Using the likelihood ratio in bloodstain pattern analysis

Daniel Attinger,Kris De Brabanter,Christophe Champod

doi:10.1111/1556-4029.14899

Daniel Attinger, Kris De Brabanter + Show 1 more

Open Access

https://doi.org/10.1111/1556-4029.14899

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Abstract

There is an apparent paradox that the likelihood ratio (LR) approach is an appropriate measure of the weight of evidence when forensic findings have to be evaluated in court, while it is typically not used by bloodstain pattern analysis (BPA) experts. This commentary evaluates how the scope and methods of BPA relate to several types of evaluative propositions and methods to which LRs are applicable. As a result of this evaluation, we show how specificities in scope (BPA being about activities rather than source identification), gaps in the underlying science base, and the reliance on a wide range of methods render the use of LRs in BPA more complex than in some other forensic disciplines. Three directions are identified for BPA research and training, which would facilitate and widen the use of LRs: research in the underlying physics; the development of a culture of data sharing; and the development of training material on the required statistical background. An example of how recent fluid dynamics research in BPA can lead to the use of LR is provided. We conclude that an LR framework is fully applicable to BPA, provided methodic efforts and significant developments occur along the three outlined directions.

Highlights

| INTRODUCTIONThere is a current trend to evaluate findings based on the concept of likelihood ratio (LR)
In evaluative reporting, there is a current trend to evaluate findings based on the concept of likelihood ratio (LR)
Recent guidelines recommending the use of LR have been issued by the UK Association of Forensic Science Providers (AFSP) [1], adapted by the European Network of Forensic Science Institutes [2], the National Institute of Forensic Science, Australia and New Zealand (NIFS) [3], and recently advised by the UK Forensic Science Regulator (FSR), the UK Charted Society of Forensic Science, and the Royal

Summary

| INTRODUCTION

There is a current trend to evaluate findings based on the concept of likelihood ratio (LR). For some areas (such as glass fragments, fibers, or footwear marks), the knowledge associated within- and between-sources variability is informed by structured data and documented knowledge At this point, it is important to stress that we are discussing the likelihood ratio approach as an overarching method useful to help the interpretation of forensic observations. The term “less complex” is not used to mean that any forensic evaluation is trivial, but it indicates that in some activities of forensic science, the methods of evaluation are established and well documented, while in others, practitioners have to resort on their personal experience through methods that are less documented in the peer-reviewed literature In this commentary, we limit the discussion to the first two types of evaluative statements, to which LRs have been applied: source-based and activity-based. TA B L E 1 List of topics of investigation and goals of BPA analysis, with methods used and type of evaluative statements

Methods used

Evaluation of the volume of a stained area

Evaluation of the same

| SUMMARY AND CONCLUSION