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Assessing typicality in forensic voice comparison: how can sociophonetics help? (and how can sociophonetics benefit?)

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Hughes, V. and Wormald, J. (2017) Assessing typicality in forensic voice comparison: how can sociophonetics help? (and how can sociophonetics benefit?). Paper presented at the Innovative Methods in Sociophonetics II Workshop (Satellite of the 4th Workshop on Sound Change). University of Edinburgh. 19th April 2017.

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Assessing typicality in forensic voice comparison: how can sociophonetics help? (and how can sociophonetics benefit?)

  1. 1. Assessing typicality in forensic voice comparison how can sociophonetics help? (and how can sociophoneticsbenefit?) Vincent Hughes Jessica Wormald
  2. 2. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Overview • What’s the problem? • Why is it a problem? • What are the solutions? • Why bother? 2
  3. 3. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 What’s the problem? 3
  4. 4. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 What’s the problem? 4 vs. known suspectunknown offender
  5. 5. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 • Voice = extremely difficult form of forensic evidence 5 1. Inherent limitations of the voice as a biometric 2. Forensically realistic materials What’s the problem?
  6. 6. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 • Speaker factors o Sources of within-speaker variation o e.g. modalities, emotion, intoxication • Technical factors o Telephone transmission o Mobile phone codecs • Situational factors o Overlapping speech o Background noise 6 What’s the problem?
  7. 7. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Forensic Voice Comparison (FVC) 1. Linguistic-phoneticapproach • Application of phonetics and sociolinguistics in forensic context • ‘Traditional’ approach • Most common method 7 What’s the problem?
  8. 8. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Forensic Voice Comparison (FVC) 2. Automatic approach • Developed within speech technology • Features of short term power spectrum (e.g. MFCCs) - holistic approach • Not currently admissible in England and Wales 8 What’s the problem?
  9. 9. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Right. Yeah, he mentioned … Right. Mm actually I don't have my driver’s licence. From Leeds yeah. Canada. No, that's what I thought it was. F- f- I felt it. I thought I had it. (?) no I did not steal the car. No, I don't have it, mate. Don't have it, mate. Are you sure it's coming out stolen? Did it come out? Wh- wh- what do y- what did you go by? (Yeah). Where was it stolen? What's he need now? Can you roll this window down a bit, I'm just claustrophobic. I'm on medication for it. Pardon? Can you let me out for a sec, I've got- I'm on medication for claustrophobia and anxiety. Pardon? Just- just the window down or something. What we waiting on? What we waiting on? Got to wait for vehicle recovery? 9 What’s the problem?
  10. 10. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Hey mate can you give me an update on the rugby union um the um Munster versus Trevis(o)- T X thirty five thirty one five. And eh college basketball uh Florida versus Louisville. And could you tell me how much time’s left please? Florida and Louisville. OK cheers mate. Hey mate. College basketball. Oregon. T X thirty five thirty one five. (that’s ?) uh just gimme an update how much time’s left. So Oregon… you know… you guys are betting on it. Seventy fifty two. Cheers mate. Hello mate. Could you- can you give me an update on the uh Gwent versus Leinster rugby union score? T X thirty five thirty one five. Right… 10 What’s the problem?
  11. 11. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 11 What’s the problem? Interesting features… • Rhoticity • Intervocalic (t) tapped • Monophthongal FACE • Monophthongal GOAT
  12. 12. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 • Unusual mix of US and Yorkshire features o consistency across samples o extremely small number of people who could have produced this evidence • David Bieber, a.k.a. Nathan Coleman o voice evidence crucial o denied murder - no comment interview o sentence to life imprisonment 12Thanks to Dom Watt for these slides What’s the problem?
  13. 13. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 13 • Strength of evidence o Similarity and typicality • Example: foot mark found at a crime scene o Size 6 o Size 13 How does this affect the strength of evidence? What’s the problem?
  14. 14. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Why is this a problem? 14
  15. 15. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Why is this a problem? How to assess the ‘typicality’ of voice features? In the past… • Eminence • Experience See e.g. Ross (2016) 15
  16. 16. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 • Paradigm Shift in forensic science (Saks and Koehler 2005) o Replicability o Scientifically defensible o Objectivity o Data-driven 16 Why is this a problem?
  17. 17. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 17 Two essential elements of the paradigm shift: • Robust estimation of typicality o Defensible statistics based on population data o Use of appropriate conclusion framework (i.e. likelihood ratios) o Not the role of the expert to ‘identify’ a speaker • Validation (i.e. error rates) o Demonstrate to the court that the methods do the job you say they do Why is this a problem?
  18. 18. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 So what’s stopping us? 18
  19. 19. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 What’s stopping us? Few available corpora • Corpora generally not collected for forensic purposes: o Some exceptions (e.g. DyViS)… o But even these are limited • Not enough focus on forensically realistic conditions • Not enough coverage of regional/social varieties • Not enough coverage of L2/ non-native varieties 19
  20. 20. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Limitations of availablecorpora • Sociolinguistic corpora: o Small o Not easily searchable (tagged) o Static (although exceptions – e.g. ONZE) o Not forensically realistic • Speech technology corpora: o Large and somewhat realistic, but… o Insufficient coverage of regional/social variation 20 What’s stopping us?
  21. 21. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Not enough population statistics • Specific features: • F0 distributions • (Long term) formant patterns • Realisations of different phonemes (acoustic + auditory) • Variation in voice quality • Within- vs. between-speaker variability • Technical variations • … 21 What’s stopping us?
  22. 22. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Absence of up-to-date descriptions of varieties • Not ‘fashionable’ (at least for English) o Some exceptions e.g. illustrations of the IPA o Usually based on one speaker • Understandable focus on theory and individual variables • Forensics: Reliance on experience or out-of-date descriptions: o e.g. Survey of English Dialects, Wells (1982)… 22 What’s stopping us?
  23. 23. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 What are the solutions? 23
  24. 24. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 What are the solutions? • Data sharing and generation of a platform for uploading recordings and transcriptions o Similar in principle to SLAAP, ONZE etc. and FAVE suite o SPADE – new projects o More general remit – community sourced (and not static) • Why bother? o Facilitates large scale socio projects o More robust statistical analyses (larger samples) o Improvement in automatic methods (see Brown et al.) 24
  25. 25. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 • Adapt data collection methods o Capture more ‘real world’ scenarios o Realistic forensic conditions o e.g. multiple recordings per speaker, technical factors • Why bother? o Better understand within-speaker variability o Better understand variation and change in real world conditions o Distinguish between patterns due to situational factors and those due to identity construction 25 What are the solutions?
  26. 26. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 • Write descriptions of language varieties o Many are outdated and based on few speakers o Up-to-date comprehensive descriptions of varieties o Journal of English Varieties • Why bother? o We need baseline data of expected patterns in order to understand variation and change o Useful for other disciplines too, e.g. SLT 26 What are the solutions?
  27. 27. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 forensics + sociolinguistics 27
  28. 28. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 The future… • Regularly updated and searchable corpora • Access to empirical reference data • Journal of descriptions of varieties • Methodological developments 28
  29. 29. Assessing typicality in forensic voice comparison IMS II - 19th April 2017 Conclusions • Greater collaboration between forensics and socio/phonetics o More essential than ever: o Paradigm shift o Automatic systems • Reciprocal benefits o Better understanding of real world variation and change o Group level patterns and how individual fits in this o Essential for development of sociolinguistics o Impact 29
  30. 30. Thanks! Any questions? vincent.hughes@york.ac.uk jessica.wormald@jpfrench.com @J_Wormald 30

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