The effect of variability on the outcome of likelihood ratios

The likelihood ratio (LR) is the “logically and legally correct” (Rose and Morrison 2009:143) framework for the estimation of strength-of-evidence under two competing hypotheses. In forensic voice comparison these considerations are reduced to the similarity and typicality of features across a pair of suspect and offender samples. However, typicality can only be judged against patterns in the relevant population (Aitken and Taroni 2004:206). In calculating numerical LRs typicality is assessed relative to a sub-section of that population. This study considers issues of variability relating to the delimitation of reference data with regard to the number of speakers and number of tokens per speaker. Using polynomial estimations of F1 and F2 trajectories from spontaneous GOOSE (Wells 1982), LR comparisons were performed against a reference set of up to 120 speakers and up to 13 tokens per speaker. Results suggest that mean same-speaker LRs are robust to such variation until the reference data is limited to small numbers of speakers and tokens. However, variance and severity of error may be continually reduced with the inclusion of more data. The definition of the relevant population with regard to regional variety is also assessed. Results for LRs are presented across four sets of test data where only one set matches the reference population for accent. In the absence of differences in levels of within-speaker variation, the magnitude of same-speaker LRs and severity of error are shown to be considerably higher for the ‘mismatch’ test sets. However, results indicate that the removal of regionally-defining acoustic information may reduce the effect of accent divergence between the evidential and reference data. This has positive implications for the application of the numerical LR approach

Front-end approaches to the issue of correlations in forensic speaker comparison

In likelihood ratio (LR)-based forensic speaker comparison it is essential to consider correlations between parameters to accurately estimate the overall strength of the evidence. Current approaches attempt to deal with correlations after the computation of LRs (back-end processing). This paper explores alternative, front-end techniques, which consider the underlying correlation structure of the raw data. Calibrated LRs were computed for a range of parameters commonly analysed in speaker comparisons. LRs were combined using (1) an assumption of independence, (2) the mean, (3) assumptions from phonetic theory, and (4) empirical correlations in the raw data. System (1), based on an assumption of independence, produced the best validity (Cllr = 0.04). Predictably, overall strength of evidence was also highest for system (1), while strength of evidence was weakest using the mean (2). Both systems (3) and (4) performed well achieving Cllr values of ca. 0.09

Public contracts as accountability mechanisms: assuring quality in public healthcare in England and Wales

Contracting in the public sector is designed to enhance the accountability of service providers to their funders. The idea is that quality is improved by the use of service specifications, monitoring of performance and imposition of contractual sanctions. Socio-legal and economic theories of contract indicate that it will be difficult to make and enforce contracts to achieve this. The results of a study of National Health Services contracting in England and Wales are reported. We conclude that contracts alone are not sufficient to improve accountability – collibration of various regulatory measures (including more hierarchical mechanisms such as performance targets) is required

What is the relevant population? Considerations for the computation of likelihood ratios in forensic voice comparison

In forensic voice comparison, it is essential to consider not only the similarity between samples, but also the typicality of the evidence in the relevant population. This is explicit within the likelihood ratio (LR) framework. A significant issue, however, is the definition of the relevant population. This paper explores the complexity of population selection for voice evidence. We evaluate the effects of population specificity in terms of regional background on LR output using combinations of the F1, F2, and F3 trajectories of the diphthong /aɪ/. LRs were computed using development and reference data which were regionally matched (Standard Southern British English) and mixed (general British English) relative to the test data. These conditions reflect the paradox that without knowing who the offender is, it is not possible to know the population of which he is a member. Results show that the more specific population produced stronger evidence and better system validity than the more general definition. However, as region-specific voice features (lower formants) were removed, the difference in the output from the matched and mixed systems was reduced. This shows that the effects of population selection are dependent on the sociolinguistic constraints on the feature analysed

Strength of forensic voice comparison evidence from the acoustics of filled pauses

