Appeal of England and Wales heard from David Baldwin, the
principal scientist at the Forensic Science Service (FSS) responsible for marks and
setting standards, and chair of the European Network of Forensic Science Insti-
tutes’ expert working group on marks, from Justin Lewis, the principal scientist at
the FSS responsible for Case Assessment and Interpretation, and from Andrew
Rennison, the UK’s Forensic Science Regulator.3The court learnt that the
likelihood-ratio framework for the evaluation of evidence was recommended not
only by these experts, but also by a growing number of forensic scientists within
the United Kingdom and that it is part of the Standards for the Formulation of
Evaluative Forensic Science Expert Opinion promulgated by the Association of
Forensic Science Providers (AFSP) of the United Kingdom and Ireland.4It also heard
explanations as to why the likelihood-ratio framework is considered to be the
logically correct framework for the evaluation of evidence. The court stated that
the Forensic Science Regulator had ‘suggested that it was not logical to adopt the
position that the Bayesian or likelihood ratio approach could be used in some
2 THE INTERNATIONAL JOURNAL OF EVIDENCE & PROOF
THE LIKELIHOOD-RATIO FRAMEWORK AND FORENSIC EVIDENCE IN COURT
T
1 D. V. Lindley, ‘Probability and Statistics’ (1977) 27 The Statistician 203 Stable URL:
<http://www.jstor.org/stable/2987898> at 210.
2 [2010] EWCA Crim 2439, [2011] 1 Cr App R 9. The judgment was given in redacted form to protect
the identity of the appellant and is reported in that form. The judgment is delivered by Thomas LJ.
3RvT[2010] EWCA Crim 2439, [2011] 1 Cr App R 9 at [47].
4 Association of Forensic Science Providers, ‘Standards for the Formulation of Evaluative Forensic
Science Expert Opinion’ (2009) 49 Science & Justice 161, doi:10.1016/j.scijus.2009.07.004. RvT[2010]
EWCA Crim 2439, [2011] 1 Cr App R 9 at [52].
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