Saturday, June 18, 2016

Traditional forensic pattern-matching testimony in Williams v. United States

The opinion of the District of Columbia's highest court in Williams v. United States, 130 A.3d 343 (D.C. 2016), is straightforward, but it is worth noting a modern example of traditional forensic pattern-matching testimony.

The case

In 2010, the bullet-riddled body of Min Soo Kang was discovered on the side of a road in Southeast D.C. Police retrieved three bullets that had penetrated the driver’s seat and fingerprints from his Cadillac Escalade in Northeast D.C. A trawl of the national Automated Fingerprint Identification System (AFIS) led police to suspect Marlon Williams. Pursuant to a search warrant, police took a High Point brand firearm from the bedroom of his apartment.

Bullets tested fired from the gun had impression from passing through the barrel that matched the ones on the bullets the car. The criminalistics evidence and “the testimony of an individual to whom Mr. Williams had made incriminating statements while they were in the courthouse cellblock formed the bulk of the government's case.” Id. at 346. The case was persuasive—the jury convicted Williams of first-degree murder and other crimes.

The firearms evidence
The firearms and toolmark examiner, Luciano Morales, testified on direct examination that when a bullet is fired from a particular gun, the gun leaves “unique” identifying marks, “similar to a fingerprint, basically.” He then testified that he had compared the markings on the bullets recovered from Mr. Kang's car with the markings on the bullets test-fired from the gun recovered from Mr. Williams's apartment (manufactured by High Point and admitted as Exhibit No. 58), and he had concluded that the bullets were fired by the same gun. On redirect, when the prosecutor asked whether there was “any doubt in [his] mind” that the bullets recovered from Mr. Kang's Escalade were fired from the gun found in Mr. Williams's room, the examiner responded, “[n]o sir.” He elaborated that “[t]hese three bullets were identified as being fired out of Exhibit No. 58. And it doesn't matter how many firearms High Point made. Those markings are unique to that gun and that gun only.” The prosecutor then asked the examiner whether, “judging from the markings that you find in 58, it's your conclusion that those three bullets were fired from 58?” The examiner was unequivocal: “Item Number 58 fired these three bullets.”
Id. The Court of Appeals held that the admission of this testimony was not error—because defense counsel did not object to it at the trial. Of course, the court recognized a standard exception to the need for a contemporaneous objection—the "plain error" rule. However, under this exception, “it is not enough for an appellant to demonstrate error; the appellant must also show that the error is ... ‘so egregious and obvious as to make the trial judge and prosecutor derelict in permitting it, despite the defendant's failure to object.’” Id. at 347.

Some readers might think that resorting to hyperbole could amount to a dereliction of a prosecutor's duty not to overstate its case and of an expert's obligation to stay within the bounds of what the expert discipline validly can claim to know. Indeed, the government conceded on appeal “that, at Mr. Williams's trial, it violated its policy ‘to only elicit firearms examiners' opinions to a reasonable degree of scientific certainty.’” Id. at 348. But the court balked at the idea that the trial judge should have demanded the phrase “reasonable scientific certainty.” It noted that
[W]e question whether this court would want to endorse a policy of “only elicit[ing] firearms examiners' opinions to a reasonable degree of scientific certainty,” in light of criticism that firearms examination does not involve any “scientific” measure of certainty. See National Research Council, Committee on Identifying the Needs of the Forensic Sciences Community, Strengthening Forensic Science in the United States: A Path Forward 154–55 (2009); National Research Council, Committee to Assess the Feasibility, Accuracy, and Technical Capability of a National Ballistics Database, Ballistic Imaging 3, 81–82 (2008).
Id. at 348 n.11.

The "reasonable scientific certainty" mantra definitely has its problems, 1/ but the better argument was that the dereliction of duty evident in Williams was not the kind of error that could satisfy the plain-error doctrine: “The government's internal policy does not constitute binding law—let alone a ‘clear’ or ‘obvious’ rule—that a trial court should be presumed to know.” Id. at 348–49 (notes omitted). “[O]nly when an error is ‘clear under current law’ can it be “plain.” Id. at 347.

But what is "current law"? Does it take a previous opinion in the same jurisdiction to make a proposition "clear under current law," or can the result clearly be immanent the fabric of such opinions, or even in those of other jurisdictions?

Although not without ambiguity, the opinion seems to accept the latter, more expansive understanding of "current law." First, it reads its previous opinions as too tentative or reserved to establish a legal rule precluding testimony of 100% confidence.
The closest this court has come to addressing this issue was in (Ricardo) Jones v. United States, 27 A.3d 1130 (D.C.2011). In that case the defense argued inter alia that toolmark and firearms examiners could not “stat[e] their conclusions with ‘absolute certainty excluding all other possible firearms.’ ” Id. at 1138. In response, the government assured this court, both in its appellate brief and at oral argument, that it was the government's policy not to present such testimony. “In light of the government's representation,” this court “assume[d], without deciding, that such experts should not be permitted to testify that they are 100% certain of a match, to the exclusion of all other firearms.” Id. at 1139. The court then determined that any such error was harmless. Id. Jones did not plainly bar the toolmark examiner in this case from testifying as he did and does not provide a foundation for a determination of plain error.
Id. at 348. Thus, “[t]here [was] no precedent in this jurisdiction that limits a toolmark and firearms examiner's testimony about the certainty of his pattern-matching conclusions.” Id. at 347-48. Although the court was willing to consider "the weight of non-binding authority outside this jurisdiction," it could locate very few appellate opinions condemning expressions of absolute certainty. Id. at 348. Hence, Williams’ objection, raised for the first time on appeal, came too late.

The future

If Williams had objected at trial, would it have made any difference to the outcome? The established view is that the expert can express total certainty in a “unique” identification. But if the expression of 100% confidence in Williams is legally (or professionally) unpalatable (as a concurring opinion by the author of the panel opinion suggested), how should a firearms examiner testify? The legal contrivance of “reasonable scientific certainty” is not a viable alternative. 1/ Statements about “likelihoods”—the probabilities of the matching patterns being seen (1) when the sample gun fires the bullet, and (2) when different guns fire the bullet—are widely recommended by students of forensic inference. Subjectively assessed likelihoods are appealing if they are frankly and transparently explained and the expertise underlying them has been demonstrated to exist. In the longer term, will efforts to achieve objective measurements and to use machine learning or other statistical tools to quantify the likelihoods for those measurements 2/ lead to a better, or at least a complementary approach?

  1. See Is "Reasonable Scientific Certainty" Unreasonable?, Forensic Sci., Stat. & L., Feb. 26, 2016, .
  2. John Song, Xiaoping Liu. A Review of NIST Projects in Surface and Topography Metrology for Firearm Evidence Identification in Forensic Science, 1 J. Sci. & Industrial Metrology 1 (2016); T.V. Vorburger et al., Topography Measurements and Applications in Ballistics and Tool Mark Identifications, Surface Topography: Metrology and Properties, Dec. 17, 2015,

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