Forensic trace DNA analysis to answer activity-level questions in a realistic-scenario activity study based upon the...

An activity-level proposition is a hypothesis informed by case information about actions at a crime scene; it aids the fact-finder in addressing the "how" and "when" of the evidence, not just the "who." Activity-level reporting focuses on probabilities of the findings [or results] from crime scene evidence given alternate propositions. Empirical data generated by simulating real-life activity scenarios can be of great value to the forensic scientist in their evaluations of crime scene evidence. In this study, experiments were designed to approximate circumstances relevant to the case of the State of Idaho vs Bryan C. Kohberger, whose DNA was found on a knife sheath at the apartment where four University of Idaho students were stabbed to death. Two competing activity-level propositions concerning the origin of the DNA on the knife sheath were examined: 1) Kohberger touched the snap (direct DNA transfer), or 2) Kohberger, the POI, shook hands with another person who touched the snap (indirect DNA transfer). Empirical data were generated by simulating both activity pathways independently, quantifying DNA recovery and generating DNA profiles to inform the probabilities used to evaluate the findings given the competing activity-level propositions. Significantly greater quantities of POI DNA were recovered following direct transfer, and major POI DNA profiles were more likely if a direct transfer occurred rather than an indirect transfer.