Combining evidence in complex cases - a practical approach to interdisciplinary casework

Activity level evaluations, although still a major challenge for many disciplines, bring a wealth of possibilities for a more formal approach to the evaluation of interdisciplinary forensic evidence. This paper proposes a practical methodology for combining evidence from different disciplines within the likelihood ratio framework. Evidence schemes introduced in this paper make the process of combining evidence more insightful and intuitive thereby assisting experts in their interdisciplinairy evaluation and in explaining this process to the courts. When confronted with two opposing scenarios and multiple types of evidence, the likelihood ratio approach allows experts to combine this evidence in a probabilistic manner. Parts of the prosecution and defence scenarios for which forensic science is expected to be informative are identified. For these so called core elements, activity level propositions are formulated. Afterwards evidence schemes are introduced to assist the expert in combining the evidence in a logical manner. Two types of evidence relations are identified: serial and parallel evidence. Practical guidelines are given on how to deal with both types of evidence relations when combining the evidence.