More than zero: accounting for error in latent fingerprint identification.

Author:Cole, Simon A.

LOUISE: I never would have guessed that he was selling fake insurance.

CANEWELL: That's what the whole idea was ... he didn't want you to guess it. If you could have guessed, then he couldn't have sold nobody no insurance.

--August Wilson, Seven Guitars (1996)


The year 2004 witnessed what was probably the most highly publicized fingerprint error ever exposed: the case of Brandon Mayfield, an Oregon attorney and Muslim convert who was held for two weeks as a material witness in the Madrid bombing of March 11, 2004, a terrorist attack in which 191 people were killed. Mayfield, who claimed not to have left the United States in ten years and did not have a passport, was implicated in this attack almost solely on the basis of a latent fingerprint found on a bag in Madrid containing detonators and explosives in the aftermath of the bombing. Unable to identify the source of the print, the Spanish National Police emailed it to other police agencies. Federal Bureau of Investigation (FBI) Senior Fingerprint Examiner Terry Green identified Mayfield as the source of the latent print. (1) Mayfield's print was in the database because of a 1984 arrest for burglary and because of his military service. The government's affidavit stated that Green "considers the match to be a 100% identification" of Mayfield. (2) Green's identification was "verified" by Supervisory Fingerprint Specialist Michael Wieners, Unit Chief, Latent Print Unit and fingerprint examiner John T. Massey, a retired FBI fingerprint examiner with over thirty years of experience.

Kenneth Moses, a well-known independent fingerprint examiner widely considered a leader in the profession, subsequently testified in a closed hearing that, although the comparison was "quite difficult," the Madrid print "is the left index finger of Mr. Mayfield." (3) A few weeks later the FBI retracted the identification altogether and issued a rare apology to Mayfield. (4) The Spanish National Police had attributed the latent print to Ouhnane Daoud, an Algerian national living in Spain.

The error occurred at a time when the accuracy of latent print identification has been subject to intense debate. Because the Mayfield case is the first publicly exposed case of an error committed by an FBI latent print examiner and the examiners were highly qualified, it was particularly sensational.

But the Mayfield case was not the first high-profile fingerprint misattribution to be exposed in 2004. (5) In January, Stephan Cowans was freed after serving six and a half years of a 30- to 45-year sentence for shooting and wounding a police officer. (6) Cowans had been convicted solely on fingerprint and eyewitness evidence, but post-conviction DNA testing showed that Cowans was not the perpetrator. (7) The Boston Police Department then admitted that the fingerprint evidence was erroneous, (8) making Cowans the first person to be convicted by fingerprint evidence and exonerated by DNA evidence. (9) As with the May field case, the Cowans misattribution involved multiple experts, including defense experts. (10)

Latent print examiners have long claimed that fingerprint identification is "infallible." (11) The claim is widely believed by the general public, as evidenced by the publicity generated by the Mayfield and Cowans cases, with newspaper headlines like "Despite Its Reputation, Fingerprint Evidence Isn't Really Infallible." (12) Curiously, the claim even appears to survive exposed cases of error, which would seem to puncture the claim of infallibility. (13) Such cases have been known since as early as 1920 and have not disturbed the myth of infallibility. (14) Today, latent print examiners continue to defend the claim of infallibility, even in the wake of the Mayfield case. (15) For example, Agent Massey commented in a story on the Mayfield case, "I'll preach fingerprints till I die. They're infallible." (16) Another examiner declared, in a discussion of the Mayfield case, "Fingerprints are absolute and infallible." (17)

