Sunday, January 17, 2016

“Statistical Lawyer’s Tricks” with DNA Mixtures in the Trial of Tommy Whack

Maryland seems to produce more than its share of significant judicial opinions on DNA evidence. Perhaps this is partly the result of having an expert Forensics Division in the Maryland Office of the Public Defender. In any event, several years ago, the state’s high court unanimously agreed that the head of the homicide unit for the Prince George's County State's Attorney's Office badly misrepresented the meaning of DNA evidence. The case, Whack v. State, 1/ is instructive in several respects:
  • the state should have undertaken further DNA testing;
  • the police department’s DNA analyst's assessment of the “statistical significance” of the DNA evidence was incomplete;
  • the defense committed the so-called “prosecutor’s fallacy” of transposing conditional probabilities; and
  • the prosecutor's closing argument included a statement about probabilities that (according to the state's own lawyers on appeal) was so “preposterous” it could not have misled the jury (because no juror could have believed it).
Shooting a Dude

Early in the morning of October 21, 2008, police received a 911 call about a shooting in Landover, Maryland. They found George Jerome White, Jr., lying on the ground next to a pick-up truck. Barely able to speak, White told police he had been robbed by an approximately six-foot-tall black male, with light or medium skin complexion, about 20 years old, with long hair or dreadlocks. White later died from two gunshot wounds to his torso.

Not long before the shooting, eight calls were exchanged between White’s cell phone and one registered to Bryant Whack. Bryant, who lived in Virginia, was in town for a funeral and staying with his cousin, Thomas (Tommy) Whack, Jr.

Bryant told the police that Tommy used the phone to call a "chat line" and that sometime after midnight, Tommy said that he planned to meet a woman. The two cousins set off together, but Bryant lost sight of Tommy. Then he heard gunshots. A few minutes later, Tommy showed up, said "it was a dude," and they went back home.

Touch DNA for the State

The police did not have much other evidence connecting Tommy to the murder. Swabs from various places on and in the truck established that White’s DNA was in the sample from the passenger seat headrest. As the Court of Appeals described it:
The chance of the major DNA profile on the headrest coming from an African American other than White was one in 212 trillion; in other words, White's DNA matched the DNA profile extracted from the passenger seat headrest.
Arguably, this represents the transposition fallacy and is a questionable use of the term “match.” A more precise statement would be that the victim’s DNA matched the major profile at every locus, and the chance of a single, randomly selected African American doing so would only be 1/(212 trillion). The probability that some African American other than White contributed the profile is probably quite small, but it could be larger (or even smaller than) than the estimated frequency of unrelated African Americans with the major profile, which is what 1/(212 trillion) is.

But whatever the probability that the victim, White, was the source of the DNA on the headrest, the match between the major profile and White does not incriminate Tommy. In fact, the state’s DNA analyst concluded that none of "the DNA profiles of other, unknown people" in the sample "could have been [the defendant's]." The link to Tommy came from a different swab. This sample, from a passenger armrest,
contained a mixture of DNA from at least four people, with White's DNA being consistent with 14 of 15 tested locations in the DNA sample and [Tommy]'s DNA consistent with 11 of the 15 tested locations in the sample. In addition, the sample disclosed the DNA of at least two additional "unknown contributors."
With this extra evidence to buttress Bryant's thin story, Tommy was indicted on charges of first-degree murder, robbery, theft, and use of a handgun in the commission of a crime of violence. At trial, the state’s DNA analyst, Jessica Charak, testified to the meaning of this partial match:
     When it comes to mixtures, in saying that someone could potentially be included as a source of the mixture, we develop a statistic just as to how strong is that statement, what does it really mean. In this particular case what we do with mixtures is we have already made the statement that all of the DNA types of the victim [White] are accounted for at 14 of the 15 locations. That's a factual statement based on the results. It is also a factual statement that [at] 11 of the 15 locations all of the DNA types of [Petitioner] are accounted for.
     Now, the statistic that we do is on the mixture as an entire whole. So we ignore the fractions that we can say that those types are accounted for and we will calculate a statistic on everything, on every single DNA type that I was able to recover in the mixture. In this case what this probability says is what are the chances that another random person may also have a DNA profile that could also be included as a potential source of the mixture? In this case it was one in 172 individuals in the African American population would also have potentially have a DNA profile that I would have to say that they also could have contributed to that mixture.
This description of the implications of the data is hardly ideal. Why did Tommy match at only 11 loci? A juror might wonder why the fact that the alleles at four of Tommy’s 15 loci were not observed in the mixture did not exclude him as a potential contributor. Presumably, the analyst believed that it was possible that four alleles “dropped out” of the profile so that the 11-locus match was sufficient for an inclusion. And if four could have dropped out, why not five? Would a 10-locus match also have led to an inclusion? A 9-locus match? What was the analyst’s criterion for the minimum number of matching loci for “another random person [to] have a DNA profile that could also be included as a potential source of the mixture?” (In theory, it could be zero. However, that low a threshold would make every person in the world a potential source. With a random match probability of 1, the DNA evidence would have been irrelevant.)

