Variability in analyst decisions during the computation of numerical likelihood ratios
Issued Date: 18 Feb 2015
Abstract
This study assesses the extent to which likelihood ratios (LRs) are affected by analyst decisions regarding the number of reference speakers, number of tokens per speaker, and degree of linguistic match between the suspect-offender and the reference data. Using F1 and F2 trajectories from spontaneous /uː/ vowels, LRs were computed against a reference set of up to 120 speakers, and between 2 and 13 tokens per speaker. LR scores proved robust when using more than 30 speakers, although validity improved as the sample size increased. Regional dialect mismatch was found to overestimate same-speaker scores by an average of one order of log10 magnitude. For different-speaker pairs, dialect mismatch generally resulted in more false hits. The results indicate that small amounts of reference data should generally be avoided, and that an awareness of the complexity of sociophonetic variation is essential in the definition of the relevant population for voice comparison.
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References
Alderman, T. (2004) The Bernard data set as a reference distribution for Bayesian likelihood ratio-based forensic speaker identification using formants. Proceedings of the 10th Australian International conference on Speech Science and Technology. 8-10 December 2004, Sydney, Australia. 510-515.
Aitken, C. G. G. and Stoney, D. A. (1991) The Use of Statistics in Forensic Science. London: Ellis Horwood.
Aitken, C. G. G. and Lucy, D. (2004) Evaluation of trace evidence in the form of multivariate
data. Applied Statistics 54: 109-122.
Aitken, C. G. G. and Taroni, F. (2004) Statistics and the Evaluation of Evidence for Forensic Scientists (2nd edition). Chichester: John Wiley.
Ash, S. (1996) Freedom of movement: /uw/-fronting in the Midwest. In J. Arnold, R. Blake, B. Davidson, S. T. Schwentner and J. Soloman (eds.) Sociolinguistic Variation: Data, Theory and Analysis – Selected Papers from NWAV 23 at Stanford. Stanford CA: Centre for the Study of Language and Information (CSLI) Publications, Stanford University. 3-23.
Bernard, J. R. (1967) Some measurements of some sounds of Australian English. PhD Dissertation, University of Sydney.
Broeders, A. P. A. (1999) Some observations on the use of probability scales in forensic identification. Forensic Linguistics 6(2): 228-241.
Byrne, C. & Foulkes, P. (2004) The mobile phone effect on vowel formants. International Journal of Speech, Language and the Law 11, 83-102.
Champod, C. and Evett, I. W. (2000) Commentary on A.P.A. Broeders (1999) ‘Some observations on the use of probability scales in forensic identification’. Forensic Linguistics 7(2): 238-243.
Chambers, J. K. (2008) Sociolinguistic Theory. Oxford: Wiley-Blackwell.
Champod, C. and Meuwly, D. (2000) The inference of identity in forensic speaker recognition. Speech Communication 31: 193-203.
Cheshire, J. (2002) Sex and gender in variationist research. In J. K. Chambers, P. Trudgill and N. Schilling-Estes (eds.) Handbook of Language Variation and Change. Oxford: Blackwell. 423-443.
Clermont, F. and Mokhtari, P. (1998) Acoustic-articulatory evaluation of the upper vowel-formant region and its presumed speaker-specific potency. Proceedings of the 5th International Conference of Spoken Language Processing (vol. 2). Sydney, Australia. 527-530.
Easton, L. and Bauer, L. (2000) An acoustic study of the vowels of New Zealand English. Australian Journal of Linguistics 20(2): 93-117.
Eckert, P. (1989) The whole woman: Sex and gender differences in variation. Language Variation and Change 1: 245-268.
Eriksson, E., Cepeda, L. F., Rodman, R. D., McAllister, D. F. and Bitzer, D. (2004) Cross-language speaker identification using spectral moments. Proceedings of the XVIIth Swedish Phonetics Conference FONETIK. 76-79.
Foulkes, P., Docherty, G. J. and Jones, M. J. (2010) Analysing stops. In M. Di Paolo and M. Yaeger-Dror. Sociophonetics: A Student’s Guide. London. Routledge.
Foulkes, P. and French, J. P. (2012) Forensic speaker comparison: a linguistic-acoustic perspective. In P. Tiersma & L. Solan (eds.) Oxford Handbook of Language and Law. Oxford: Oxford University Press. 557-572.
Foulkes, P., Scobbie, J. and Watt, D. (2010) Sociophonetics. In W. Hardcastle, J. Laver & F. Gibbon (eds.) Handbook of Phonetic Sciences (2nd ed.). Oxford: Blackwell. 703-754.
