Validating Conclusion Scales in the Forensic Sciences

The goal of this project is to calibrate the conclusions made by examiners in the forensic pattern disciplines and determine how expanded scales might be used by examiners. These expanded scales add precision to the determination made by examiners, but also may be misinterpreted by detectives, prosecutors, or the Court. In addition, definitive conclusions such as ‘Identification’ have been interpreted by many members of the public to mean ‘to the exclusion of all others’, and many authors have suggested alternative language in the form of strength-of-evidence statements such as ‘Strong Support for Common Source’. The need to calibrate the language used for forensic conclusions was noted both by a subcommittee of the Organization of Scientific Area Committees (OSAC) and the National Research Council of the National Academies. The first part of the present project builds on existing work to explore the use of expanded conclusion scales and strength-of-evidence statements in the disciplines of fingerprints, footwear, and tool marks. We propose to test practicing examiners using casework-like materials to determine how expanded scales would compare to the traditional conclusion scale. The data is collected using online tools designed to mimic those used in casework, and the results are analyzed using mathematical models based on Signal Detection Theory. This will demonstrate how the evidence thresholds change with different conclusion scales. In the second part of the project, we use a methodology that builds on existing approaches in the literature to determine the perceived strength of evidence for various conclusions statements. We will test statements currently in use against strength-of-evidence alternatives. Participants will use a graphical interface to dynamically adjust various statements along a scale that represents the strength of the evidence of each statement. Both the relative and absolute position of these statements will be used to draw conclusions about how these different phrases are interpreted by examiners and laypersons. The results of this study will calibrate the language used in forensic examinations, and determine whether experts and laypersons have different interpretations of the definitive conclusions. This project contains a research and/or development component, as defined in applicable law, and complies with Part 200 Uniform Requirements - 2 CFR 200.210(a)(14). CA/NCF