Monday, February 15, 2016

Approximating Individualization: The ASTM's Standard Terminology for Digital Evidence

Forensic scientists portraying existing standards for evidence testing and evaluation have been known to praise the “rigorous standard development process of ASTM,” 1/ an internationally recognized standards development organization. Having looked over the organization's "Standard Terminology for Digital and Multimedia Evidence Examination" 2/ (not to mention some of its other standards), 3/ I wonder if the results are as rigorous (or as comprehensible) as they should be.

Consider the definition of "individualization":
Individualization, n—theoretically, a determination that two samples derive from the same source; practically, a determination that two samples derive from sources that cannot be distinguished within the sensitivity of the comparison process. (Compare identification.) DISCUSSION—Theoretical individualization is the asymptotic upper bound of the sensitivity of a source identification process.
The definition presents individualization as a theoretical construct that cannot be fully attained. But lots of things can be sorted down to the individual level—telephone, passport, and social security numbers are obvious examples. Surnames and given names are not individualizing at the national level, but they are among the students in almost every class that I have taught. As these examples suggest, individualization can only be defined for the elements of a set. 4/ If the set is enumerated and all its elements available for inspection, then it is possible to "individualize"—not just theoretically or "asymptotically," but practically and precisely.

Thus, the domain of the ASTM definition must be cases in which no exhaustive list of the elements is available. Even with this modification, however, the definition of "individualization" as "a determination that two samples derive from sources that cannot be distinguished within the sensitivity of the comparison process" is flawed for two reasons.

First, "sensitivity," not "specificity," must be what is intended. Sensitivity is the probability that the process will declare that an item comes from a source when it really does come from the source. The least upper bound on sensitivity (or any other probability) is 1. A process that always declares a positive association will have a sensitivity of 1 because it always will declare a positive association when there is one. The degree of source discrimination within a set of potential sources is the specificity. Only when the specificity equals 1 is exact individualization possible.

Second, the ASTM definition of "individualization" fails to state a crucial presupposition. If the specificity of the test verges on 1, then (by definition) the probability that a claim of individualization will be correct when the target item is the individual so identified also verges on 1. This is the approximate individualization that the ASTM is trying to define. But the definition as written does not require that the specificity be close to 1. An analyst following the words of the definition could claim to have "individualized" even when neither ideal nor approximate individualization exists.  As long as the specificity is not 1, "a determination that two samples derive from sources that cannot be distinguished" only shows that the item is an element of a class of indistinguishable items.

  1. Jay Siegel, Forensic Chemistry: Fundamentals and Applications 230 (2015).
  2. ASTM E2916-13, Standard Terminology for Digital and Multimedia Evidence Examination (2013), available for $44 at
  3. E.g., Broken Glass, Mangled Statistics, Forensic Science, Statistics & the Law, Feb. 3, 2016.
  4. David H. Kaye, Identification, Individuality, and Uniqueness: What's the Difference?, 8 Law, Probability & Risk 85 (2009); David H. Kaye, Probability, Individualization, and Uniqueness in Forensic Science Evidence: Listening to the Academies, 75 Brooklyn L. Rev. 1163 (2010).

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