Microscopic comparison of evidence
slice marks in bullet and known marks
Courtesy of Jack Dillon
Pattern matching is the process of determining whether or not the details of striated marks or impressions on two objects correspond. Early forensic scientists and their successors, (including Calvin Goddard, Julian S. Hatcher, Gerald Burrard, J. Howard Mathews) practiced pattern matching. Today, examiners continue to rely on their extensive experience, training, and education to perform the specialized pattern identification represented by toolmark identification.
The examiner compares the class characteristics of the two objects; if all class characteristics correspond, the examiner proceeds to compare the individual characteristics. During the comparison of the individual characteristics, the examiner looks for repetitive marks occurring on both objects in the same location and position. The examiner may test the tool in an effort to re-create the marks for comparison with the evidence marks.
This is a somewhat subjective process performed by the examiner. Pattern identification relies on the experience, training, and education of the examiner. The quality of training is the single most important element of the examiner’s competence and expertise. Long-term hands-on experience is gained from working side-by-side with an experienced examiner in a structured training program.
The most difficult aspect of this mental process is the articulation of how an identification can be determined. This process has traditionally been more qualitative than quantitative, and therefore difficult to convey to a jury.
What was needed was a structured theoretical basis to assist in
- establishing an identification as to a common source,
- conveying results to a jury,
- justifying results to a jury.