Validating strength-of-support conclusion scales for fingerprint, footwear, and toolmark impressions

In the pattern comparison disciplines such as fingerprints, footwear, and toolmarks, the results of a comparison are communicated by examiners in the form of categorical conclusions such as Identification or Exclusion. These statements have been criticized as requiring knowledge of prior probabilities by the examiners and being overinterpreted by laypersons. Alternative statements based on strength-of-support language have been proposed. The current study compares traditional conclusion scales against strength-of-support scales to determine how these new statements might be used by examiners in casework. Each participant completed 60 comparisons within their discipline, which were designed to approximate casework conditions, using either a traditional or a strength-of-support conclusion scale. The scale used on each trial was randomly assigned, and participants knew the scale for that trial as they began the comparison. Fingerprint examiners were much less likely to use Extremely Strong Support for Common Source than Identification. Footwear examiners treated the traditional and strength-of-support scales similarly, but toolmark examiners were much less likely to use Extremely Strong Support for Common Source than Identification, similar to fingerprint examiners. A separate group of fingerprint examiners used Identification less often when an expanded scale was available. The results demonstrate that expanded scales may result in the highest conclusion category being used less often by examiners when other alternatives are possible, and the term "extremely strong support" may introduce risk aversion on the part of examiners.