Statistical Validation on the Individuality of Tool Marks Due to the Effect of Wear, Environment Exposure and Partial Evidence

• NCJ Number: 227929
• Author(s): Benjamin Bachrach
• Date Published: June 2009
• Length: 78 pages
• Grant Number(s): 2006-DN-BX-K029
• Agencies: NIJ-Sponsored
• Publication Type: Report (Study/Research)

Annotation

The primary goal of this study was to evaluate an objective, quantifiable method for determining the effect of wear, environmental conditions, and partial information on the reliability of tool-mark evidence in its ability to meet admissibility criteria for having been made by a tool in the suspect's possession.

Abstract

The study advises that the conclusions reported apply only to the specific tool considered in this study, which was a diagonal cutter tool that made marks on copper wire exposed to salt water of various concentrations. The preliminary experiment showed that the tool marks deteriorated slowly in air and tap water, but deteriorated dramatically in salt water. The most significant wear on the cutting tool occurred within the first individual cuts (one or two), after which the effect of tool wear lessened rapidly. The study verified that the tool marks created by diagonal cutters are easily identifiable after 300 cuts, and the rate of change of the similarity measure between tool marks suggests that identification of the tool from the marks it makes would be possible well beyond 300 cuts. On the other hand, the experiment showed that although the rate of change of the tool's working surface is slow, the tool marks created in close proximity to each other after a small number of cuts will have greater similarity than tool marks made many cuts apart. The study confirmed researchers' expectations that identification of a tool mark exposed to the environment becomes more challenging in proportion to the time of exposure and the corrosive properties of the exposure media. In addition, the study of partial tool marks validated the premise that a partial tool mark can be as reliable as a complete tool mark in matching the mark to a given tool. Extensive tables and figures, 34 references, and appended description of tool-mark preparation