Embracing likelihood ratios and highlighting the principles of forensic interpretation

Abstract

Due to the police commissioning of expert forensic services worldwide, the police may specify the level of forensic service they need; thus, forensic scientists can be put under pressure to present scientific findings as quickly as possible and in simple and definitive terms. Frequently, this means reporting forensics results out of the context of the case, as forensic practitioners may not be given relevant case information. However, to do so can be misleading, inaccurate and potentially lead to miscarriages of justice. Thus, whilst well-intentioned, efforts to accelerate and simplify forensic science evidence can undermine the justice system. However, providing that all forensic scientists and practitioners follow three basic forensic interpretation principles based on the formulation of the likelihood ratio component of Bayes Theorem approach, and are given case context, then the chances of miscarriages of justice arising from forensic science should be minimised. Principle #1: Always consider at least one alternative hypothesis. Principle #2: Always consider the probability of the evidence given the proposition and not the probability of the proposition given the evidence. Principle #3: Always consider the framework of circumstance. The expression of a likelihood ratio lies at the core of a pre-assessment that, if used correctly, should minimise bias in the forensic investigation. Several disciplines use likelihood ratios to interpret their findings and place them in the context of the case. There are still several forensic science areas where this is not undertaken, and therefore they may be a source of concern.

Keywords

• Forensic interpretation
• Likelihood ratio
• Bayes’ theorem
• Bias