The logic of inference and decision for scientific evidence

Uncertainty is an inevitable complication encountered by members of the judiciary who face inference and decision-making as core aspects of their daily activities. Inference, in this context, is mainly inductive and relates to the use of incomplete information, to reason about propositions of interest. Applied to scientific evidence, this means, for example, to reason about whether or not a person of interest is the source of a recovered evidential material. More so, fact-finders are required to make decisions about ultimate issues, e.g. regarding a defendant’s guilt. The distinct, but related roles of inference and decision require a logical assistance because unaided human reasoning is liable to fallacious conclusions. This represents a critical cause of concern because incoherent reasoning in legal proceedings places defendants at risk and can lead to miscarriages of justice. In this respect, the role of forensic scientists, whose duty is to help assess the probative value of scientific findings, is subjected to ongoing scrutiny. Scientists (should) provide assistance by offering to mandating authorities conclusions that are scientifically sound and logically defensible. The choice of a normative framework – that is a coherent standard of reasoning – thus is widely advocated. In the current understanding, such a framework intends to implement several desirable properties for evidential assessment, such as logic, balance, robustness and transparency. Within this general evidential context, this chapter addresses two fundamental questions. The first is ‘How is one to manage scientific information?’, and the second is ‘How is one to justify a decision?’. The focus will be on presenting and discussing the extent to which probability theory and decision theory, can contribute toward answering these questions. This chapter will acknowledge and distinguish between, on the one hand, perspectives on evidence evaluation in forensic science literature and, on the other hand, legal evidence and proof processes on the level of cases as a whole.