The authors’ goals for this paper are to discuss proposition formation in mixed DNA casework and to differentiate the process followed depending on role of investigator or evaluator; their motivation for the research reported in this paper was a murder case involving four related people, where a bloodstain was recovered associated with the accused that could be explained as a mixture of all four deceased.
The Bayesian paradigm is the preferred approach to evidence interpretation. It requires the evaluation of the probability of the evidence under at least two propositions. The value of the findings (i.e., the authors’ LR) will depend on these propositions and the case information, so it is crucial to identify which propositions are useful for the case at hand. Previously, a number of principles have been advanced and largely accepted for the evaluation of evidence. In the evaluation of traces involving DNA mixtures there may be more than two propositions possible. The authors apply these principles to some exemplar situations. They also show that in some cases, when there are no clear propositions or no defendant, a forensic scientist may be able to generate explanations to account for observations. In that case, the scientist plays a role of investigator, rather than evaluator. The authors believe that it is helpful for the scientist to distinguish those two roles. (Publisher Abstract Provided)