Sunday, December 22, 2013

Forensic Science’s Latest Proof of Uniqueness

A federally funded study on the "Determination of Unique Fracture Patterns in Glass and Glassy Polymers" affirms that fracture-pattern matches are unique. The researchers believe their work permits experts to continue to provide their "usually conclusive" testimony about cracked glass and plastic.

"The purpose of the research" undertaken at the University of California at Davis's graduate program in forensic science was "to provide a first, objective scientific background that will illustrate that repetitive fractures, under controlled conditions on target materials such as glass window panes and glass bottles, are in fact different and unique. In this phase of our study, we fractured glass window panes, glass bottles (clear wine bottles), and polymer tail light lens covers. Each and every fracture was documented in detail for subsequent inter-comparison and to illustrate the uniqueness of the fracture pattern." (Tulleners et al. 2013, p. 7).

Not surprisingly, the researchers found that all their fractures were distinguishable. In all, they conducted 5,310 pairwise comparisons by examining the fracture patterns in all pairs formed within each of the three groups of 60 items. This finding, they concluded, "should aid the practitioner in any court testimony involving the significance of fracture matching of broken glass and polymers materials." (Ibid., p. 23).

What testimony might this be? "For the forensic community, the ability to piece together glass fragments in order to show a physical fit or a 'Physical Match' is the strongest evidentiary finding of an association." (Ibid., p. 6) "The usual statement is that 'the evidence glass fragment was physically matched to another glass establishing thus both share a common origin.'" (Ibid.) This testimony, the researchers suggest, is just fine: "we are substantiating the individuality of glass and polymer fractures under closely controlled conditions." (Ibid., p. 3, emphasis added). Thus, "[t]his research should enhance the capability of the analyst to testify in a court of law as to the uniqueness of a fracture." (Id., p. 61, emphasis added).

But why would the analyst want to claim universal uniqueness? Forensic science’s hoary division of its world into two parts -- "unique" feature sets and "class" characteristics -- is an article of faith. (E.g., Kaye 2009). The latest study certainly is of some use in confirming the intuition that fracture patterns are highly variable. The existence of varying patterns is one fact that makes "fractography" evidence, as it is called in the field, probative. But the study’s explanation of how it proves that every pattern is unique seems like a parody of scientific reasoning. The explanation is this:
In this research, it is hypothesized that every fracture forms a unique and nonreproducible fracture pattern. Alternately, it may be that some fracture patterns may be reproduced from time to time. If it is found that each fracture forms a unique and nonreproducible fracture pattern, then this finding will support the theory that coincidental duplication of fracture patterns cannot be attained. However, if duplicate fracture patterns are found, this would falsify the null hypothesis and show that some fracture patterns may be reproduced from time to time.
(Ibid., p. 27). Such is the power of the unique-vs-class thinking. This impoverished dichotomy collapses a spectrum of possible states of nature into two discrete states. Combined with a cartoon-like version of Sir Karl Popper’s criterion of falsification, it leads the researchers to believe that their failure to find a class characteristic proves the "null hypothesis" of uniqueness.

True, the failure to find "duplicate fracture patterns" in a small sample "support[s] the theory that coincidental duplication of fracture patterns cannot be attained." (Or it would in a study in which the analyst deciding on whether two patterns were the same did not already know that all of them came from different objects.)

But it also supports the alternative theory that coincidental duplication can be attained. Instead of taking no-duplication-is-possible as the “null hypothesis,” we could postulate that, on average, 1 in every 10,000 fractures of the items tested would produce indistinguishable fracture patterns. Or, we could hypothesize that the mean duplication rate is 1/100,000. Since we just spinning out hypotheses, we could pick still other rates.

A great many such hypotheses seem compatible with the finding of no duplicates among 180 fractures. Observing a unique set of patterns in the sample supports (to varying degrees) a wide range of hypotheses about the duplication probability. To indulge an overly simplistic model, if we were to assume that the probability of detecting a duplicated pattern in each of the 5,310 comparisons were some identical, albeit small, number, then the 95% confidence interval for this duplication probability would go from zero (uniqueness) all the way up to 1/1770. (See Eypasch et al. 1995). To testify that the experiment supports only “the theory that coincidental duplication of fracture patterns cannot be attained” would be foolish. A more accurate statement would be that it supports the theory that duplication occurs at an unknown, but not very large, rate.

To be sure, there is reason to believe that duplication is improbable, and the UC-Davis study adds to our knowledge of fracture patterns. However, fractographers should think twice (or more!) before they testify that the study demonstrates the utter uniqueness of all fractures. They gain little by embracing the claim of universal uniqueness (Cole 2009; Kaye et al. 2011), and this study does not deliver on the promise of "objective criteria to determine the uniqueness of a fit." (Tulleners et al., p. 7).

  • Simon A. Cole, 2009. Forensics Without Uniqueness, Conclusions Without Individualization: The New Epistemology of Forensic Identification. Law, Probability and Risk 8:233-255
  • Ernst Eypasch, Rolf Leferinga, C K Kuma, Hans Troid, 1995. Probability of Adverse Events That Have Not Yet Occurred: A Statistical Reminder. Brit. Med. J. 311:619, available at
  • David H. Kaye, David E. Bernstein & Jennifer L. Mnookin, 2011. The New Wigmore, A Treatise on Evidence: Expert Evidence. New York: Aspen Pub. Co. (2d ed.)
  • David H. Kaye, 2009. Identification, Individuality, and Uniqueness: What's the Difference? Law, Probability & Risk 8:85-89, (abstract)
  • Frederic A. Tulleners, John Thornton & Allison C. Baca, 2013. Determination of Unique Fracture Patterns in Glass and Glassy Polymers, available at

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