Cited by 47Statistical Methods and Bayesian InferenceLegal processes and jurisprudenceStatistical Methods and Applications

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Avoiding overstating the strength of forensic evidence: Shrunk likelihood ratios/Bayes factors

Open access
Morrison, Geoffrey Stewart, Poh, Norman·2018·Science & Justice

Abstract

When strength of forensic evidence is quantified using sample data and statistical models, a concern may be raised as to whether the output of a model overestimates the strength of evidence. This is particularly the case when the amount of sample data is small, and hence sampling variability is high. This concern is related to concern about precision. This paper describes, explores, and tests three procedures which shrink the value of the likelihood ratio or Bayes factor toward the neutral value of one. The procedures are: (1) a Bayesian procedure with uninformative priors, (2) use of empirical lower and upper bounds (ELUB), and (3) a novel form of regularized logistic regression. As a benchmark, they are compared with linear discriminant analysis, and in some instances with non-regularized logistic regression. The behaviours of the procedures are explored using Monte Carlo simulated data, and tested on real data from comparisons of voice recordings, face images, and glass fragments.

Overview

AI snapshot

Written from the paper’s full text.

TL;DR

This paper describes and tests three shrinkage procedures—Bayesian uninformative Jeffreys priors, empirical lower and upper bounds (ELUB), and regularized logistic regression—for avoiding overstatement of forensic likelihood ratios/Bayes factors, compared with LDA and non-regularized logistic regression. On simulated and real voice, face, and glass data, shrinkage reduced overestimation but often at a cost in Cllr, and the authors prefer regularized logistic regression for casework when data are sparse or distributional assumptions are violated.

Key findings

  • In Monte Carlo simulations (10 or 100 training samples per category; population means separated by 2 or 4 variance units; 1000 training sets; test set of 1000 per category), the Bayesian procedure outperformed or was not substantially worse than the best procedure across all conditions on RMS error and Cllr; LDA was best when training data were large; ELUB usually performed worst; regularized logistic regression was generally not as good as the Bayesian procedure.
  • For real forensic voice comparison, GMM-UBM scores gave Cllr values of 0.410 (LDA), 0.413 (Bayesian), 0.412 (ELUB), 0.430 (regularized logistic regression), and 0.413 (non-regularized logistic regression); i-vector PLDA scores gave 0.357, 0.374, 0.361, 0.290, and 0.261 respectively. In the sisters case, LDA produced extremely large log likelihood ratios (e.g., up to 3×10^40), Bayesian values reached 4876, ELUB gave 5, and regularized logistic regression gave values around 1.6–26.6.
  • For face-image comparison, GMM-UBM scores gave Cllr values of 0.220 (LDA), 0.215 (Bayesian), 0.206 (ELUB), 0.181 (regularized logistic regression), and 0.169 (non-regularized logistic regression); MLP scores gave 0.167, 0.159, 0.143, 0.115, and 0.104 respectively. Logistic regression gave the best Cllr, and regularization effected shrinkage with a concomitant decrease in accuracy.
  • For glass-fragment comparison, excluding/including very small scores gave Cllr values of 0.391/0.242 (LDA), 0.413/0.236 (Bayesian), 0.389/0.244 (ELUB on LDA), 0.009 (ELUB on tailored model, included only), 0.104/0.111 (regularized logistic regression), and 0.021/0.007 (non-regularized logistic regression). Non-regularized logistic regression performed best; regularized logistic regression was substantially biased in this extreme case.

Methods

  • The paper describes and tests three shrinkage procedures—Bayesian model with uninformative Jeffreys reference priors, ELUB applied to LDA output, and a novel regularized logistic regression using a uniform “prior” weighted by κψ pseudodata points—against LDA as a baseline and, on some datasets, non-regularized logistic regression.
  • Simulated data were Gaussian same-origin and different-origin score distributions with means of +1 and −1 and common variance 1 (2 variance units), or means separated by 4 variance units; 10 or 100 training samples per category were generated, with 1000 training sets and a test set of 1000 points per category. Performance was assessed using RMS error against reference log likelihood ratios and Cllr.
  • Real data came from forensic voice comparison (sisters case with 5 recordings per sister; GMM-UBM and i-vector PLDA scores from 105 speakers for first-stage training and 61 additional speakers for second-stage training/testing, yielding 111 same-speaker and 9720 different-speaker scores), face-image comparison (XM2VTS: 225 faces, 8 images each, 600 same-origin and 40,000 different-origin scores), and glass-fragment comparison (LA-ICP-MS: 979 sources, 659 for first-stage training and 320 for second-stage training/testing, yielding 320 same-origin and 51,040 different-origin scores, of which 41,108 were very small). Cross-validation was used to avoid training and testing on the same data.