Fraser, J. and Williams, R. (2009) The Handbook of Forensic Sciences. London: Willan.
French, J. P., Nolan, F., Foulkes, P., Harrison, P. and McDougall, K. (2010) The UK position statement on forensic speaker comparison: a rejoinder to Rose and Morrison. International Journal of Speech, Language and the Law 17, 143-152.
Gold. E and Hughes, V. (2013) Issues and opportunities for the application of the numerical likelihood ratio framework to forensic speaker comparison. Paper presented at the International Association of Forensic Phonetics and Acoustics (IAFPA) Conference. University of South Florida, USA. 21-24 July 2013.
Gordon, E., Maclagan, M. and Hay, J. (2007) The ONZE corpus. In J. C. Beal, K. P. and H. Moisl (eds.) Models and Methods in the Handling of Unconventional Digital Corpra: Volume 2, Diachronic Copora. London: Palgrave. 82-104.
Greisbach, R., Osser, E. and Weinstock, C. (1995) Speaker identification by formant contours. In A. Braun and J. Köstner (eds.) Studies in Forensic Phonetics. Beiträge zur Phonetik und Linguistik 64. Trier: Wissenschaftlicher Verlag Trier. 49-55.
Haddican, B. (2008-2013) A comparative study of language change in northern Englishes. Economic and Social Research Council (ESRC) of Great Britain. RES-061-25-0033.
Haddican, B., Foulkes, P., Hughes, V. and Richards, H. (2013) Interaction of social and linguistic constraints on two vowel changes in northern England. Language Variation and Change 25(3): 371-403.
Hall-Lew, L. (2005) One shift, two groups: When fronting alone is not enough. University of
Pennsylvania Working Papers in Linguistics 10(2): 105-116.
Hawkins, S. and Midgley, J. (2005) Formant Frequencies of RP monophthongs in four age-groups of speakers. Journal of International Phonetic Association 35(2): 183—199.
Hay, J., Maclagan, M. and Gordon, E. (2008) Dialects of English: New Zealand English. Edinburgh: Edinburgh University Press.
Hughes, V. (in progress) The effects of variability on the outcome of numerical likelihood ratios for forensic speaker comparison. PhD in progress. University of York, UK.
Hughes, V., Foulkes, P., Haddican, B. and Richards, H. (2011) Vowel variation in Manchester: a dynamic approach. Paper presented at the 8th UK Conference on Language Variation and Change (UKLVC8). Edge Hill University, UK. 12-14 September 2011.
Hughes, V. and Foulkes, P. (2013) Sociolinguistic considerations in the default definition of the relevant population when computing numerical likelihood ratios. Paper presentated at the International Association of Forensic Phonetics and Acoustics (IAFPA) conference. University of South Florida, USA. 21-24 July 2013.
Hughes, V., Brereton, A. and Gold, E. (submitted) Sample size and the computation of numerical likelihood ratios using articulation rate. York Papers in Linguistics.
Ingram, J. C. L., Prandolini, R. and Ong, S. (1996) Formant trajectories as indices of phonetic variation for speaker identification. Forensic Linguistics 3(1): 129-145.
Ishihara, S. and Kinoshita, Y. (2008) How many do we need? Exploration of the Population Size Effect on the performance of forensic speaker classification. Paper presented at the 9th Annual Conference of the International Speech Communication Association (Interspeech). Brisbane, Australia. 1941-1944.
Johnson, K. (2003). Acoustic and auditory phonetics (2nd edition). Oxford: Blackwell.
Jones, D. (1966) The Pronunciation of English (4th Edition). Cambridge: Cambridge University Press.
Kahn, J. (2009) Race, genes and justice: a call to reform the presentation of forensic DNA evidence in criminal trials. Brooklyn Law Review 74(2): 325-375.
Kaye, D. H. (2004) Logical relevance: problems with the reference population and DNA mixtures in People v. Pizarro. Law, Probability and Risk 3: 211-220.
Kaye, D. H. (2008) DNA probabilities in People v. Prince: When are racial and ethnic statistics relevant? In T. Speed and D. Nolan (eds.) Probability and Statistics: Essays in Honour of David A Freedman. Beachwood, OH: Institute of Mathematical Statistics. 289-301.
Kinoshita, Y. and Ishihara, S. (2012) The effect of sample size on the performance of likelihood ratio-based forensic voice comparison. Proceedings of the 14th Australasian International Conference on Speech Science and Technology. Sydney, Australia. 3-6 December 2012.
