title: Discussion on how to implement a verbal scale in a forensic laboratory: Benefits, pitfalls and suggestions to avoid misunderstandings.
abstract: In a recently published guideline for evaluative reporting in forensic science, the European Network of Forensic Science Institutes (ENFSI) recommended the use of the likelihood ratio for the measurement of the value of forensic results. As a device to communicate the probative value of the results, the ENFSI guideline mentions the possibility to define and use a verbal scale, which should be unified within a forensic institution. This paper summarizes discussions held between scientists of our institution to develop and implement such a verbal scale. It intends to contribute to general discussions likely to be faced by any forensic institution that engages in continuous monitoring and improving of their evaluation and reporting format. We first present published arguments in favour of the use of such verbal qualifiers. We emphasise that verbal qualifiers do not replace the use of numbers to evaluate forensic findings, but are useful to communicate the probative value, since the weight of evidence in terms of likelihood ratio are still apprehended with difficulty by both the forensic scientists, especially in the absence of hard data, and the recipient of information. We further present arguments that support the development of the verbal scale that we propose. Recognising the limits of the use of such a verbal scale, we then discuss its disadvantages: it may lead to the spurious view according to which the value of the observations made in a given case is relative to other cases. Verbal qualifiers are also prone to misunderstandings and cannot be coherently combined with other evidence. We therefore recommend not using the verbal qualifier alone in a written statement. While scientists should only report on the probability of the findings - and not on the probability of the propositions, which are the duty of the Court - we suggest showing examples to let the recipient of information understand how the scientific evidence affects the probabilities of the propositions. To avoid misunderstandings, we also advise to mention in the statement what the results do not mean. Finally, we are of the opinion that if experts were able to coherently articulate numbers, and if recipients of information could properly handle such numbers, then verbal qualifiers could be abandoned completely. At that time, numerical expressions of probative value will be appropriately understood, as other numerical measures that most of us understand without the need of any further explanation, such as expressions for length or temperature.
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: raymond.marquis@unil.ch.
authors.forename: Raymond
authors.initials: R
authors.lastname: Marquis
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: alex.biedermann@unil.ch.
authors.forename: Alex
authors.initials: A
authors.lastname: Biedermann
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: liv.cadola@unil.ch.
authors.forename: Liv
authors.initials: L
authors.lastname: Cadola
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: christophe.champod@unil.ch.
authors.forename: Christophe
authors.initials: C
authors.lastname: Champod
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: line.gueissaz@unil.ch.
authors.forename: Line
authors.initials: L
authors.lastname: Gueissaz
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: genevieve.massonnet@unil.ch.
authors.forename: Geneviève
authors.initials: G
authors.lastname: Massonnet
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: williams.mazzella@unil.ch.
authors.forename: Williams David
authors.initials: WD
authors.lastname: Mazzella
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: franco.taroni@unil.ch.
authors.forename: Franco
authors.initials: F
authors.lastname: Taroni
authors.affiliation: School of Criminal Justice, University of Lausanne, 1015 Dorigny, Switzerland. Electronic address: nathalie.hickschampod@unil.ch.
authors.forename: Tacha
authors.initials: T
authors.lastname: Hicks
mesh_terms: D003142:Communication; D044707:Forensic Sciences; D006801:Humans; D016013:Likelihood Functions; D009626:Terminology as Topic
keywords: Interpretation; Likelihood ratio; Probative value; Statement; Verbal scale