Limitations

  • Stated: the choice of κψ for regularized logistic regression is essentially arbitrary; κψ = 5 may have induced greater shrinkage than preferred; regularized logistic regression was biased for the glass data because the extreme variance difference affected not only the slope but also the intercept; only an equal-variance Gaussian Bayesian model with Jeffreys reference priors was tested; Cllr does not account for the utility of not overstating strength of evidence, and no such metric was developed.
  • Stated: procedures including shrinkage were less accurate and had higher Cllr than their non-shrinkage counterparts; ELUB has a cliff-edge truncation effect; the Bayesian procedure was non-monotonic and produced relatively large Bayes factors that may be difficult to justify given small sample data.
  • Evident from the text: the Monte Carlo simulations met the LDA and Bayesian Gaussian equal-variance assumptions, whereas most real datasets violated them; the face-image data used controlled lighting and background and are not reflective of challenging forensic conditions such as CCTV; the sisters case had only 5 recordings per speaker.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1016/j.scijus.2017.12.005

Cites

76 of 76 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Figures

Image 1
CC-BY · From the PDF
Fig. 1 (a) Sample-based Gaussian distributions (solid blue lines), and Bayesian posterior predictive distributions (dashed red lines). (b) Score to log likelihood ratio or score to log Bayes factor transformation functions based on: Sample-based Gaussian distributions (solid blue line), Bayesian posterior predictive distributions (dashed red line), ELUB (dotted green line), and regularized logistic regression (dot-dashed magenta line). For interpretation of the references to colour in fi gure captions, the reader is referred to the electronic version of this article.
Fig. 1 (a) Sample-based Gaussian distributions (solid blue lines), and Bayesian posterior predictive distributions (dashed red lines). (b) Score to log likelihood ratio or score to log Bayes factor transformation functions based on: Sample-based Gaussian distributions (solid blue line), Bayesian posterior predictive distributions (dashed red line), ELUB (dotted green line), and regularized logistic regression (dot-dashed magenta line). For interpretation of the references to colour in fi gure captions, the reader is referred to the electronic version of this article.CC-BY · From the PDF
Fig. 2 (a) Example of logistic regression fi tted without regularization. (b) Example of logistic regression fi tted with a small amount of regularization to avoid numerical problems. (c) Example of logistic regression fi tted with a large amount of regularization to induce shrinkage. The large symbols represent sample data, and the small symbols represent weighted pseudodata with an uninformative uniform distribution.
Fig. 2 (a) Example of logistic regression fi tted without regularization. (b) Example of logistic regression fi tted with a small amount of regularization to avoid numerical problems. (c) Example of logistic regression fi tted with a large amount of regularization to induce shrinkage. The large symbols represent sample data, and the small symbols represent weighted pseudodata with an uninformative uniform distribution.CC-BY · From the PDF
Image 4
CC-BY · From the PDF
Image 5
CC-BY · From the PDF
Fig. 7 Boxplots of RMS error values for each procedure in each condition: (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. Note that the y -axis scale is di ff erent for each panel.
Fig. 7 Boxplots of RMS error values for each procedure in each condition: (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. Note that the y -axis scale is di ff erent for each panel.CC-BY · From the PDF
Fig. 8 Boxplots of C llr values for each procedure in each condition: (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. The dotted lines indicate the C llr values given the reference likelihood ratio values. Note that the y -axis scale is di ff erent for each panel.
Fig. 8 Boxplots of C llr values for each procedure in each condition: (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 10 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 2 variance units. (a) 100 training samples per category and separation between the means in the Monte Carlo population distributions of 4 variance units. The dotted lines indicate the C llr values given the reference likelihood ratio values. Note that the y -axis scale is di ff erent for each panel.CC-BY · From the PDF
Fig. 9 Data from the forensic voice comparison case involving two sisters: CLDF values for the elder-sister recordings (blue circles), younger-sister recordings (red triangles), and the questioned-speaker recording (green cross), along with pooled-variance Gaussian distributions fi tted to the known-speaker data. (For interpretation of the references to colour in this fi gure legend, the reader is referred to the web version of this article.)
Fig. 9 Data from the forensic voice comparison case involving two sisters: CLDF values for the elder-sister recordings (blue circles), younger-sister recordings (red triangles), and the questioned-speaker recording (green cross), along with pooled-variance Gaussian distributions fi tted to the known-speaker data. (For interpretation of the references to colour in this fi gure legend, the reader is referred to the web version of this article.)CC-BY · From the PDF
Fig. 10 Histograms of score data from (a) the GMM-UBM forensic voice comparison system, and (b) the i-vector PLDA forensic voice comparison system. Both systems were applied to the same voice-recording data.
Fig. 10 Histograms of score data from (a) the GMM-UBM forensic voice comparison system, and (b) the i-vector PLDA forensic voice comparison system. Both systems were applied to the same voice-recording data.CC-BY · From the PDF
Fig. 11 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the GMM-UBM forensic voice comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.
Fig. 11 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the GMM-UBM forensic voice comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.CC-BY · From the PDF
Fig. 12 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the i-vector PLDA forensic voice comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.
Fig. 12 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the i-vector PLDA forensic voice comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.CC-BY · From the PDF
Fig. 13 Histograms of score data from (a) the GMM-UBM face-image comparison system, and (b) the MLP face-image comparison system. Both systems were applied to the same faceimage data.
Fig. 13 Histograms of score data from (a) the GMM-UBM face-image comparison system, and (b) the MLP face-image comparison system. Both systems were applied to the same faceimage data.CC-BY · From the PDF
Fig. 14 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the GMM-UBM face-image comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.
Fig. 14 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the GMM-UBM face-image comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.CC-BY · From the PDF
Fig. 15 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the MLP face-image comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.
Fig. 15 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the MLP face-image comparison system. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) Non-regularized logistic regression.CC-BY · From the PDF
Fig. 16 Histograms of score data from the 2-level MVKD system applied to the glassfragment data.
Fig. 16 Histograms of score data from the 2-level MVKD system applied to the glassfragment data.CC-BY · From the PDF
Fig. 17 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the 2-level MVKD system applied to the glass-fragment data. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) ELUB applied to the output of a tailored model. (f) Non-regularized logistic regression. For all panels except (e) the results presented are those when very small score values were excluded from both training and testing.
Fig. 17 Tippett plots resulting from fi tting the score to log likelihood ratio or score to log Bayes factor transformation functions to the score data from the 2-level MVKD system applied to the glass-fragment data. (a) LDA. (b) Bayesian procedure with uninformative Je ff reys reference priors. (c) ELUB applied to LDA output. (d) Regularized logistic regression. (e) ELUB applied to the output of a tailored model. (f) Non-regularized logistic regression. For all panels except (e) the results presented are those when very small score values were excluded from both training and testing.CC-BY · From the PDF

