School Seminar Series - What did Bain really Say?
What did Bain really Say? A forensic analysis of the disputed utterance based on data, acoustic analysis, statistical models, calculation of likelihood ratios, and testing of validity
Geoffrey Stewart MorrisonDirector, Forensic Voice Comparison Laboratory, School of Electrical Engineering & Telecommunications, University of New South Waleshttp://geoff-morrison.net/
Date: 2nd March 2012 (Friday)
Time: 1.30pm
Location: EEG3
The David Bain legal case is (in)famous in New Zealand. In 1995 Bain was convicted of murdering his family. He maintained his innocence and a retrial was eventually held in 2009. He was found not guilty. Prior to the retrial, a police officer thought he heard the words "I shot the prick" in the telephone call that Bain made to the emergency services shortly after the murders. The defence contended that these were not the words spoken. An alternative hypothesis which emerged was that Bain had said "I can't breathe". A number of forensic experts offered their opinion, but all except one focussed on what they heard. Rose, in contrast, pointed out that what anyone hears is irrelevant, what matters is what Bain said, and the latter should be assessed via acoustic analysis not auditory perception. Further, the proper way for a forensic scientist to evaluate the strength of evidence is via a likelihood ratio: What is the probability of getting the acoustic properties of the disputed utterance if Bain had said "I shot the prick" versus what is the probability of getting the acoustic properties of the disputed utterance if he had said "I can't breathe". The key segment of the recording which Rose focussed on was the part immediately following "I". Were the acoustics of this segment more likely if what had been said was a postalveolar fricative (a realisation of the first phoneme in "shot") or were they more likely if what had been said was a palatal fricative (a realisation of the first phoneme in "can't")? Rose concluded that the likelihood of the latter was greater than that of the former, but at the time he did not go as far as building a database, and performing acoustic and statistical analyses to calculate a numeric likelihood ratio, and to test the validity of such procedures. I have now conducted a preliminary version of such an analysis and in this presentation provide a step-by-step exposition of how to calculate and test the validity of a numeric likelihood ratio for the disputed utterance in the Bain case.
Dr Geoffrey Stewart Morrison received his PhD from the University of Alberta in 2006. His dissertation focussed on statistical modelling of speech data. He began working on forensic voice comparison in 2007 as a Research Associate with Dr Philip Rose at the Australian National University. Since 2010 he has been a Senior Researcher and the Director of the Forensic Voice Comparison Laboratory, School of Electrical Engineering & Telecommunications, University of New South Wales. He is also an Invited Lecturer in the Judicial Phonetics Specialisation, Master in Phonetics and Phonology Programme, Spanish National Research Council / MenÈndez Pelayo International University, an Adjunct Associate Professor in the Department of Linguistics, University of Alberta, and is Chair of the Forensic Acoustics Subcommittee of the Acoustical Society of America. In collaboration with the Australian Federal Police, New South Wales Police, Queensland Police, National Institute of Forensic Sciences, Australasian Speech Science and Technology Association, and the Guardia Civil, he is the lead researcher on an Australian Research Council Linkage Project on making demonstrably valid and reliable forensic voice comparison a practical everyday reality in Australia. He has published on forensic voice comparison and on the evaluation of forensic evidence in leading journals in acoustics, speech processing, forensic science, and law.