Reporting scales for forensic source comparisons

Music City Center 

In most forensic science disciplines, there is no objective way to determine whether two fingerprints, bullets, or handwritten documents come from the same person or not. Instead, it is the responsibility of individual analysts to make subjective conclusions and communicate their results to a judge or jury. Some examiners appear to be well-calibrated with each other and utilize the full range of possible outcomes, while other examiners display a tendency to overuse some categories and underuse others.  Recently, there have been proposals to shift from a 3-category scale (e.g., "same source", "different source", and "inconclusive") to 5 or 7 category scales (e.g., "strong support for same source", "moderate support for same source", etc.). Using Item Response Theory-based tools from standardized testing, we quantify these differences and illustrate potential implications of different reporting scales using simulation. Since examiner conclusions can influence investigator, judge, and jury decisions, it is important to measure and understand the range of individual differences in reporting styles before adopting a more complicated scale.