Hughes, V. and Foulkes, P. (2014) Defining the relevant population according to class and age when computing numerical likelihood ratios for forensic voice comparison. Paper presented at BAAP 2014 Colloquium, University of Oxford. 7-9 April 2014.
5. 1.0 Introduction
5
• likelihood ratio (LR) involves assessment of both
similarity and typicality
– it matters “whether the values found matching …
are vanishingly rare, or sporadic, or near universal”
(Nolan 2001:16)
• typicality = defined by patterns in the relevant
population (Aitken & Taroni 2004)
– quantified relative to a sample of the population
– outcome = value centered on 1
Hughes & Foulkes
BAAP 2014
12. 2.2 Method
Hughes & Foulkes
BAAP 2014
12
• structure:
– 1 set of test data (multiple pairs of same- (SS) and
different-speaker (DS) pairs of samples
– multiple sets of reference data
• matched with test data for social factor of interest
• mismatched with test data for social factor of interest
• mixed: no control over social factor of interest
• LRs computed using MVKD (Aitken & Lucy 2004)
– transformed using base-10 logarithm (0 = neutral evidence)
15. 2.3 Experiment 3: class & age
Hughes & Foulkes
IAFPA 2013
15
• test set = 29 young professionals
• reference sets:
Condition Speakers Hd
Matched 20 young prof …another young
professional…”
Mixed 5 young prof/5 old prof/
5 young non-prof/ 5 old non-prof
…another man…”
Mismatched
(both)
20 older non-prof …another older non-
professional…”
Mismatched
(class-only)
20 young non-prof …another young non-
professional…”
✓
(-)
✗
✗
16. 16
Hughes & Foulkes
BAAP 2014 16
Raw LR Log10 LR Verbal expression
>10000 4à5 Very strong evidence
1000à10000 3à4 Strong evidence
100à1000 2à3 Moderately strong evidence
10à100 1à2 Moderate evidence
1à10 0à1 Limited evidence
1à0.1 0à-1 Limited evidence
0.1à0.01 -1à-2 Moderate evidence
0.01à0.001 -2à-3 Moderately strong evidence
0.001à0.0001 -3à-4 Strong evidence
<0.0001 -4à-5 Very strong evidence
Champod and Evett (2000)
Hp
Hd
24. 4.0 Discussion
24
• effects on Cllr appear systematic :
– mixed = under optimistic Cllr relative to matched
– mismatch = over optimistic Cllr relative to matched
– age a more significant factor than class
• ‘getting it wrong’ more problematic than
‘keeping it general’
– distributions of LRs and Cllr closer to baseline in
mixed condition than in mismatched
Hughes & Foulkes
BAAP 2014