Verification

Claims the paper makes

41 Own result · 27 No basis given · 26 Own interpretation · 11 Sources not found · 4 Own method

Show all 56 claims

  • It may state two separate claims as one. 38
  • The quote only partly states this claim. 3
  • Probably not a substantial claim. 15
53 of 53
  • The authors state this as an assumption.

    The sample data used in the Bayesian model are from a Gaussian distribution with unknown mean and variance.

    “using sample data assumed to be from a Gaussian distribution with unknown mean and variance”
  • The authors state this as an assumption.

    The models fitted to the population distributions are appropriate.

    “assuming models appropriate for fitting the population distributions”
  • The authors state this as an assumption.

    The data used for LDA have Gaussian distributions with equal variance.

    “If the assumptions for LDA hold that the data have Gaussian distributions with equal variance”
  • The paper relies on this without support.

    The real data sets used in Section 5 are reflective of casework conditions.

    “In order to explore the generalizability to conditions more reflective of casework, the present section applies the procedures to real data from comparisons of voice recordings, face images, and glass fragments.”
  • The paper relies on this without support.

    The range of amount of sample data and range of separation between categories used in the simulations is sufficient to understand the relative behaviour of the procedures and to interpolate and extrapolate within and beyond these ranges.

    “We believe that this range of amount of sample data and range of separation between the categories is sufficient to gain an understanding of the relative behaviour of the procedures and to conceptually interpolate and extrapolate within and beyond these ranges.”
  • The paper relies on this without support.

    LDA and Bayesian procedures depend on assumptions of Gaussian distributions with equal variance.

    “LDA and the Bayesian procedure depend on assumptions of Gaussian distributions with equal variance”
  • The paper relies on this without support.

    Deviations from Gaussian distributions with equal variance are common in score data.

    “Since deviations from these distributional assumptions are common in score data, we prefer regularized logistic regression in general.”
  • The authors state this as an assumption.

    The analysis assumes only one consequential misleading likelihood ratio at LR_th.

    “We have simplified by assuming that we only add one consequential misleading likelihood ratio at LR th .”
  • The authors state this as an assumption.

    The Bayesian procedure model assumed equal-variance Gaussian distributions.

    “For simplicity in the present paper, we only tested a model which assumed equal-variance Gaussian distributions and used Jeffreys reference priors.”
9 of 9

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Metadata

Authors
Morrison, G. S., & Poh, N.
Affiliations
  • Aston University (GB)
  • University of Surrey (GB)
Corresponding author
Morrison, Geoffrey Stewart
Venue
Science & Justice
Publisher
Elsevier BV
Published
2018-05
Volume
58
Issue
3
Pages
200-218
ISSN
1355-0306 (ISSN-L)
OpenAlex
W2777942909
Cited by
47
References
74
Citations by year
2026: 5 · 2025: 3 · 2024: 12 · 2023: 4 · 2022: 6 · 2021: 6 · 2020: 6 · 2019: 4 · 2018: 1
FWCI
1.86
Type
journal article
Language
en
Open access
Locations
  • Science & Justice (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • Aston Publications Explorer (Aston University) (submittedVersion)
Funders
EPSRC (EP/K032208/1)
Topics
  • Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods and Bayesian Inference
  • Social Sciences › Law › Legal processes and jurisprudence
  • Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods and Applications
Keywords
likelihood ratios, Bayes factor, shrinkage, regularized logistic regression, non-informative priors, small sample size, sampling variability, precision, voice comparison, linear discriminant analysis

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