Künzel, H. J. (2001) Beware of the ‘telephone effect’: the influence of telephone transmission on the measurement of formant frequencies. Forensic Linguistics 8(1): 80–99.
Labov, W. (1971) The study of language in its social context. In J. A. Fishman (ed.) Advances in the Sociology of Language (vol. 1). The Hague: Mouton. 152-216.
Lewontin, R. C. and Hartl, D. L. (1991) Population genetics in forensic DNA typing. Science 254(5039): 1745-1750.
Loakes, D. (2006) A forensic phonetic investigation into the speech patterns of identical and non-identical twins. PhD Dissertation, University of Melbourne.
McDougall, K. (2004) Speaker-specific formant dynamics: an experiment on Australian English /aɪ/. International Journal of Speech, Language and the Law 11(1): 103-130.
McDougall, K. (2006) Dynamic features of speech and the characterisation of speakers: towards a new approach using formant frequencies. International Journal of Speech, Language and the Law 13(1): 89-126.
McDougall, K. and Nolan, F. (2007) Discrimination of speakers using the formant dynamics of /uː/ in British English. In J. Trouvain and W. J. Barry (eds.) Proceedings of the 16th International Congress of Phonetic Sciences. 6-10 August 2007, Saarbrücken: Universität des Saarlandes. 1825-1828.
Milroy, L., Milroy, J. and Docherty, G. (1994-1997) Phonological Variation and Change in Contemporary British English. Economic and Social Research Council (ESRC) of Great Britain. RES-R000234892.
Morrison, G. S. (2007) MatLab implementation of Aitken and Lucy’s (2004) forensic likelihood-ratio software using multivariate-kernel density estimation. Retrieved on 6 June 2011 from: http://geoff-morrison.net/Software/multivar_kernel_LR.m
Morrison, G. S. (2010) Forensic voice comparison. In I. Freckleton and H. Selby (eds.) Expert Evidence (Ch. 99). Sydney: Thomson Reuters.
Morrison, G. S. (2011) A comparison of procedures for the calculation of forensic likelihood ratios from acoustic-phonetic data: multivariate kernel density (MVKD) versus Gaussian mixture model-universal background model (GMM-UBM). Speech Communication 53: 242-256.
Nolan, F. (2001) Speaker identification evidence: its forms, limitations, and roles. Proceedings of the Law and Language: Prospect and Retrospect Conference. 12-15 December 2001, Levi, Finland.
Robertson, B. and Vignaux, G. A. (1995) Interpreting Evidence: Evaluating Forensic Science in the Courtroom. Chichester: John Wiley.
Rose, P. (2004) Technical forensic speaker identification from a Bayesian linguist's perspective. Keynote paper, Forensic Speaker Recognition Workshop, Speaker Odyssey ’04. 31 May-3 June 2004, Toledo, Spain. 3-10.
Rose, P. (2010) Bernard’s 18 - vowel inventory size and strength of forensic voice comparison evidence. Proceedings of the 13th Australian International Conference on Speech and Technology. Melbourne, Australia. 14-16 December 2010. 30-33.
Rose, P. (2011) Forensic voice comparison with Japanese vowel acoustics – a likelihood ratio-based approach using segmental cepstra. Proceedings of the 17th International Congress of Phonetic Sciences. 17-21 August 2011, Hong Kong. 1718-1721.
Rose, P. (2012) The likelihood ratio goes to Monte Carlo: the effect of reference sample size on likelihood ratio estimates. Paper presented at the UNSW Forensic Speech Science Conference. Sydney, Australia. 3 December 2012.
Rose, P. (2013) More is better: likelihood ratio-based forensic voice comparison with vocalic segmental cepstra frontends. International Journal of Speech, Language and the Law 20(1): 77-116.
Rose, P., Kinoshita, Y. and Alderman, T. (2006) Realistic extrinsic forensic speaker discrimination with the diphthong /aɪ/. Proceedings of the 11th Australasian International Conference on Speech Science and Technology. University of Auckland, New Zealand, 6-8 December 2006. 329-334.
Tabachnick, B. G. and Fidell, L. S. (2007) Using Multivariate Statistics (5th Edition). New York: Harper Collins.
Tagliamonte, S. (1996-1998) Roots of identity: variation and grammaticalisation in contemporary British English. Economic and Social Research Council (ESRC) of Great Britain. RES-R000221842.
Torgersen, E. N. and Kerswill, P. (2004) Internal and external motivation in phonetic change: dialect levelling outcomes for an English vowel shift. Journal of Sociolinguistics 8(1): 23-53.
