Bones of Contention: A Standard for Analyzing Skeletal Trauma in Forensic Anthropology

The Academy Standards Board (ASB) of the American Association of Forensic Sciences (AAFS) posted the second proposed draft of a "Standard for Analyzing Skeletal Trauma in Forensic Anthropology" for public comment. The standard does not go far toward standardizing procedures or showing that the procedures to which it applies have been scientifically tested. Of course, it could well be that ample, well designed studies have demonstrated that forensic anthropologists can consistently and accurately classify skeletal defects in human remains according to the categories the standard mentions. But the standard contains no bibliography and no citations to show that this is the case. 

It contains some negative injunctions and a few positive suggestions about reporting -- for example:

• Forensic anthropologists shall not determine cause or manner of death.
• Practitioners shall not estimate the temperature or duration of heat exposure based on thermal defects to bone.
• Practitioners may report the minimum number of traumatic events (e.g., blunt impacts, projectile entry defects, or sharp defects) observed skeletally, but shall not report a definitive maximum number of impacts, as skeletal trauma evidence may not reflect all impacts to the body.
• When a suspect tool is submitted for analysis, similarities between the tool and defect may be reported; conclusions shall be reported in terms of an exclusion or failure to exclude.

As such, ASB 147-21 is not without any redeeming legal value. Nevertheless, it does not articulate any analytical process by which the classifications it calls for should be made (cf. "vacuous standards"); it requires no reporting of the uncertainty in this process; it does not contemplate the possibility of evidence-based rather than conclusion-based statements of the implications of the data; and it refers to an all-inclusive list of methods as "acceptable." If I may elaborate:

Is "Interpretation" Limited to an Opinion on the Inference (Conclusion) from the Data?

The revision defines "trauma interpretation" as "Opinion regarding the mechanism of, timing, direction of impact(s) or minimum number of impacts associated with skeletal defect(s) based on quantitative and/or qualitative observations." The phrase "based on ... observations" indicates that the opinion expresses a belief in the truth, falsity, or probability of an inference being drawn from the data. Interpretation should include the possibility of describing the strength of the evidence in favor of the inference rather than opining on the truth, falsity, or probability of the conclusion itself. In addition, if the opinion-statement is an assertion that the hypothesis about what happened is true or false (either categorically or to some probability), it is not just based on the data but on a prior probability for the hypothesis as well.

Despite these definitions, the standard sanctions "interpretation" in the form of rudimentary statements about the extent to which the data prove the hypothesis in question. Section 6 notes that "Trauma interpretation shall be clearly identified in the report using terms such as ‘indicative of’ and ‘consistent with’ or by using a subheading titled ‘Interpretation.’"These phrases have their problems, but they are one manner of referring to the probability of the evidence given the truth of certain probabilities rather than vice versa.

Is Interpretation Based on Non-scientific Evidence and Inference?

The revision introduces the following (non)criteria for deciding that blasts or explosions caused skeletal trauma: "Blasts/explosive events often cause blunt (including concussive) and projectile trauma to the body. When the trauma pattern and circumstantial information support a blast event, the trauma mechanism should be classified as 'blast trauma'”. The undefined notion of "support" is too vague to give any guidance. Is "consistent with" considered "support"? Let's hope not -- patterns can be "consistent with" one hypotheses (it could occur when the hypothesis is true) but much more probable under the opposite hypothesis.

And then there is the green light this recommendation gives to presenting a conclusion based on nonscientific "circumstantial evidence" as if it were based on expertise involving the skeletal evidence. Knowing that a blast occurred can drive the conclusion that the damage to the skeleton is "blast trauma." Should there also be a report on the skeletal evidence from an analyst blinded to the other information uncovered in the investigation?

Is Everything Acceptable?

ASB 147-21 states that "Skeletal trauma shall be examined. Acceptable methods to examine trauma include gross, microscopic, radiographic, and other analytical methods." This formulation deems every conceivable analytical method as "acceptable" no matter how poorly conceived it may be. Labeling everything as "acceptable" is troublesome in a standard that does not include criteria and procedures for performing the analysis and that does not lead the reader to any evidence of the reliability and validity of the undefined "analytical procedures."

Of course, forensic anthropologists know that some procedures do not work well, and only an outlier would use them. The drafters of ASB 147-21 undoubtedly appreciate the need for suitable methods (and hence prohibit certain conclusions that cannot be drawn with any existing method). Well motivated and informed forensic anthropologists will not be led astray if they consult the standard. But outliers do appear in court. Remember Louise Robbins. Unless the dubious method yields one of the explicitly prohibited statements in this standard, the outlier witnesses could maintain that they have proceeded exactly as the standard requires. Standards with this potential for abuse should be reformed.They should strive to standardize the methods they govern, and they should state what is known about the accuracy and reliability of these methods.