Understanding forensic expert evaluative evidence: A study of the perception of verbal expressions of the strength of evidence

Verbal expressions of evidential strength are routinely used when presenting forensic expert evaluative evidence. The degree to which these verbal expressions are interpreted uniformly among different individuals requires further empirical study. This study focussed on groups of individuals with different roles within the criminal justice system and individuals with varying degrees of expertise and knowledge. Three groups of individuals were identified: laypeople, legal professionals and those with some forensic or investigative knowledge. The participants in the study (n=230) were provided with a case summary to which a verbal expression of the strength of evidence was randomly assigned. Participants were subsequently invited to indicate their perception of the strength of the evidence on a scale that was provided. Generally, across the study groups, the trend was one of increased perceived strength of evidence as the intended strength of the verbal expression was increased, with some notable exceptions. In general, there was good concordance between the groups in the way the different expressions were perceived. It was found that participants performed poorly when it came to differentiating between expressions at the 'strong' end of the scale ('strong', 'very strong' and 'extremely strong'). The findings resonate with calls for validated and robust communication frameworks for evaluative opinions. Further empirical research in this area is warranted and that such research can represent an important contribution towards improving the communication and presentation of forensic evidence.