Wardaugh, R. (2010) An Introduction to Sociolinguistics (6th ed). Oxford: Wiley-Blackwell.
Wells, J. C. (1982) The Accents of English (3 vols). Cambridge: Cambridge University Press.
Zhang, C., Morrison, G. S. and Thiruvaran, T. (2011) Forensic voice comparison using Chinese /iau/. Proceedings of the 17th International Congress of Phonetic Sciences. 17-21 August 2011, Hong Kong. 2280-2283.
Aitken, C. G. G. and Stoney, D. A. (1991) The Use of Statistics in Forensic Science. London: Ellis Horwood.
Aitken, C. G. G. and Lucy, D. (2004) Evaluation of trace evidence in the form of multivariate
data. Applied Statistics 54: 109-122.
Aitken, C. G. G. and Taroni, F. (2004) Statistics and the Evaluation of Evidence for Forensic Scientists (2nd edition). Chichester: John Wiley.
Ash, S. (1996) Freedom of movement: /uw/-fronting in the Midwest. In J. Arnold, R. Blake, B. Davidson, S. T. Schwentner and J. Soloman (eds.) Sociolinguistic Variation: Data, Theory and Analysis – Selected Papers from NWAV 23 at Stanford. Stanford CA: Centre for the Study of Language and Information (CSLI) Publications, Stanford University. 3-23.
Bernard, J. R. (1967) Some measurements of some sounds of Australian English. PhD Dissertation, University of Sydney.
Broeders, A. P. A. (1999) Some observations on the use of probability scales in forensic identification. Forensic Linguistics 6(2): 228-241.
Byrne, C. & Foulkes, P. (2004) The mobile phone effect on vowel formants. International Journal of Speech, Language and the Law 11, 83-102.
Champod, C. and Evett, I. W. (2000) Commentary on A.P.A. Broeders (1999) ‘Some observations on the use of probability scales in forensic identification’. Forensic Linguistics 7(2): 238-243.
Chambers, J. K. (2008) Sociolinguistic Theory. Oxford: Wiley-Blackwell.
Champod, C. and Meuwly, D. (2000) The inference of identity in forensic speaker recognition. Speech Communication 31: 193-203.
Cheshire, J. (2002) Sex and gender in variationist research. In J. K. Chambers, P. Trudgill and N. Schilling-Estes (eds.) Handbook of Language Variation and Change. Oxford: Blackwell. 423-443.
Clermont, F. and Mokhtari, P. (1998) Acoustic-articulatory evaluation of the upper vowel-formant region and its presumed speaker-specific potency. Proceedings of the 5th International Conference of Spoken Language Processing (vol. 2). Sydney, Australia. 527-530.
Easton, L. and Bauer, L. (2000) An acoustic study of the vowels of New Zealand English. Australian Journal of Linguistics 20(2): 93-117.
Eckert, P. (1989) The whole woman: Sex and gender differences in variation. Language Variation and Change 1: 245-268.
Eriksson, E., Cepeda, L. F., Rodman, R. D., McAllister, D. F. and Bitzer, D. (2004) Cross-language speaker identification using spectral moments. Proceedings of the XVIIth Swedish Phonetics Conference FONETIK. 76-79.
Foulkes, P., Docherty, G. J. and Jones, M. J. (2010) Analysing stops. In M. Di Paolo and M. Yaeger-Dror. Sociophonetics: A Student’s Guide. London. Routledge.
Foulkes, P. and French, J. P. (2012) Forensic speaker comparison: a linguistic-acoustic perspective. In P. Tiersma & L. Solan (eds.) Oxford Handbook of Language and Law. Oxford: Oxford University Press. 557-572.
Foulkes, P., Scobbie, J. and Watt, D. (2010) Sociophonetics. In W. Hardcastle, J. Laver & F. Gibbon (eds.) Handbook of Phonetic Sciences (2nd ed.). Oxford: Blackwell. 703-754.
Fraser, J. and Williams, R. (2009) The Handbook of Forensic Sciences. London: Willan.
French, J. P., Nolan, F., Foulkes, P., Harrison, P. and McDougall, K. (2010) The UK position statement on forensic speaker comparison: a rejoinder to Rose and Morrison. International Journal of Speech, Language and the Law 17, 143-152.
Gold. E and Hughes, V. (2013) Issues and opportunities for the application of the numerical likelihood ratio framework to forensic speaker comparison. Paper presented at the International Association of Forensic Phonetics and Acoustics (IAFPA) Conference. University of South Florida, USA. 21-24 July 2013.