The question of the "error rate" of forensic fingerprint identification has become a topic of considerable legal debate in recent years. "Error rate" has been enshrined as one of the non-definitive criteria for admissible scientific evidence under the United States Supreme Court's decision Daubert v. Merrell Dow Pharmaceuticals. (18) In discussing how trial judges should exercise their "gatekeeping" responsibility to ensure that "any and all scientific testimony or evidence admitted is not only relevant, but reliable," (19) the Court noted that "in the case of a particular scientific technique, the court ordinarily should consider the known or potential rate of error." (20) In Kumho Tire v. Carmichael, (21) the Court decided that the "gatekeeping" responsibility extended to non-scientific expert evidence and reiterated the same non-definitive checklist it enumerated in Daubert. (22) Though courts have found that latent print identification is non-scientific expert evidence, Kumho prevents such a determination from becoming a loophole through which latent print identification could evade Daubert's requirement that judges ensure its reliability. Indeed, the Court specifically noted that even the case of "experience-based" testimony--which, presumably, is what latent print identification is, if it is not science--it is relevant to know the error rate. (23) Although the Supreme Court was careful to note that its proposed checklist was merely illustrative, courts frequently treat it as a de facto litmus test for admissibility. Since criminal defendants began challenging the admissibility of forensic fingerprint evidence in 1999, (24) the error rate of fingerprint evidence has been extensively discussed and litigated.

Curiously, it would appear that the Court's inclusion of error rate in Daubert/Kumho, rather than having the palliative effect of encouraging latent print examiners to measure their error rate, has had the unintended consequence of tempting them to make even less sustainable claims. (25) Thus, in response to a challenge to the admissibility of latent print evidence under Daubert/Kumho, the government and latent print examiners advanced the "breathtaking" (26) claim that the error rate of forensic fingerprint identification is zero. (27)

As with infallibility, latent print examiners defend the claim of a zero error rate even when confronted with known cases of misattribution in real cases. In a 60 Minutes interview about the Jackson case, Agent Meagher demonstrated an identification to reporter Leslie Stahl:

MEAGHER: The latent print is, in fact, identical with the known exemplar.

STAHL: It's identical?


STAHL: You can tell that?


STAHL: What are the chances that it's still not the right person?


STAHL: Zero.

MEAGHER: It's a positive identification. (28)

How can a process commit errors and yet be considered infallible? How can the "error rate" of any technique, let alone one that has been known to commit errors, be considered zero? In this article, I will argue that the coexistence of these two contradictory notions is not merely a product of simple "doublethink." (29) Rather, I will show that it the product of a rhetorical strategy to isolate, minimize, and otherwise dismiss all exposed cases of error as "special cases," or "one-offs," (30) and therefore as irrelevant.

After a brief legal and technical background discussion in Part I, Part II of this paper discusses what we do know about the error rate of latent print identification. Part II.A catalogs twenty-two cases of fingerprint misattribution that have been reported in the public record. An analysis of these cases shows that they are most likely only the tip of the proverbial iceberg of actual cases of fingerprint misattribution. Part II.B discusses the results of proficiency testing of latent print examiners. These tests also show a non-zero error rate. In Part III, I discuss what might be called "the rhetoric of error." This Part analyzes rhetorical efforts by fingerprint advocates and courts to minimize, dismiss, and explain away the evidence of error revealed in Part II. Fingerprint practitioners seek to create an error-free aura around fingerprint identification that has the potential to dangerously mislead finders of fact. At the end of Part III, I discuss some more defensible ways of conceptualizing fingerprint error. Far from being "one-offs," I suggest that the cases of error are more likely the product of routine practice. Whatever special circumstances exist in the misattribution cases are more likely to account for the exposure of the misattribution than the misattribution itself. I conclude by arguing that it is necessary to confront, analyze, and understand error if we ever hope to reduce it.



      Latent print identification is a process of source attribution. (31) Latent print examiners compare "latent" prints taken from crime scenes to prints of known origin. Although prints taken from suspects using ink or scanners are typically of good quality--and can be re-taken if they are not--latent prints are typically partial, smudged, or otherwise distorted. It is the poor quality of many latent prints that makes latent print identification problematic. The most valuable aspect of the latent print testimony in criminal justice proceedings is the attribution of the latent print to the defendant. Although latent print testimony is often phrased as claiming that the latent print and the known print of the defendant are "identical," this is not strictly true; all fingerprint impressions, including those taken from the same finger, are in some way unique. (32) The true import of latent print testimony is not that the unknown print and the known print are "identical" but rather that they derive from a common source. (33) Since the source of the known print is known to be the defendant (because someone in the...

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