The standard for IR spectroscopy presupposes that the goal is to “to determine whether any significant differences exist between the known and questioned samples,” where a “significant difference” is “a difference between two samples that indicates that the two samples do not have a common origin.” The criminalist then is expected to declare whether “[s]pectra are dissimilar,” “indistinguishable,” or “inconclusive.”
Although categorical judgments have the benefit of simplicity and familiarity, most literature on forensic inference now maintains that analysts should present statements about the weight of the evidence rather than categorical conclusions about source hypotheses. By considering and presenting the degree to which the observations support one hypothesis as compared to another without dictating the conclusion that must be drawn, the analyst supplies the most information. It is not clear whether the standard rejects this view and is intended to preclude experts from using a weight-of-evidence approach to the comparison process.
The categorical approach that the standard adopts is notable for its vagueness. On its face, the definition of “significant difference” permits analysts to declare that differences with almost no discriminating power are so significant that two samples “do not have a common origin.” This lack of guidance arises because any difference that occurs more frequently among two samples with different origins than among two same-source samples “indicates” different origins and hence is “significant.” For example, a difference that arises 1,000 times more often for different-source samples is indicative of difference sources. But so is a difference that arises only 10% more often for different-source samples. Both “indicate” non-association. They differ only in the magnitude of the measure of non-association. The 1,000-times-more-often quantity is a strong indication of non-association, whereas the 10% figure is a weak indication. But in both cases, the differences indicate (to some degree) non-association relative to association.
To avoid this looseness, one might try to read “indicates” as connoting “strongly indicates” or “establishes,” but there is no reason to promulgate an ambiguous standard that requires readers in the fields of forensic science and law to struggle to discern and supply its intended meaning. And, if “establishes” is the intended meaning, then more guidance is needed to help analysts determine, on the basis of objective data about the range of differences seen in same-source and in different-source samples, when a difference is “significant” in the sense of discriminating between the former and the latter types of samples. That is, the standard should supply a validated decision rule; it should present the conditional error probabilities of this decision rule; and it should refer specifically to the studies that have validated it. These features of standards are not absolute requirements for admitting scientific evidence, but they would go far to assuring courts and counsel that the criteria of “known or potential rate of error” and “standards controlling the technique's operation” enumerated in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579, 594 (1993), militate in favor of admissibility (and persuasiveness of the testimony if a case goes to trial).
Section 10.6.1.1 of the ASTM Standard does not begin to do this. It offers an unbounded “rule of thumb” — “that the positions of corresponding peaks in two or more spectra be within ±5 cm^-1. For sharp absorption peaks one should use tighter constraints. One should critically scrutinize the spectra being compared if corresponding peaks vary by more than 5 cm^-1. Replicate collected spectra may be necessary to determine reproducibility of absorption position.” What is the basis for “critical scrutiny”? How many replicates are necessary? When are they necessary? What is the accuracy of examiners who follow the open-ended “rule of thumb”?
Given the lack of standards for deciding what is “significant,” the definitions of “dissimilar,” “indistinguishable,” and “inconclusive” are indeterminate. They read:
- 10.7.1 Spectra are dissimilar if they contain one or more significant differences.
- 10.7.2 Spectra are indistinguishable if they contain no significant differences.
- 10.7.3 A spectral comparison is inconclusive if sample size or condition precludes a decision as to whether differences are significant.