One of the most influential philosophies behind modern forensic science, commonly known as Locard's Exchange Principle, states that, "with contact between two items, there will be an exchange." True with Fingerprints and DNA, this is also true with impression evidence, an important and sometimes overlooked aspect of the criminal investigation process.
Impression evidence may include shoe prints and tire tracks, both two- and three- dimensional. When properly collected, such evidence can later be compared with other like impressions or suspected origins in the St. Louis County Police Crime Laboratory. The range of conclusions for such examinations includes:
– This is the highest degree of association expressed by a footwear and tire impression examiner. The questioned impression and the known footwear or tire share agreement of class and randomly acquired characteristics of sufficient quality and quantity.
High degree of association
– The questioned impression and known footwear or tire must correspond in the class characteristics of design, physical size, and general wear. For this degree of association there must also exist: (1) wear that, by virtue of its specific location, degree and orientation make it unusual and/or (2) one or more randomly acquired characteristics.
Association of class characteristics
– The class characteristics of both design and physical size must correspond between the questioned impression and the known footwear or tire. Correspondence of general wear may also be present.
Limited association of class characteristics
– Some similar class characteristics were present; however, there were significant limiting factors in the questioned impression that did not permit a stronger association between the questioned impression and the known footwear or tire. These factors may include but were not limited to: insufficient detail, lack of scale, improper position of scale, improper photographic techniques, distortion or significant lengths of time between the date of the occurrence and when the footwear or tires were recovered that could account for a different degree of general wear. No confirmable differences were observed that could exclude the footwear or tire.
Indications of non-association
– The questioned impression exhibits dissimilarities when compared to the known footwear or tire; however, the details or features were not sufficiently clear to permit an exclusion.
– This is the highest degree of non-association expressed in footwear and tire impression examinations. Sufficient differences were noted in the comparison of class and/or randomly acquired characteristics between the questioned impression and the known footwear or tire.
Unsuitable or Inconclusive
– The impression in question lacks sufficient detail for comparison purposes.
After a case is worked, an Examiner will write a detailed report concerning scientific findings related to the impression evidence analysis. Then, upon request, the Examiner will testify in criminal proceedings based upon these findings as an expert witness in the area of Impressions examinations.
Shoe and Impression (photo courtesy of Zalman992/wikimedia)