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Validating Conclusion Scales in the Forensic Sciences

NCJ Number
Thomas Busey
Date Published
74 pages

This is the Final Report of a project that combined two sets of studies to validate the conclusion scales in the fingerprint, footwear, and toolmark disciplines.


The first study measured how fingerprint examiners and members of the general public interpreted various articulation statements. The second set of studies measured how fingerprint, footwear, and toolmark examiners would use articulation statements expressed in strength-of-evidence language rather than as source attribution statements. By combining across the two sets of studies, this project demonstrated how statements are both used in casework comparisons and how the articulation language is interpreted by the consumers of forensic evidence. Pattern comparison disciplines use categorical statements to present conclusions. The first study measured the strength of evidence for six scales with members of the general public and fingerprint examiners. The statements came from different types of scales and included categorical conclusions, likelihoods, strength of support statements, and random match probabilities. An online interface was used that required participants to first correctly sort the statements in a given conclusion scale, and then place each statement on a single evidence axis that ranged from most support imaginable for same source to most support imaginable for different sources. The data were analyzed using both the raw values and a Thurstone-Mosteller model based on ordinal values. Systematic differences were found between examiners and members of the general public. Laypersons can distinguish between statements meant to represent moderate compared with strong evidence, but tend to place categorical conclusions above statements that involve numerical values. The second set of studies compared traditional conclusion statements with statements phrased as strength of evidence for different propositions. Guidance is provided on how expert witnesses for each of the forensic disciplines of fingerprint, footwear, and toolmark evidence can most effectively present their confidence levels to laypersons on juries. 22 figures, 14 tables, and 40 references