A Statistical Validation of the Individuality and Repeatability of Striated Tool Marks: Screwdrivers and Tongue and Groove Pliers*

Tool mark identification relies on the premise that microscopic imperfections on a tool's working surface are sufficiently unique and faithfully transferred to enable a one-to-one association between a tool and the tool marks it creates. This paper presents a study undertaken to assess the validity of this premise. As part of this study sets of striated tool marks were created under different conditions and on different media. The topography of these tool marks was acquired and the degree of similarity between them was quantified using well-defined metrics. An analysis of the resulting matching and nonmatching similarity distributions shows nearly error-free identification under most conditions. These results provide substantial support for the validity of the premise of tool mark identification. Because the approach taken in this study relies on a quantifiable similar- ity metric, the results have greater repeatability and objectivity than those obtained using less precise measures of similarity. The ability to perform tool mark to tool mark comparisons based on microscopic features observed on the tool mark' ss urface is at the core of tool mark identification. Supreme Court decisions such as Daubert versus Merrill Dow (1) and Kumho Tire versus Carmi- chael (2) are making it increasingly necessary to further formalize scientific evidence presented in court. Furthermore, the develop- ment of DNA identification techniques and the level of accuracy achievable in the estimation of the associated error rates have raised the expectations for the quantitative precision that may be achieved in forensic analysis. Quantitative evidence regarding the validity of the basic premise of tool mark comparison would pro- vide additional support for the admissibility of tool mark evidence. The Federal Bureau of Investigation (FBI) and Intelligent Automa- tion, Inc. (IAI) have undertaken an extensive study to verify the premise that the microscopic features transferred from a tool's working surface to the marks created by it are sufficiently unique and repeatable to enable the association of a tool with its marks. This paper reports the results of this study for the case of striated tool marks (a paper reporting the results for impressed tool marks is in preparation). In particular, we consider two types of tools: screwdrivers and tongue and groove pliers. In addition to consider- ing the comparison of striated tool marks created under the same conditions, we also evaluated the effect of the media onto which