Describing communication during a forensic investigation using the Pebbles on a Scale metaphor.

During the investigation of a crime, evidence is collected, analyzed, interpreted, and discussed by various stakeholders. This article examines the communication that may occur between two of these stakeholders: detectives and forensic analysts, and how their interaction influences the interpretation of evidence as the investigation proceeds and the theory of the case evolves. Such communication can be understood as sets of actions that are inter-dependent: for example, a request for a specific analysis by a detective leads to analyses and conclusions that the analyst shares with the detective, which leads to an assessment of these conclusions relative to the theory of the case, which leads to further analysis requests, and so forth. We present the Pebbles on a Scale metaphor, which describes how communication and the understanding of evidence takes place between the detective and analysts, and the different ways in which they consider the information as a function of their roles in the investigation. Using a hypothetical case for illustration, we discuss communicative challenges, the evolving theory of the case, the language that is used by analysts to discuss "yes", "no" and "I don't know" conclusions, and how those conclusions are used by detectives during the progression of the investigation.

STRmix™ collaborative exercise on DNA mixture interpretation.

An intra and inter-laboratory study using the probabilistic genotyping (PG) software STRmix™ is reported. Two complex mixtures from the PROVEDIt set, analysed on an Applied Biosystems™ 3500 Series Genetic Analyzer, were selected. 174 participants responded. For Sample 1 (low template, in the order of 200 rfu for major contributors) five participants described the comparison as inconclusive with respect to the POI or excluded him. Where LRs were assigned, the point estimates ranging from 2 × 104 to 8 × 106. For Sample 2 (in the order of 2000 rfu for major contributors), LRs ranged from 2 × 1028 to 2 × 1029. Where LRs were calculated, the differences between participants can be attributed to (from largest to smallest impact): This study demonstrates a high level of repeatability and reproducibility among the participants. For those results that differed from the mode, the differences in LR were almost always minor or conservative.