In all, 20 laboratories (15 domestic, two foreign, plus USAMRIID, Dugway, and Battelle) submitted to the FBIR a total of 1,070 (USDOJ, 2010, p. 24) samples of stocks derived from the Ames strain of Bacillus anthracis. Of these, 1,059 samples were screened and results reported for the presence or absence of the four mutant genotypes—A1, A3, D, and E (FBI Documents, B2M10D2)—using the assays described in Chapter 5. The FBI told the committee that the other 11 samples were not viable, failed to grow the Ames strain of B. anthracis, or failed to grow B. anthracis at all. The results and interpretation of this screening are described below.
It is important, however, to recognize not only several inherent limitations of the FBIR collection that make it difficult to assess the evidence in any formal statistical sense, but also the effects of the decision to require growth of the samples before testing for the four mutant genotypes. In addition to issues of representativeness, there are issues with the independence of the samples.
First, statistical analyses typically assume that samples are taken at random from a defined population. The FBI aimed to create a comprehensive repository that encompassed the entire population of stocks derived from the Ames strain, rather than a representative fraction thereof. Given uncertainties in the extent of the entire population of Ames stocks worldwide (highlighted by concern about the possibility of clandestine stocks held by terrorist organizations—see section 3.4.3), the lack of specificity in the subpoena protocol, the uncertainties in compliance with the subpoena protocol, the incomplete information on transfers of Ames-derived stocks between laboratories, and the possibility that some stocks were produced but later destroyed, the repository was unlikely to have been comprehensive.2 DOJ states in its Summary, “The collection of Ames isolates from laboratories both from the United States and abroad that constitute the FBIR are a comprehensive representation of the Ames strain” (USDOJ, 2010, p. 28). Section 6.5 provides further discussion of this issue, along with the implications of the violation of this assumption of “representativeness” for the statistical inferences
Second, there were complex and varying degrees of genetic relationships among the stocks, reflecting their common descent from the original Ames isolate and the history of transfers, single-colony isolations, and mixtures of materials within and between institutions. As a consequence, some sets of related stocks were likely to be represented by many samples and others by few samples, rendering it impossible to assess the relevant frequencies of genotypes across the population of interest.
Third, statistical analyses are critically dependent on replication to provide measures of the sensitivity and specificity of assays, and such replication should
2 The issue of overseas samples discussion in Chapter 3, section 3.4.3 raises additional questions about the comprehensiveness of the FBIR: but as stated previously, these issues were beyond the scope of this committee.