A Framework for Interpreting Evidence
This chapter considers the underlying claims of several alternative approaches to the presentation of evidence. These will be termed the frequentist approach, the likelihood ratio approach, and the full Bayesian approach. The frequentist approach in forensic science has never been formalized and hence is quite hard to discuss. It appears to have grown as a framework by a set of intuitive steps. The approach will be subdivided into two parts: the coincidence probability and the exclusion probability. The similarity between the coincidence approach and hypothesis testing is the former’s greatest claim to prominence. The exclusion probability approach calculates and reports the exclusion or inclusion probability. Both the coincidence approach and the exclusion probability approach can be based on either frequentist or subjectivist probabilities. The chapter argues that laypersons will have difficulties going from source-level propositions to activity-level propositions, except in very simple cases where a large amount of blood or semen from a single source is recovered.