The notion that fragments that fit together must have come from the same larger piece of glass sounds like common sense. Almost all of us have broken glass or ceramic materials at one time or another and seen that the fragments (at least the major ones) can be reassembled. Apparently, criminalists have had the same experience. The supporting documentation for the ULTR notes that “[i]t has long been reported by forensic glass examiners that two glass objects that physically fit together were once part of the same broken object.” 1/
But what type of scientific inquiry would establish the truth of the common sense idea? Let’s do a series of thought experiments. I have ten drinking glasses in my cupboard. (Actually, six are in the dishwasher, but I’ll pretend all are clean and back in the cupboard.) They sure look alike to me, being the same size, shape, and color.
- Experiment 1. I take one glass out and strike it with a hammer. Then I pick up the pieces and discover that all pairs of fragments that were adjacent in the unbroken glass fit together.
- Experiment 2. I take out a second glass and strike it with a hammer. Then I put all the pieces in a bag with those from the first glass and mix them up. Will I find any pairs of fragments that seem to fit together that are Glass1-Glass2 (heteroginous, if I can coin a word that means “of different origin”), or will all the pairs that fit together be Glass1-Glass1 or Glass2-Glass2 (homoginous, a neologism for "of the same origin")? Let’s suppose that the subset of all the possible pairs that I find to match are indeed homoginous.
- Experiment 3. You guessed it. I take a third glass, strike it with the hammer, and examine all possible pairs from all the fragments from the three glasses. This is getting very time-consuming, for the number of pairs to consider is growing exponentially, but I am persistent. Can we conclude, on the basis of common sense, that each and every physically matching pair will be homoginous?
Nonetheless, it is quite reasonable to conjecture that only a tiny fraction (which could include 0) of apparently matching fragments that have complex edge patterns are heteroginous, and thus that finding a physical fit is powerful evidence of homoginality. 2/ Of course, the reasonableness of the conjecture depends on the criteria for a fit and the method for declaring one—neither of which are mentioned in the DOJ materials—but let’s assume the existence and maintenance of rigorous criteria and a reliable method. Can criminalists currently estimate how often the conclusion of homoginality is correct when the fragments (unbeknownst to the analyst) actually come from different pieces of glass?
Even if the answer to this question is in the negative, there are ways to present the physical match without overclaiming. Why must examiners “state or imply that the glass fragments were once part of the same broken object”? Examiners who believe that there is a physical fit can contribute a great deal simply by documenting and exhibiting the fit itself. If, additionally, they have some special expertise in moving from the fit to the conclusion of homoginality, then doing also could assist the fact finder. But if their inference is simply that of common sense, the expert aspect of the probative value of that part of the testimony is questionable. In these circumstances, the criminalist might be better advised to show how well the fragments fit together and leave it to the judge or jury to assess the weight of this demonstrable fact.
Another alternative to testimony “to the exclusion of all other sources” (see note 1) is a statement that the observed match is more probable when fragments really come from the same broken object than when they come from different broken objects. I do not know whether specific studies have been performed to justify this likelihood-ratio-type statement, but it should not be outside the capacity of forensic science to generate data that would support this rather modest statement. Indeed, the claim probably is too weak. My intuition is that the likelihood ratio is much greater than 1—it is a heck of a lot more probable to generate a matching pair of fragments by breaking the same object than by breaking different ones. A lawyer can appeal to give the evidence substantial weight on the basis of similar beliefs. If criminalists have experimental or observational studies to quantify how much more probable the physical match is for homoginous fragments, then they can provide that likelihood ratio.
Based on the material the DoJ has supplied, however, the more ambitious criminalists who want to “render an opinion of positive identity” and asservate “to the exclusion of all other sources” (see note 1), as the proposed ULTR apparently allows, should be required to express the limitation that no matter how positive they are in their opinion, the core of that opinion is a common sense judgment of what it means for pieces of glass to fit together rather than the product of elaborate scientific experimentation.
- An accompanying footnote cites four sources:
- Kirk, P., Density and Refractive Index: Their Application in Criminal Identification, Charles C. Thomas, Springfield, IL, 1951, p. 4-5.
- “Don’t Overlook Evidentiary Value of Glass Fragments”, Law Enforcement Bulletin, Vol. 33, No. 10, October 1964, p. 19.
- Bottrell, M.C., “Forensic Glass Comparison: Background Information Used in Data Interpretation”, Forensic Science Communications [Online], (April, 2009). Available: http://www.fbi.gov/about-us/lab/forensic-science-communications/fsc/april2009/review/2009_04_review01.htm. accessed on August 15, 2013.
- Koons, R. D., Buscaglia, J., Bottrell, M., and Miller, E. T. Forensic glass comparisons. In: Forensic Science Handbook. vol. I, 2nd ed. Richard Saferstein, Ed., Prentice Hall, Upper Saddle River, New Jersey, 2002, p. 161–213.
I do not have Paul Kirk’s 1951 book at hand. The Law Enforcement Bulletin is not generally regarded as a substantial scientific publication. The 2009 publication is a generally informative and comprehensive review article from the FBI. However, it contains no reports of examiners' experiences or experiments with physically fitting fragments of glass back together. In fact, only two sentences even refer to physically fitting fragments—and neither of them explains the basis for the conclusion that the ULTR endorses. (The first sentence is this: “Only physically matching two or more broken glass fragments allows for their association with each other to the exclusion of all other sources (Scientific Working Group for Materials Analysis [SWGMAT] 2005c).” The second is equally devoid of information on the foundation for the conclusion. It reads: “The ... Trace Evidence Unit Quality Assurance Manual (2006) states [that a] glass association is defined as two or more glass samples that can be fracture fitted together, or that exhibit indistinguishable observable properties and/or range overlap in all measured properties.”)
Finally, the FBI chapter in Richard Saferstein’s handbook discusses “types of fractures” (pp. 173-77) and later, “the mechanical fit.” The authors write that
The mechanical fit is one of the most desirable of forensic glass examinations because the examiner can render an opinion of positive identity—that two or more pieces of glass were once a portion of, and were broken from, the same pane or object. Glass is particularly suitable for this type of examination. It is amorphous and brittle, is neither stretched nor distorted by breakage, and can be reassembled to its original configuration. Because it is amorphous, no two glass objects will break in exactly the same way. (P. 179).As has been discussed extensively for other types of matching evidence, the claim of uniqueness at some possibly unattainable level of precision of measurement does not imply the truth of an unqualifiedly positive opinion of identity.
- See generally David H. Kaye, Beyond Uniqueness: The Birthday Paradox, Source Attribution, and Individualization in Forensic Science Testimony, 12 Law, Probability & Risk 3 (2013); David H. Kaye, Probability, Individualization, and Uniqueness in Forensic Science Evidence: Listening to the Academies, 75 Brooklyn L. Rev. 1163 (2010); David H. Kaye, Identification, Individuality, and Uniqueness: What's the Difference?, 8 Law, Probability & Risk 85 (2009).