The evaluation of exclusionary DNA results: a discussion of issues in R v. Drummond

A recent appeal ruling by the Supreme Court of South Australia, based in large part on the significance of the absence of an individual's DNA on an item, has been successful in overturning a conviction. An issue of interest to the court in the original trial was the probability that two people could struggle and DNA not be detected on each other's clothing. In the parlance of the hierarchy of propositions, the question of interest to the court was about potential activities, however using the DNA results the expert could only provide source level information and in the absence of DNA results such reporting is meaningless. In this article the circumstances of the case, the evidence at trial and the expert reports are examined to understand the circumstances that led to the initial conviction and subsequent successful appeal. The main body of this article is dedicated to helping evaluate the DNA profiling results considering competing posited activities to determine the value of the exclusionary evidence to the questions posed in court. I demonstrate the application of Bayesian theory and relevant literature studies on DNA transfer to assign a likelihood ratio, which in this case supported the defence version of events.