Communicating forensic evidence: Is it appropriate to report posterior beliefs when DNA evidence is obtained through a database search?

Forensic scientists lack a uniform system and common vocabulary for presenting conclusions about the sources of the traces or the weight of the evidence associated with these traces. Practices vary both across fields and sometimes within them. Several modes of presentation coexist, such as categorical conclusions (e.g. ‘the trace has been identified as coming from Mr. X’), or less definitive but easily misunderstood observations (such as ‘the trace is consistent with coming from Mr. X’ or ‘the trace matches Mr. X’). In some fields, practitioners qualify their conclusions about the identity of sources with verbal probabilities (such as ‘there is a strong probability than Mr. X left the trace’). In other fields, such as forensic DNA analysis, statistics are used to express the rarity of the features in traces or the ‘random match probability’. Most of these conclusion types cannot be logically derived from the information available to forensic scientists. In addition, there is no reason, other than historical, for reporting different evidence types using different types of conclusions. In other words, there is clearly a need for reporting forensic evidence in a consistent and scientifically rigorous manner, and, at the same time, for ensuring that the information resulting from forensic analyses is understood and used in the most rational way by factfinders. This issue, while raised periodically, has not been satisfactorily addressed.