Quantifying the uniqueness of footwear impressions from the same footwear source

The field of impression evidence analysis employs the concept of uniqueness, in order to arrive at a conclusion of association, between an evidence and a reference sample. However, the idea of uniqueness and its application in this field of forensic science, is considered conceptual by a few practitioners and courts, as it fails to be supported by sound scientific studies and experimentation. This study aims to provide a scientific basis for the idea of uniqueness and its validity in footwear impression evidence analysis. Additionally, this study also aims to determine the presence or the lack of, a variation, between outsole impression size and the outsole size of the footwear that created the impression. The study required 3 volunteers to each, create 30 touch impressions and 10 step impressions with the same footwear, utilizing the EZID^TM Footwear Impression System manufactured by Sirchie®. Using Adobe Photoshop^TM CS4, the 30 touch impressions were analyzed by sequentially overlaying the impressions onto each other, in an effort to determine the area of overlay agreement between the impressions being analyzed. Subsequently, the 10 step impressions were examined by measuring the size of the impression created on the EZID Impression test cards, from the toe area to the heel area of the outsole impression. The data obtained indicate that, even under controlled settings and while attempting to maintain constant pressure through the creation of footwear impressions, it is impossible to generate prints that overlay perfectly. Further, the data also shows that a negligible variation occurs between the footwear outsole length and the length of the impression it creates, using the EZID^TM Footwear Impression System. This variation, however, is too small to cause any major hindrances in the estimation of footwear size