Gordon, E., Maclagan, M. and Hay, J. (2007) The ONZE corpus. In J. C. Beal, K. P. and H. Moisl (eds.) Models and Methods in the Handling of Unconventional Digital Corpra: Volume 2, Diachronic Copora. London: Palgrave. 82-104.
Greisbach, R., Osser, E. and Weinstock, C. (1995) Speaker identification by formant contours. In A. Braun and J. Köstner (eds.) Studies in Forensic Phonetics. Beiträge zur Phonetik und Linguistik 64. Trier: Wissenschaftlicher Verlag Trier. 49-55.
Haddican, B. (2008-2013) A comparative study of language change in northern Englishes. Economic and Social Research Council (ESRC) of Great Britain. RES-061-25-0033.
Haddican, B., Foulkes, P., Hughes, V. and Richards, H. (2013) Interaction of social and linguistic constraints on two vowel changes in northern England. Language Variation and Change 25(3): 371-403.
Hall-Lew, L. (2005) One shift, two groups: When fronting alone is not enough. University of
Pennsylvania Working Papers in Linguistics 10(2): 105-116.
Hawkins, S. and Midgley, J. (2005) Formant Frequencies of RP monophthongs in four age-groups of speakers. Journal of International Phonetic Association 35(2): 183—199.
Hay, J., Maclagan, M. and Gordon, E. (2008) Dialects of English: New Zealand English. Edinburgh: Edinburgh University Press.
Hughes, V. (in progress) The effects of variability on the outcome of numerical likelihood ratios for forensic speaker comparison. PhD in progress. University of York, UK.
Hughes, V., Foulkes, P., Haddican, B. and Richards, H. (2011) Vowel variation in Manchester: a dynamic approach. Paper presented at the 8th UK Conference on Language Variation and Change (UKLVC8). Edge Hill University, UK. 12-14 September 2011.
Hughes, V. and Foulkes, P. (2013) Sociolinguistic considerations in the default definition of the relevant population when computing numerical likelihood ratios. Paper presentated at the International Association of Forensic Phonetics and Acoustics (IAFPA) conference. University of South Florida, USA. 21-24 July 2013.
Hughes, V., Brereton, A. and Gold, E. (submitted) Sample size and the computation of numerical likelihood ratios using articulation rate. York Papers in Linguistics.
Ingram, J. C. L., Prandolini, R. and Ong, S. (1996) Formant trajectories as indices of phonetic variation for speaker identification. Forensic Linguistics 3(1): 129-145.
Ishihara, S. and Kinoshita, Y. (2008) How many do we need? Exploration of the Population Size Effect on the performance of forensic speaker classification. Paper presented at the 9th Annual Conference of the International Speech Communication Association (Interspeech). Brisbane, Australia. 1941-1944.
Johnson, K. (2003). Acoustic and auditory phonetics (2nd edition). Oxford: Blackwell.
Jones, D. (1966) The Pronunciation of English (4th Edition). Cambridge: Cambridge University Press.
Kahn, J. (2009) Race, genes and justice: a call to reform the presentation of forensic DNA evidence in criminal trials. Brooklyn Law Review 74(2): 325-375.
Kaye, D. H. (2004) Logical relevance: problems with the reference population and DNA mixtures in People v. Pizarro. Law, Probability and Risk 3: 211-220.
Kaye, D. H. (2008) DNA probabilities in People v. Prince: When are racial and ethnic statistics relevant? In T. Speed and D. Nolan (eds.) Probability and Statistics: Essays in Honour of David A Freedman. Beachwood, OH: Institute of Mathematical Statistics. 289-301.
Kinoshita, Y. and Ishihara, S. (2012) The effect of sample size on the performance of likelihood ratio-based forensic voice comparison. Proceedings of the 14th Australasian International Conference on Speech Science and Technology. Sydney, Australia. 3-6 December 2012.
Künzel, H. J. (2001) Beware of the ‘telephone effect’: the influence of telephone transmission on the measurement of formant frequencies. Forensic Linguistics 8(1): 80–99.
Labov, W. (1971) The study of language in its social context. In J. A. Fishman (ed.) Advances in the Sociology of Language (vol. 1). The Hague: Mouton. 152-216.
Lewontin, R. C. and Hartl, D. L. (1991) Population genetics in forensic DNA typing. Science 254(5039): 1745-1750.
