characteristics. (In the field of forensic tire evidence, class characteristics often refer to such things as design, pattern, size, shape, mold variations, etc.).”43 Regardless of the type of impression evidence, class characteristics are not sufficient to conclude that any one particular shoe or tire made the impression. That latter step—which is not always possible—requires comparison of the individual identifying characteristics on the impression evidence with those on a shoe or tire that is suspected of leaving the impression. These individual characteristics occur during the normal use of an item, sometimes called wear and tear,44 and are created by “random, uncontrolled processes.”45 For footwear, Bodziak writes that “individual identifying characteristics are characteristics that result when something is randomly added to or taken away from a shoe outsole that either causes or contributes to making that shoe outsole unique.”46 Such characteristics might include cuts, scratches, gouges, holes, or random inclusions that result from manufacturing, such as bubbles, and those that result from adherent substances, such as rocks, chewing gum, papers, or twigs.

Following analysis of the impression, an identification is determined or ruled out according to the number of individual characteristics the evidence has in common with the suspected source. But there is no defined threshold that must be surpassed, nor are there any studies that associate the number of matching characteristics with the probability that the impressions were made by a common source. It is generally accepted that the specific number of characteristics needed to assign a definite positive identification depends on the quality and quantity of these accidental characteristics and the criteria established by individual laboratories.47 According to Cassidy, many factors and accidental characteristics are required before a positive identification can be established; however, the most important are the examiner’s experience, the clarity of the impression, and the uniqueness of the characteristic.48 Proficiency testing for examiners of impression evidence is available through Collaborative Testing Service, Inc., but the proficiency tests for footwear impressions include samples that are either a match or not a match49—that is, none of the samples included in the tests have the sort of ambiguities that would lead an experienced examiner to an “inconclusive”

43

Ibid.

44

M.J. Cassidy. 1980. Footwear Identification. Quebec, Canada: Government Printing Office Centre.

45

K. Inman and N. Rudin. 2001. Principles and Practice of Criminalistics. Boca Raton, FL: CRC Press, p. 129.

46

Ibid., p. 335.

47

Liukkonen, Majamaa, and Virtanen, op. cit.

48

Cassidy, op. cit.

49

H. Majamaa and Y. Anja. 1996. Survey of the conclusions drawn of similar footwear cases in various crime laboratories. Forensic Science International 82:109-120.



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