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Review of the Scientific Approaches Used During the FBI's Investigation of the 2001 Anthrax Letters (2011) / Chapter Skim
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Appendix C: Committee Evaluation of Statistical Analysis Report
Pages 185-192

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From page 185...
... Results on 1,059 of the 1,070 samples were tabulated in the Statistical Analysis Report. Eight samples tested positive for all four mutations; seven of these eight samples came from one institution (USAMRIID)
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From page 186...
... Because the FBIR is not a representative and random collection of independent samples, the results on the assays from the repository may be biased. Virtually all statistical procedures assume that the units on which measurements are made comprise a random, representative col lection from the target population.
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From page 187...
... Twenty-one of these 112 sample that were eliminated from the statistical analysis assayed positive for 1, 2, or 3 mutations. Table C-1 lists these samples.
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From page 188...
... : 052-026 + + + inc - A1, A3, MRI-D 053-010 var + + + + A3, MRI-D, IITRI-D, E 054-008 inc + + inc + A3, MRI-D, E 054-066 + Inc + + + A1, MRI-D, IITRI-D, E The following four samples revealed positive assays for 2 of the 4 mutations, in addition to the 11 samples noted among the 947 samples (ordered by FBIR number) : 005-022 + var - inc + A1, E 044-020 - + - inc + A3, E 053-004 var + - - + A3, E 054-068 - var + - + MRI-D, E DILUTION EXPERIMENTS Dilution experiments were conducted to assess the sensitivity of the assays to various concentrations.
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From page 189...
... tabulates the results of the D assays from the two facilities: IITRI-D MRI-D Inconclusive Negative No growth Pending Positive Total Inconclusive 0 22 12 0 0 34 Negative 17 909 1 1 12 940 Negative-u 1 20 0 0 0 21 Positive 6 12 0 0 46 64 TOTAL 24 963 13 1 58 1,059 The Statistical Analysis Report combined the "negative-u" results with the "negative" results, and eliminated the 12 samples that showed "no-growth" by IITRI-D and "inconclusive" by MRI-D as well as the one "pending" sample, to yield the following table: IITRI-D MRI-D Inconclusive Negative Positive Total Inconclusive 0 22 0 22 Negative 18 929 12 959 Positive 6 12 46 64 TOTAL 24 963 58 1,045 Eliminating the 14 "no-growth" and "pending" samples, the concordance rate is 975/1045 = 0.933, with a 95 percent confidence interval (0.916, 0.947)
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From page 190...
... "SIGNIFICANCE" OF SEVEN (++++) SAMPLES FROM INSTITUTION F The Statistical Analysis Report notes in its conclusions: "In summation, though the random chance of occurrence of the sample type (++++)
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From page 191...
... samples appeared in Institution F is not completely typical, it also could hardly be considered extreme.
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Key Terms

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  • assayed positive
  • dilution experiments
  • institutions submitted
  • inconclusive negative

  • total inconclusive
  • key assumptions
  • variation indicates
  • fewer samples
  • sample transfers

  • assay procedure
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  • assays fbir
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  • fbi retained
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  • sample type
  • report notes


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