A more complete and comprehensible analysis would compare (1) the probability of the data (including the heights rather than just the locations of all the peaks) on the assumption that White, Tommy, and two unknown individuals contributed to the sample and (2) the probability of the data on the assumption that White and three unknown individuals contributed. (Still more hypotheses could be considered if the number of contributors was uncertain.) Such “likelihood ratios” have been admitted in many cases, but they were not used here.

Transposition and Other Statistical Errors of the Defense

Let’s put these worries about the analysis and presentation of the partial match to the side — apparently, they were not raised at trial or on appeal. Let’s assume that 1/172 is the probability of an inclusion in the mixture for a randomly selected African American using whatever (perhaps arbitrary) criteria the analyst here employed. Even taking this figure at face value, both defense counsel and the state’s expert clearly committed the transposition fallacy (indicating that the common synonym of "prosecutor’s fallacy" is not always apt). On cross-examination, the defense lawyer asked:
Q: So you identified possibly that [Tommy] contributed to that sample, right?
A: Right.
Q: But the possibilities that some other African American contributed to that sample would be one in 172?
A: Right.
The correct answer would have been “No, I cannot tell you the probability that some other African American contributed to the sample. I am only saying that if some other, unrelated African American contributed and the defendant did not, then the probability that the defendant would have been included is 1/172.”

In closing argument, defense counsel made another statistically fallacious claim — that “[e]very 173rd person would have their DNA alleles in that car. So out of 1700 people, you have ten people.” Of course, it would very odd if exactly every 172d person would be identified as consistent with the mixture — just as it would very odd if a fair coin always came up heads every other toss. But this error is minor. It can be corrected by adding the words “on average” and “approximately” to the sentences.

“A Statistical Lawyer’s Trick” and the Prosecution’s “Preposterous” Reply

The big problem resided in the prosecutor’s rebuttal argument. For some reason, the state neglected to determine whether Bryant Whack, like his cousin Tommy, had a DNA profile that was consistent with the alleles observed in the mixture. Naturally, the defense tried to exploit this glaring gap in the state's case. Counsel pointed out that if 1 in 172 unrelated African Americans would be included, then the chance of including a cousin would be even higher. Bryant might have been the murderer! This DNA evidence, the defense emphasized, was nothing like the match of the victim to the headrest sample, which had an infinitesmal random match probability of 1/(212 trillion). A figure of 1/172 or more leaves plenty of room for doubt.

The prosecution’s responded by calling the comparison “a statistical lawyer trick”:
Remember, [Tommy] is the fourth sample, the only place where we have four samples. So if you remember, as Ms. Charak testified, that we know George White, one hundred percent George White, and [defense counsel] agrees is in that headrest sample, when I add two more 50 million times less. When we add [Tommy], and we know it's [Tommy], when we add [Tommy] that is why the number is 172. It is statistics again. It is a statistical lawyer trick. [Defense counsel] wants you to say don't believe the statistics because the science says he is there, but this 172 is no less strong than that 212. [Tommy] is there. [Tommy] left that DNA.
The jury acquitted Tommy of most of the charges — first-degree premeditated murder, first-degree felony murder, robbery with a dangerous weapon, robbery, and use of a handgun in the commission of a crime of violence. But the jury convicted him of second-degree murder, and he appealed.

The first appellate court, Maryland's Court of Special Appeals, apparently did not see any problem with the prosecution's presentation. It rejected the defendant’s claim that the state’s closing argument — to which counsel objected — was unfair, without bothering to write an opinion for publication.

