Variability in analyst decisions during the computation of numerical likelihood ratios


  • Vincent Hughes University of York
  • Paul Foulkes University of York



Likelihood ratio, forensic voice comparison, relevant population, regional variation, sample size


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 /u?/ 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.

Author Biographies

Vincent Hughes, University of York

Vincent Hughes is a post-doctoral Research Assistant on the project Voice and Identity - source, filter, biometric (funded by the UK Arts and Humanities Research Council #AH/M003396/1, 2015 17). He conducted his PhD at the University of York, focusing on the effects of variability on likelihood ratio estimations for forensic voice comparison. He also holds undergraduate (BA German and Linguistics) and postgraduate (MSc Forensic Speech Science) degrees from the University of York. He is a member of the International Association of Forensic Phonetics and Acoustics.

Paul Foulkes, University of York

Paul Foulkes is Professor in the Department of Language and Linguistic Science, University of York. His teaching and research interests include forensic phonetics, laboratory phonology, phonological development, and sociolinguistics. His current collaborators include Cathi Best, Jean-Pierre Chevrot, Gerry Docherty, Bronwen Evans, Peter French, Bill Haddican, Jen Hay, Vincent Hughes, Jason Shaw, Marilyn Vihman, and Kim Wilson. He has worked on over 200 forensic cases from the UK, Ghana, and New Zealand.



How to Cite

Hughes, V., & Foulkes, P. (2015). Variability in analyst decisions during the computation of numerical likelihood ratios. International Journal of Speech, Language and the Law, 21(2), 279–315.




Most read articles by the same author(s)