Evaluation of alternate materials and methods for the collection of identifying characteristics of footwear
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Footwear impression evidence is often recovered at a crime scene and is either in the form of a two dimensional and/or a three-dimensional impression. Two-dimensional impressions occur when a shoe coated with a substance such as dust, blood, or ink, leaves behind a silhouette of the sole on a surface. Three-dimensional impressions contain depth information of the sole patterns that include three-dimensional characteristics that are transferred into a substrate such as dirt, mud, or sand. From these impressions, a sufficient number of identifying characteristics can be identified and used to individualize an impression to a particular footwear on the premise that it is a practical impossibility to have two shoes with the same set of randomly acquired features. This requires the capture of unknown impressions with both photography and casting; however, there are a variety of three-dimensional capture methods that can be employed. By evaluating different materials and techniques on the basis of their permanency, elasticity, and statistical ability to acquire defined identifying characteristics, a more significant comparison of successful methods can be made based on the likelihood ratios of the average occurrence of individualizing characteristics. This provides a powerful description of resolution of one particular method and can rationalize the difference between methods through statistical analysis. This research addresses a conservative statistical model of random occurrence of individual characteristics in a defined area (sole of footwear) by using binomial coefficients to evaluate possible performance discrepancies between field collection techniques and laboratory analysis of alternate materials and methods (alginate, moulage, silicone, thixotropic plastic, polyurethane foam, polymer gypsum mix) in terms of their ability to capture identifying characteristics from impressions made by a reference footwear, and allows a comparison to contemporary forensic methods utilized in the field and laboratory analysis.