This study investigates the evidential value of filled pauses (FPs, i.e. um, uh) as variables in forensic voice comparison. FPs for 60 young male speakers of standard southern British English were analysed. The following acoustic properties were analysed: midpoint frequencies of the first three formants in the vocalic portion; ‘dynamic’ characterisations of formant trajectories (i.e. quadratic polynomial equations fitted to nine measurement points over the entire vowel); vowel duration; and nasal duration for um. Likelihood ratio (LR) scores were computed using the Multivariate Kernel Density formula (MVKD; Aitken and Lucy, 2004) and converted to calibrated log10 LRs (LLRs) using logistic-regression (Brümmer et al., 2007). System validity was assessed using both equal error rate (EER) and the log LR cost function (Cllr; Brümmer and du Preez, 2006). The system with the best performance combines dynamic measurements of all three formants with vowel and nasal duration for um, achieving an EER of 4.08% and Cllr of 0.12. In terms of general patterns, um consistently outperformed uh. For um, the formant dynamic systems generated better validity than those based on midpoints, presumably reflecting the additional degree of formant movement in um caused by the transition from vowel to nasal. By contrast, midpoints outperformed dynamics for the more monophthongal uh. Further, the addition of duration (vowel or vowel and nasal) consistently improved system performance. The study supports the view that FPs have excellent potential as variables in forensic voice comparison cases

Strength of forensic voice comparison evidence from the acoustics of filled pauses

This study investigates the evidential value of filled pauses (FPs, i.e. um, uh) as variables in forensic voice comparison. FPs for 60 young male speakers of standard southern British English were analysed. The following acoustic properties were analysed: midpoint frequencies of the first three formants in the vocalic portion; ‘dynamic’ characterisations of formant trajectories (i.e. quadratic polynomial equations fitted to nine measurement points over the entire vowel); vowel duration; and nasal duration for um. Likelihood ratio (LR) scores were computed using the Multivariate Kernel Density formula (MVKD; Aitken and Lucy, 2004) and converted to calibrated log10 LRs (LLRs) using logistic-regression (Brümmer et al., 2007). System validity was assessed using both equal error rate (EER) and the log LR cost function (Cllr; Brümmer and du Preez, 2006). The system with the best performance combines dynamic measurements of all three formants with vowel and nasal duration for um, achieving an EER of 4.08% and Cllr of 0.12. In terms of general patterns, um consistently outperformed uh. For um, the formant dynamic systems generated better validity than those based on midpoints, presumably reflecting the additional degree of formant movement in um caused by the transition from vowel to nasal. By contrast, midpoints outperformed dynamics for the more monophthongal uh. Further, the addition of duration (vowel or vowel and nasal) consistently improved system performance. The study supports the view that FPs have excellent potential as variables in forensic voice comparison cases

Formant dynamics and durations of um improve the performance of automatic speaker recognition systems

We assess the potential improvement in the performance of MFCC-based automatic speaker recognition (ASR) systems with the inclusion of linguistic-phonetic information. Likelihood ratios were computed using MFCCs and the formant trajectories and durations of the hesitation marker um, extracted from recordings of male standard southern British English speakers. Testing was run over 20 replications using randomised sets of speakers. System validity (EER and Cllr) was found to improve with the inclusion of um relative to the baseline ASR across all 20 replications. These results offer support for the growing integration of automatic and linguistic-phonetic methods in forensic voice comparison

Anomalous diffusion in correlated continuous time random walks

We demonstrate that continuous time random walks in which successive waiting times are correlated by Gaussian statistics lead to anomalous diffusion with mean squared displacement ~t^{2/3}. Long-ranged correlations of the waiting times with power-law exponent alpha (0<alpha<=2) give rise to subdiffusion of the form ~t^{alpha/(1+alpha)}. In contrast correlations in the jump lengths are shown to produce superdiffusion. We show that in both cases weak ergodicity breaking occurs. Our results are in excellent agreement with simulations.Comment: 6 pages, 6 figures. Slightly revised version, accepted to J Phys A as a Fast Track Communicatio