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Forensic science expressions and legal proof

Evaluative forensic science opinions should be presented in a form that can be accommodated within that process of criminal proof demanded by judges and applied by juries. This is a non-mathematical inductive process that seeks ‘the inference to best explanation’ to a standard of proof beyond reasonable doubt. The question posed is not the mathematical probability of the prosecution hypothesis but whether, having regard to all the evidence properly before the court, the prosecution hypothesis is the only explicable hypothesis, in the sense that no reasonably possible defence hypothesis remains open. To ensure that a jury remains focused on applying this legal standard it is argued that evaluative forensic science evidence tendered by the prosecution should be expressed not as a likelihood ratio that emphasises the greater likelihood of finding evidence if the prosecution’s contentions are true, but in a form that emphasises the ranges of explanations for the evidence consistent with the defence case and that must be excluded as a reasonable possibility having regard to all the evidence before the accused can be convicted. It is argued that this can be achieved through expression in terms of a frequency or range.