Again, Tommy appealed, this time to the court of last resort in Maryland. Before the Court of Appeals, the state adopted an intriguing position. It admitted that “the prosecutor's statement regarding the probability statistics was factually incorrect,” but it contended that it was so astonishingly incorrect that the mistake did not matter. The Court of Appeals summarized this argument:
The State argues, however, that the prosecutor's statement, that "this 172 is no less strong than that 212," was "preposterous" and no jury would ever draw the conclusion that the two statistics were the same. The State contends that this statement was meant to suggest only that the science underlying both statistics was equally strong. The State acknowledges that the prosecutor's rebuttal argument may have been "inartfully argued," but the State maintains that the prosecution did not mischaracterize the evidence. Moreover, the State notes that the expert's report was admitted into evidence, allowing the jury to examine it further if there was any confusion.
The Court of Appeals’ Analysis

The Court of Appeals discerned two fatal flaws in the closing argument. First, it held that “the prosecutor went too far in stating emphatically that Petitioner's DNA was present in the truck.” But should not a party should be allowed to portray its theory of the events that unfolded as facts when it has presented evidence to support that theory? Emphatically asserting that Tommy leaned on the armrest and shot White is the state's theory of the case. To be sure, the evidence as a whole seems thin, but as the Court of Appeals acknowledged, "[t]he prosecutor is allowed liberal freedom of speech and may make any comment that is warranted by the evidence or inferences reasonably drawn therefrom."

Still, the statement that “the science says he is there” is far stronger than what "the science," as represented by Ms. Charak, said. Her testimony was that Tommy was among the "one in 172 individuals in the African American population [who] would also have potentially have a DNA profile that I would have to say that they also could have contributed to that mixture."

Even so, had the prosecutor asserted that the science proved that Tommy was there -- because it was unlikely (only a 1/172 chance) that a randomly selected African American would be included as a contributor -- there would have been no reversible error. The real problem is the second flaw that the Court of Appeals identified -- “overstating the statistical significance of the DNA evidence by equating the odds of one in 172 with one in 212 trillion.” The Court of Appeals conceded that “no jury was likely to believe that one in 172 was literally the same as one in 212 trillion,” but it still felt that “the prosecutor's statement could have seriously misled the jury” because “[t]he declaration that this one in 172 figure was ‘no less strong’ than the one in 212 trillion figure suggests that Petitioner's DNA ‘matched’ the DNA taken from the armrest to the same extent that White's DNA ‘matched’ a sample taken from the headrest.”

It may not be that unusual for prosecutors to push as hard as they can at trial, figuring that a conviction will stick on appeal (unless an appellate court perceives that the defendant might well have been innocent). In this case, however, the court explicitly announced a policy of DNA exceptionalism:
[C]ounsel have a responsibility to take extra care in describing DNA evidence, particularly when it comes to statistical probabilities. ... The prosecutor wrongly asserted that Petitioner's DNA was definitely on the armrest when the evidence demonstrated only that it might be present. The prosecutor also suggested that the statistical analysis backed up this assertion, urging jurors to draw an equivalency between the [near] mathematical certainty that White's DNA was in the truck with the probability that Petitioner's DNA was located there. These remarks were highly improper because the statements misrepresented complicated scientific evidence that was a key part of the prosecution's case.
Prosecutors in Maryland and elsewhere should heed this warning. They should not portray moderately small probabilities as overwhelmingly small. They should present their evidence for what it is worth.

The Rest of the Story

Although the court remanded for a new trial, that never came to pass. Instead, the prosecutor, Wes Adams, entered into a plea agreement. Tommy Whack was sentenced to 11 years in prison. 2/

Adams then entered the race for state's attorney in nearby Anne Arundel county. In a bitter election, he unseated the incumbent by a wide margin. 3/ When his performance in the Whack case became an issue, Adams insisted that he never mislead the jury and that he only decided against a new trial because of changes in the way DNA evidence is reported. "Hogwash," said his opponent. 4/

As the current County State's Attorney, Adams is “employing the highest ethical standards in vigorously prosecuting the guilty, protecting the innocent, and representing the interests of the State of Maryland in court.” 5/

  1. 433 Md. 728, 73 A.3d 186 (2013).
  2. Tim Pratt, State's Attorney Race Pits Leitess Against Adams, Capital Gazette, Oct. 6, 2014.
  3. Kelcie Pegher, Adams Ousts Incumbent Leitess for State's Attorney, Capital Gazette, Nov. 5, 2014.
  4. Id. 
  5. Wes Adams, Anne Arundel County State's Attorney,

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