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Strengthening Forensic Science in the United States: A Path Forward
standards and no statistical foundation for estimation of error rates.63 The National Academies report, Ballistic Imaging, while not claiming to be a definitive study on firearms identification, observed that, “The validity of the fundamental assumptions of uniqueness and reproducibility of firearms-related toolmarks has not yet been fully demonstrated.” That study recognized the logic involved in trying to compare firearms-related toolmarks by noting that, “Although they are subject to numerous sources of variability, firearms-related toolmarks are not completely random and volatile; one can find similar marks on bullets and cartridge cases from the same gun,” but it cautioned that, “A significant amount of research would be needed to scientifically determine the degree to which firearms-related toolmarks are unique or even to quantitatively characterize the probability of uniqueness.”64
Toolmark and firearms analysis suffers from the same limitations discussed above for impression evidence. Because not enough is known about the variabilities among individual tools and guns, we are not able to specify how many points of similarity are necessary for a given level of confidence in the result. Sufficient studies have not been done to understand the reliability and repeatability of the methods. The committee agrees that class characteristics are helpful in narrowing the pool of tools that may have left a distinctive mark. Individual patterns from manufacture or from wear might, in some cases, be distinctive enough to suggest one particular source, but additional studies should be performed to make the process of individualization more precise and repeatable.
Recent research has attempted to develop a statistical foundation for assessing the likelihood that more than one tool could have made specific marks by assessing consecutive matching striae, but this approach is used in a minority of cases. See A.A. Biasotti. 1959. A statistical study of the individual characteristics of fired bullets. Journal of Forensic Sciences 4:34; A.A. Biasotti and J. Murdock. 1984. “Criteria for identification” or “state of the art” of firearms and tool marks identification. Journal of the Association of Firearms and Tool Mark Examiners 16(4):16; J. Miller and M.M. McLean. 1998. Criteria for identification of tool marks. Journal of the Association of Firearms and Tool Mark Examiners 30(1):15; J.J. Masson. 1997. Confidence level variations in firearms identification through computerized technology. Journalof the Association of Firearms and Tool Mark Examiners 29(1):42. For a critique of this area and a comparison of scientific issues involving toolmark evidence and DNA evidence, see A. Schwartz. 2004-2005. A systemic challenge to the reliability and admissibility of firearms and tool marks identification. Columbia Science and Technology Law Review 6:2. For a rebuttal to this critique, see R.G. Nichols. 2007. Defending the scientific foundations of the firearms and tool mark identification discipline: Responding to recent challenges. Journal of ForensicSciences 52(3):586-594.
All quotes from National Research Council. 2008. Ballistic Imaging. Washington, DC: The National Academies Press, p. 3.