• “could have made” (significant association of multiple class characteristics)

  • “inconclusive” (limited association of some characteristics)

  • “probably did not make” (very high degree of nonassociation)

  • “elimination” (definite exclusion)

  • ”unsuitable” (lacks sufficient detail for a meaningful comparison).

Additionally, SWGTREAD discourages the use of once common terminology, such as “consistent with” (acceptable when used to describe a similarity of characteristics), “match/no match,” “responsible for/not responsible for,” and “caused with/not caused with.”56 Neither the IAI nor SWGTREAD address the statistical evaluation of impression evidence.

Summary Assessment

The scientific basis for the evaluation of impression evidence is that mass-produced items (e.g., shoes, tires) pick up features of wear that, over time, individualize them. However, because these features continue to change as they are worn, elapsed time after a crime can undercut the forensic scientist’s certainty. At the least, class characteristics can be identified, and with sufficiently distinctive patterns of wear, one might hope for specific individualization. However, there is no consensus regarding the number of individual characteristics needed to make a positive identification, and the committee is not aware of any data about the variability of class or individual characteristics or about the validity or reliability of the method. Without such population studies, it is impossible to assess the number of characteristics that must match in order to have any particular degree of confidence about the source of the impression.

Experts in impression evidence will argue that they accumulate a sense of those probabilities through experience, which may be true. However, it is difficult to avoid biases in experience-based judgments, especially in the absence of a feedback mechanism to correct an erroneous judgment. These problems are exacerbated with the less common types of impression evidence. For example, a European survey found that 42 laboratories conducted 28,093 shoeprint examinations and 41 laboratories conducted 591 tire track examinations, but only 14 laboratories conducted a total of 21 lip print examinations and 17 laboratories conducted a total of 100 ear print examinations.57 Although one might argue that those who perform the


SWGTREAD. 2006. Standard Terminology for Expressing Conclusions of Forensic Footwear and Tire Impression Examinations. Available at www.theiai.org/guidelines/swgtread/terminology_final.pdf.


Liukkonen, Majamaa, and Virtanen, op. cit.

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