The interpretation of forensic conclusions by criminal justice professionals: The same evidence interpreted differently

Elmarije K Van Straalen,Christianne J De Poot,Marijke Malsch,Henk Elffers

doi:10.1016/j.forsciint.2020.110331

Abstract

Forensic reports use various types of conclusions, such as a categorical (CAT) conclusion or a likelihood ratio (LR). In order to correctly assess the evidence, users of forensic reports need to understand the conclusion and its evidential strength. The aim of this paper is to study the interpretation of the evidential strength of forensic conclusions by criminal justice professionals. In an online questionnaire 269 professionals assessed 768 reports on fingerprint examination and answered questions that measured self-proclaimed and actual understanding of the reports and conclusions. The reports entailed CAT, verbal LR and numerical LR conclusions with low or high evidential strength and were assessed by crime scene investigators, police detectives, public prosecutors, criminal lawyers, and judges. The results show that about a quarter of all questions measuring actual understanding of the reports were answered incorrectly. The CAT conclusion was best understood for the weak conclusions, the three strong conclusions were all assessed similarly. The weak CAT conclusion correctly emphasizes the uncertainty of any conclusion type used. However, most participants underestimated the strength of this weak CAT conclusion compared to the other weak conclusion types. Looking at the self-proclaimed understanding of all professionals, they in general overestimated their actual understanding of all conclusion types.

Highlights

In the process of investigating and proving a crime, many different types of evidence may play a role, including witness statements, DNA, digital traces, fingerprints, observations, and shoeprints
Since professionals in the Netherlands are confronted with reports on fingerprint examinations using a categorical or likelihood ratio conclusion, we examine the interpretation of these different forensic conclusions in fingerprint examination reports
Before looking at the actual understanding, we will look at the alleged comprehension of those conclusion types: do professionals think they understand the reports and conclusions?

Summary

Introduction

In the process of investigating and proving a crime, many different types of evidence may play a role, including witness statements, DNA, digital traces, fingerprints, observations, and shoeprints. It is impossible, and undesirable, for all professionals working with or making decisions based on this evidence to have expertise in all these different areas. Besides the assumption that reports are correct, it is important that a report clearly states what context information was used for the assessment and the conclusion, and what context information was left out of the assessment By describing this, it is possible for the users of the reports to make a thought-through decision based on all available evidence

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Conclusion