Critical Analysis of Forensic Cut-Offs and Legal Thresholds: A Coherent Approach to Inference and Decision

Forensic Science International, 2018, 288, 72-80, DOI/10.1016/j.forsciint.2018.04.030

14 Pages Posted: 13 Jun 2018

See all articles by Alex Biedermann

Alex Biedermann

University of Lausanne

Franco Taroni

University of Lausanne

S. Bozza

Ca Foscari University of Venice - Dipartimento di Economia

Marc Augsburger

University of Lausanne

C. G. G. Aitken

University of Edinburgh - School of Mathematics

Date Written: May 27, 2018

Abstract

In this paper we critically discuss the definition and use of cut-off values by forensic scientists, for example in forensic toxicology, and point out when and why such values – and ensuing categorical conclusions – are inappropriate concepts for helping recipients of expert information with their questions of interest. Broadly speaking, a cut-off is a particular value of results of analyses of a target substance (e.g., a toxic substance or one of its metabolites in biological sample from a person of interest), defined in a way such as to enable scientists to suggest conclusions regarding the condition of the person of interest. The extent to which cut-offs can be reliably defined and used is not unanimously agreed within the forensic science community, though many practitioners – especially in operational laboratories – rely on cut-offs for reasons such as ease of use and simplicity. In our analysis, we challenge this practice by arguing that choices made for convenience should not be to the detriment of balance and coherence. To illustrate our discussion, we will choose the example of alcohol markers in hair, used widely by forensic toxicologists to reach conclusions regarding the drinking behaviour of individuals. Using real data from one of the co-authors’ own work and recommendations of cut-offs published by relevant professional organisations, we will point out in what sense cut-offs are incompatible with current evaluative guidelines (e.g., [31]) and show how to proceed logically without cut-offs by using a standard measure for evidential value. Our conclusions run counter to much current practice, but are inevitable given the inherent definitional and conceptual shortcomings of scientific cut-offs. We will also point out the difference between scientific cut-offs and legal thresholds and argue that the latter – but not the former – are justifiable and can be dealt with in logical evaluative procedures.

Keywords: Toxicological Analyses, Forensic Science, Interpretation, Cut-Offs, Legal Thresholds

Suggested Citation

Biedermann, Alex and Taroni, Franco and Bozza, S. and Augsburger, Marc and Aitken, C. G. G., Critical Analysis of Forensic Cut-Offs and Legal Thresholds: A Coherent Approach to Inference and Decision (May 27, 2018). Forensic Science International, 2018, 288, 72-80, DOI/10.1016/j.forsciint.2018.04.030, Available at SSRN: https://ssrn.com/abstract=3185770

Alex Biedermann (Contact Author)

University of Lausanne ( email )

Faculty of Law, Criminal Justice and Public Admin.
Lausanne, Vaud 1015
Switzerland

HOME PAGE: http://www.unil.ch/unisciences/alexbiedermann

Franco Taroni

University of Lausanne ( email )

Quartier Chambronne
Lausanne, Vaud CH-1015
Switzerland

S. Bozza

Ca Foscari University of Venice - Dipartimento di Economia ( email )

Cannaregio 873
Venice, 30121
Italy

Marc Augsburger

University of Lausanne ( email )

Quartier Chambronne
Lausanne, Vaud CH-1015
Switzerland

C. G. G. Aitken

University of Edinburgh - School of Mathematics ( email )

United Kingdom
0131 650 4877 (Phone)

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