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Forensic Analysis Weighing Bullet Lead Evidence
crime-scene and the suspect’s bullets. The FBI examiner applies statistical tests to compare the elements in each crime-scene fragment with the elements in each of the suspect’s bullets. If any of the fragments and suspect’s bullets are determined statistically to be analytically indistinguishable for each of the elemental concentration means, the examiner’s expert court testimony currently will indicate that the fragments and bullets probably came from the same “source.”
The Federal Bureau of Investigation (FBI) asked the National Research Council to conduct an impartial scientific assessment of the soundness of the principles underlying CABL, the optimal manner for conducting an examination with CABL, and the scientifically valid conclusions that can be reached with CABL. In particular, the FBI asked the National Research Council to address the following three subjects and specific questions:
Analytical method. Is the method analytically sound? What are the relative merits of the methods currently available? Is the selection of elements used as comparison parameters appropriate? Can additional useful information be gained by measurement of isotopic compositions?
Statistics for comparison. Are the statistical tests used to compare two samples appropriate? Can known variations in compositions introduced in manufacturing processes be used to model specimen groupings and provide improved comparison criteria?
Interpretation issues. What are the appropriate statements that can be made to assist the requester in interpreting the results of compositional bullet lead comparison, for both indistinguishable and distinguishable compositions? Can significance statements be modified to include effects of such factors as the analytical technique, manufacturing process, comparison criteria, specimen history, and legal requirements?
The committee’s assessment of these questions and its overarching recommendations are summarized below. Its complete recommendations are found in the body of the report and collected in Chapter 5. The full report provides clear comments on the validity of the chemical and statistical analyses utilized in CABL, and on what can and cannot validly be stated in court regarding CABL evidence. It is up to prosecutors and judges to use the conclusions of this report to decide whether CABL evidence has enough value to be introduced in any specific case.
The current analytical instrumentation used by the FBI is appropriate and is the best available technology with respect to both precision and accuracy for the elements analyzed in a lead matrix. No other technique for this application provides as good or better quantitative, multi-element capability; wide linear dynamic range; limited interferences; and low (parts per billion) detection and quantitative limits. Furthermore, the elements selected by the FBI for analysis