Date of Graduation
Eberly College of Arts and Sciences
Forensic and Investigative Science
A footwear impression collected at a crime scene can provide information about the shoe that created it, yielding investigative leads, excluding persons-of-interest, and linking evidence between cases. The analysis of footwear evidence often requires the preparation of test impressions from known exemplar shoes created under controlled laboratory conditions for comparison to questioned impressions. The goal of these comparisons is to evaluate the degree of similarities and differences between the questioned impression and potential sources.
Given that this comparison leads to an assessment of the nature and degree of agreement between questioned and known impressions, it is important to characterize the variation that may exist between replicate test impressions created in the laboratory. This study aimed to evaluate the variation that exists within and between test impressions prepared using a static benchtop and a dynamic walking method, as well as explore the potential influence of the wearer’s foot size when using the walking method.
To examine this variation, twenty-three participants were recruited to prepare test impressions of two different shoe makes and models in four different manufacturer’s sizes. Five replicate benchtop impressions per make/model/size and three replicate walking impressions per participant/make/model/size were created, resulting in more than 500 test impressions. A total of 150 quality control copies were blindly mixed into the larger dataset, increasing the total to almost 700 test impressions. Reproducible and reliable ground control points, such as the edge of a tread element, defined points of interest on impressions, and the distance between pairs of points was computed as a function of method and foot/shoe size match or mismatch.
Physical size differences between measurements from benchtop impressions versus walking impressions of the same shoe were computed, and the largest physical size difference measured was 4.18 mm, which is concerning since the smallest reported physical size difference between manufacturer’s half-sizes is 4.20 mm . Bland-Altman analyses were performed to determine estimates of agreement of measurements and possible bias between methods of impression creation. The methods revealed a systematic bias in that benchtop impressions were always longer and narrower than walking impressions. Additionally, the largest upper bound on the limit of agreement was 4.75 mm, indicating that differences in measurements between benchtop and walking impressions could be as large as 4.75 mm. Analysis of covariance was used to test for a numerical significance between each method used to create test impressions, while controlling for the wearer’s weight. Differences in length measurements from benchtop impressions and from walking impressions created when the manufacturer’s reported shoe size was two sizes smaller than the wearer’s foot size were statistically significantly different from almost all other experimental groups for all impressions. Conversely, the study failed to detect statistically significant differences in width measurements from impressions, indicating that the factor of wearer foot size had a larger impact on toe-to-heel length measurements than on medial-to-lateral ball of the toe width measurements. Results also indicated that of the two outsoles included in this study (the Nike® Downshifter 11 and the Asics® Gel Dedicate 7), differences between Nike® benchtop impressions and Nike® walking impressions were always greater than differences between Asics® benchtop impressions and Asics® walking impressions. Since major differences in outsole chemical composition did not exist for these shoes, the greater change in physical size for the Nike® impressions is hypothesized to be due to other factors beyond the scope of this investigation, such as the depth of tread elements, the composition of the midsole, the geometric tread arrangement, and/or intended-end-use for the shoe. In summary, this research characterized the variation that can be expected when test impressions are created using different methods and when a foot/shoe size match or mismatch is present using the walking method. Based on the findings presented in this paper, best practice dictates that footwear analysts creating test impressions using the walking method should avoid wearing a manufacturer’s reported shoe size that is two or more sizes smaller than their foot size.
Brady, Samantha K., "Quantifying Variation in High-Quality Footwear Replicate Impressions" (2023). Graduate Theses, Dissertations, and Problem Reports. 12134.