Loakes, D. (2006) A forensic phonetic investigation into the speech patterns of identical and non-identical twins. PhD Dissertation, University of Melbourne.
McDougall, K. (2004) Speaker-specific formant dynamics: an experiment on Australian English /aɪ/. International Journal of Speech, Language and the Law 11(1): 103-130.
McDougall, K. (2006) Dynamic features of speech and the characterisation of speakers: towards a new approach using formant frequencies. International Journal of Speech, Language and the Law 13(1): 89-126.
McDougall, K. and Nolan, F. (2007) Discrimination of speakers using the formant dynamics of /uː/ in British English. In J. Trouvain and W. J. Barry (eds.) Proceedings of the 16th International Congress of Phonetic Sciences. 6-10 August 2007, Saarbrücken: Universität des Saarlandes. 1825-1828.
Milroy, L., Milroy, J. and Docherty, G. (1994-1997) Phonological Variation and Change in Contemporary British English. Economic and Social Research Council (ESRC) of Great Britain. RES-R000234892.
Morrison, G. S. (2007) MatLab implementation of Aitken and Lucy’s (2004) forensic likelihood-ratio software using multivariate-kernel density estimation. Retrieved on 6 June 2011 from: http://geoff-morrison.net/Software/multivar_kernel_LR.m
Morrison, G. S. (2010) Forensic voice comparison. In I. Freckleton and H. Selby (eds.) Expert Evidence (Ch. 99). Sydney: Thomson Reuters.
Morrison, G. S. (2011) A comparison of procedures for the calculation of forensic likelihood ratios from acoustic-phonetic data: multivariate kernel density (MVKD) versus Gaussian mixture model-universal background model (GMM-UBM). Speech Communication 53: 242-256.
Nolan, F. (2001) Speaker identification evidence: its forms, limitations, and roles. Proceedings of the Law and Language: Prospect and Retrospect Conference. 12-15 December 2001, Levi, Finland.
Robertson, B. and Vignaux, G. A. (1995) Interpreting Evidence: Evaluating Forensic Science in the Courtroom. Chichester: John Wiley.
Rose, P. (2004) Technical forensic speaker identification from a Bayesian linguist's perspective. Keynote paper, Forensic Speaker Recognition Workshop, Speaker Odyssey ’04. 31 May-3 June 2004, Toledo, Spain. 3-10.
Rose, P. (2010) Bernard’s 18 - vowel inventory size and strength of forensic voice comparison evidence. Proceedings of the 13th Australian International Conference on Speech and Technology. Melbourne, Australia. 14-16 December 2010. 30-33.
Rose, P. (2011) Forensic voice comparison with Japanese vowel acoustics – a likelihood ratio-based approach using segmental cepstra. Proceedings of the 17th International Congress of Phonetic Sciences. 17-21 August 2011, Hong Kong. 1718-1721.
Rose, P. (2012) The likelihood ratio goes to Monte Carlo: the effect of reference sample size on likelihood ratio estimates. Paper presented at the UNSW Forensic Speech Science Conference. Sydney, Australia. 3 December 2012.
Rose, P. (2013) More is better: likelihood ratio-based forensic voice comparison with vocalic segmental cepstra frontends. International Journal of Speech, Language and the Law 20(1): 77-116.
Rose, P., Kinoshita, Y. and Alderman, T. (2006) Realistic extrinsic forensic speaker discrimination with the diphthong /aɪ/. Proceedings of the 11th Australasian International Conference on Speech Science and Technology. University of Auckland, New Zealand, 6-8 December 2006. 329-334.
Tabachnick, B. G. and Fidell, L. S. (2007) Using Multivariate Statistics (5th Edition). New York: Harper Collins.
Tagliamonte, S. (1996-1998) Roots of identity: variation and grammaticalisation in contemporary British English. Economic and Social Research Council (ESRC) of Great Britain. RES-R000221842.
Torgersen, E. N. and Kerswill, P. (2004) Internal and external motivation in phonetic change: dialect levelling outcomes for an English vowel shift. Journal of Sociolinguistics 8(1): 23-53.
Wardaugh, R. (2010) An Introduction to Sociolinguistics (6th ed). Oxford: Wiley-Blackwell.
Wells, J. C. (1982) The Accents of English (3 vols). Cambridge: Cambridge University Press.
Zhang, C., Morrison, G. S. and Thiruvaran, T. (2011) Forensic voice comparison using Chinese /iau/. Proceedings of the 17th International Congress of Phonetic Sciences. 17-21 August 2011, Hong Kong. 2280-2283.
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