This reasoning is not very different from saying that a pitcher who does not strike out every batter couldn’t possibly be a reliable pitcher; that a polling firm that fails to correctly predict every election must be using methods that couldn’t possibly be reliable; or that a test for heart disease that sometimes errs couldn’t possibly be reliable. Without considering the success as well as the failure rate of the method, it is impossible to say that it is unreliable—or, by the same token, that it is reliable (in the sense of being worth relying on). 1/
Yet, the argument from cases of exonerations—lacking any comparison group—are legion in discourse on the use of trace evidence for identification. The latest example I encountered comes from a Washington Post blog site. Two days ago, Radley Balko wrote that
Mr. Balko went on to write that “not only did the courts continue to allow bite mark matching into evidence, every single time a defendant challenged its validity, that defendant lost.” 2/
Both the reasoning and the description of legal history are not quite right. To begin with, even absolute proof of innocence only shows that the test has a nonzero false-positive error rate (no surprise there) and that the witness should not have claimed to a certainty that no one else could have left the mark in question.
To be sure, some evidence that has found breathing space in the courtroom should be squeezed out entirely. But let’s face it—no scientific test meets the standard of perfection. If every case in which evidence that has produced false convictions meant “that the expert and his methods couldn’t possibly be reliable,” there could be no evidence. Overselling has occurred with every type of forensic evidence—from bitemarks to toolmarks to fingerprints to DNA. Courts, scientists, and criminalists should do their best to prevent this. Thus, whether through rules of evidence or through education and monitoring of analysts, testimony must be calibrated to the power of the scientific technique. The testimony should fairly express the known probative value of evidence from a validated method.
An example of testimony that violates this precept comes from Ege v. Yukins. 3/ In that case, a discredited dental expert testified as follows:
The eventual outcome in the case contradicts the assertion that no challenge to the admission of bitemark evidence has succeeded. The state trial judge in Ege realized that the testimony was improper and only “denied [postconviction] relief because of the lack of a contemporaneous objection and a view that the showing of prejudice was insufficient.” 5/
A federal district court also concluded that “expert testimony identifying the petitioner as the only possible perpetrator of the alleged bite mark in the Detroit metropolitan area was improperly admitted.” 6/
The U.S. Court of Appeals for the Sixth Circuit agreed “with the district court that ‘Dr. Warnick's opinion that the petitioner was the only person in the entire Detroit metropolitan area who could have made the mark on the corpse carried an aura of mathematical precision pointing overwhelmingly to the statistical probability of guilt, when the evidence deserved no such credence.’” 7/ It affirmed the order for a new trial.
In short, the argument that a method of forensic identification that has been proved to be fallible is, for that reason alone, inadmissible proves too much. Likewise, the claim that no challenge to bitemark evidence has ever prevailed is exaggerated (although not by much). 8/
Please do not misunderstand me. The series of articles on bitemark evidence from which the remarks I have quoted were taken is impressive and useful. In offering these corrections to two small parts that seem a bit extreme, I am not arguing that bitemark analysis, which has little claim to validity, is either reliable (in the statistical sense that repeated analyses of the same marks give the same answers) or valid (in the sense that the answers are more often correct when marks from the same source are analyzed than when marks from different sources are compared). From the writing I have seen, bitemark analysis does not cut it.
I also believe that cases of false convictions should be studied and that the existence of a given type of scientific evidence in these cases should not be ignored. Finding a large number of false convictions with such evidence present is a warning signal. The evidence may come from a method that has a large false-positive rate, 9/ and that possibility must be investigated to decide whether the evidence should be excluded across the board or whether juries should receive the information -- together with an honest and clear explanation of the uncertainty in the results.
- See infra note 9.
- Radley Balko, A High-ranking Obama Official Just Called for the “Eradication” of Bite Mark Evidence, The Watch, Wash. Post, July 22, 2015.
- 485 F.3d 364 (6th Cir. 2007).
- Ege v. Yukins, 380 F.Supp. 2d 852, 871 (E.D. Mich. 2005), affirmed in part, reversed in part, 485 F.3d 364 (6th Cir. 2007).
- Id. at 857–58.
- Id. at 858
- 485 F.3d at 376.
- The federal courts in Ege treated the answer to the 3.5 million people as "probability testimony" without questioning Michigan's general rule that bitemark identifications are admissible. A true (and wrongly decided) case of bitemark probability evidence is State v. Garrison, 585 P.2d 563 (Ariz. 1978).
- The false-positive probability is P(+|O), where + is a positive statement ("the defendant left the mark") and O is the fact that some other person left the mark. Even if this probability is small, a disturbing number of false convictions could involve this evidence. Suppose that P(+|O) = 0.02, that 1,000 tests are performed in a set of cases with marks, and guilty defendants left the marks in 60% of these cases. The expected number of false positives is (0.02)(400) = 8. Assume that the probability of a true positive is P(+|S) = 0.96, where S means that the defendant is the source of the mark. Then the expected number of true positives is (600)(0.96) = 576. If defendants are convicted in all these cases, 8 convictions will be false (assuming that the culprit left the mark), and the many true positives will not be seen in the cases of exonerations of the innocent defendants. As indicated at the outset of these remarks, other data than exonerations are required to judge whether the test is reliable and valid.