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Forensic Analysis Weighing Bullet Lead Evidence
ensure the stability of the measurement process; only standard deviations within control-chart limits are eligible for use in pooled estimates.
In choosing a statistical test to apply when determining a “match,” the goal was to choose a test that had good performance properties as measured by (1) its rate of false non-matches and (2) its rates of false matches, evaluated at a variety of separations between the concentrations of the CS and the PS bullets. The latter corresponds to the probability of providing false evidence of guilt, which our society views as important to keep extremely low.
Given arguments of statistical efficiency that translate into lower error rates, it is attractive to consider either the T2 test statistic, or the successive t-test statistics procedure, since they are more representative of current statistical practice. The application of both procedures is illustrated using some sample data in Appendix K.
Recommendation: The committee recommends that the FBI use either the T2 test statistic or the successive t-test statistics procedure in place of the 2-SD overlap, range overlap, and chaining procedures. The tests should use pooled standard deviations and correlations, which can be calculated from the relevant bullets that have been analyzed by the FBI Laboratory. Changes in the analytical method (protocol, instrumentation, and technique) will be reflected in the standard deviations and correlations, so it is important to monitor these statistics for trends and, if necessary, to recalculate the pooled statistics.
The committee recognizes that some work remains in order to provide additional rigor for the use of this testing methodology in criminal cases. Further exploration of the several issues raised in this chapter should be carried out. As part of this effort, it will be necessary to further mine the extant data resources on lead bullet composition to establish an empirical base for the methodology’s use. In addition, this analysis may discover deficiencies in the extant data resources, thereby identifying additional data collection that is needed.
Recommendation: To confirm the accuracy of the values used to assess the measurement uncertainty (within-bullet standard deviation) in each element, the committee recommends that a detailed statistical investigation using the FBI’s historical data set of over 71,000 bullets be conducted. To confirm the relative accuracy of the committee’s recommended approaches to those used by the FBI, the cases that match using the committee’s recommended approaches should be compared with those obtained with the FBI approaches, and causes of discrepancies between the two approaches—such as excessively wide intervals from larger-than-expected estimates of the standard deviation, data from specific time periods, or examiners—should be identified. As the FBI adds new bullet data to its