consistent with the hypothesis of “no match.” But if the seven averages are “close,” the data would be more consistent with the hypothesis that the two bullets “match.” The role of statistics is to determine how close, that is, to determine limits beyond which the bullets are deemed to have come from sources that have different mean concentrations and within which they are deemed to have come from sources that have the same mean concentrations.

1.1. Statistical Hypothesis Tests

The classical approach to deciding between the two hypotheses was developed in the 1930s. The standard hypothesis-testing procedure consists of these steps:

  1. Set up the two hypotheses. The “assumed” state of affairs is generally the null hypothesis, for example, “drug is no better than placebo.” In the compositional analysis of bullet lead (CABL) context, the null hypothesis is “bullets do not match” or “mean concentrations of materials from which these two bullets were produced are not the same” (assume “not guilty”). The converse is called the alternative hypothesis, for example, “drug is effective” or in the CABL context, “bullets match” or “mean concentrations are the same.”

  2. Determine an acceptable level of risk posed by rejecting the null hypothesis when it is actually true. The level is set according to the circumstances. Conventional values in many fields are 0.05 and 0.01; that is, in one of 20 or in one of 100 cases when this test is conducted, the test will erroneously decide on the alternative hypothesis (“bullets match”) when the null hypothesis actually was correct (“bullets do not match”). The preset level is considered inviolate; a procedure will not be considered if its “risk” exceeds it. We consider below tests with desired risk levels of 0.30 to 0.0004. (The value of 0.0004 is equivalent to 1 in 2,500, thought by the FBI to be the current level.)

  3. Calculate a quantity based on the data (for example, involving the sample mean concentrations of the seven elements in the two bullets), known as a test statistic. The value of the test statistic will be used to test the null hypothesis versus the alternative hypothesis.

  4. The preset level of risk and the test statistic together define two regions, corresponding to the two hypotheses. If the test statistic falls in one region, the decision is to fail to reject the null hypothesis; if it falls in the other region (called the critical region), the decision is to reject the null hypothesis and conclude the alternative hypothesis.

The critical region has the following property: Over the many times that this protocol is followed, the probability of falsely rejecting the null hypothesis does not exceed the preset level of risk. The recommended test procedure in Section 4



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