Williams v. Illinois (Part I: Just the Facts)

The Supreme Court heard oral argument last week in Williams v. Illinois. The case could have been just another of the thousands of rape cases that work their way through state courts across the country. But the prosecution decided to take a shortcut to convict Sandy Williams. Rather than present the laboratory analyst who produced the DNA profiles of the victim and the rapist, it had an analyst from a completely different laboratory testify in a bench trial. This gap in the state's case eventually attracted the attention of the U.S. Supreme Court. Indeed, Williams is the third case in as many years in which the Supreme Court has agreed to review criminal convictions relying on the findings of laboratory workers who do not appear at trial.

Unsurprisingly, the parties frame the issue before the Court differently. On one hand, according to Williams the question is

Whether the prosecution violates the Confrontation Clause when it presents . . . the substance of a testimonial forensic laboratory report through the trial testimony of an expert witness who took no part in the reported forensic analysis, where the defendant had no opportunity to confront the analysts who authored the report.

Brief for Petitioner at i. On the other hand, according to the state, the issue is

Whether a criminal defendant’s Sixth Amendment right to confront witnesses against him is satisfied where a prosecution expert testifies live at trial to her independent, expert opinions and is subject to unrestricted cross-examination.

Brief for Respondent at i.

How can the expert’s opinions be “independent” if she simply took what someone else told her as true in forming them? What is the value of “unrestricted cross-examination” when the witness’s knowledge of what transpired is so severely restricted? Understanding what information the testifying witness relied on seems crucial to an informed resolution of the case. Yet, the prosecution elicited no detailed information on this at trial. In this posting, I shall describe the facts of the case more fully and what I was able to extract by reading the trial transcript about the “data” (as the state’s witness called it), used in forming her “independent, expert opinions” (as Illinois calls them). Later postings will comment on the oral argument and the legal issues.

The Rape Kit

The Supreme Court of Illinois outlined the events leading to the submission of biological evidence to the state laboratory:

On February 10, 2000, 22–year–old L.J. worked until 8 p.m. as a cashier at a clothing store in Chicago. On her way home . . . [a]s she passed an alley, the defendant came up behind her and forced her to sit in the backseat of a beige station wagon, where he [sexually assaulted her]. He then pushed L.J. out of the car while keeping L.J.'s coat, money, and other items. After L.J. ran home, her mother opened the door and saw her in tears, partially clothed with only one pant leg on. [H]er mother called the police.

Shortly after 9 p.m., Chicago police officers arrived at the home . . . . After L.J. told the officers what had transpired, the officers issued a “flash” message for a black male, 5 foot, 8 inches tall, wearing a black skull cap, a black jacket and driving a beige station wagon. An ambulance transported L.J. and her mother to the emergency room. [V]aginal swabs . . . were . . . placed into a criminal sexual assault evidence collection kit along with L.J.'s blood sample. The kit was sent to the Illinois State Police (ISP) Crime Lab for testing and analysis.
On February 15, 2000, [a] forensic biologist . . . performed tests that confirmed the presence of semen. . . .

People v. Williams, 939 N.E.2d 268, 270–71 (Ill. 2010).

To Maryland and Back

Like too many police laboratories, the ISP lab was behind in processing rape kits and other DNA samples. So after letting the rape kit sit for nine months, it sent the vaginal swab that it knew contained semen along with the reference sample of LJ’s blood to a private company, Cellmark Diagnostics, in Germantown, Maryland, via Federal Express. Cellmark received the samples the next day (November 29, 2000). Another fours months went by before Cellmark returned the samples and supplied a report (on April 3, 2001).

The analysis of the reference sample of the victim’s blood at Cellmark should have been straightforward. That sample had plenty of DNA purely from LJ. Nevertheless, no one testified about the electropherogram. Was it clean, with clear peaks, or did it have blobs, pull-up, or off-ladder peaks? The transcript of the testimony, reproduced in relevant parts at the end of this posting, does not contain such questions. The testifying expert never looked at this electropherogram or the data underlying it.

The transcript does suggest, however, that the vaginal swab was not so easily analyzed. Vaginal swabs from rape victims often contain epithelial cells from the victim and some number of sperm cells. Extracting the DNA yields a female fraction from the victim’s epithelial cells and a male fraction from the sperm cells. The two DNA profiles will be mixed together, and someone must deduce which male profiles are consistent with the mixture (and the probability that each such profile is present in the mixture).

Sometimes this “mixture deconvolution” can be avoided by a procedure that extracts the female DNA first and then extracts the male DNA (because sperm cells are harder to break open than are epithelial cells). Under ideal conditions, the latter extract contains DNA from the semen cells only. Unfortunately, the differential extraction failed. The second extract still was a mixture of DNA from LJ and the unknown rapist.

So the Cellmark analyst did his or her best to decipher it by “subtracting” the victim’s peaks (as ascertained from the victim’s reference blood sample) and attributing the remaining peaks to the rapist. This was not as simple as it sounds, because the individual STR alleles are not that uncommon, and the victim and the rapist probably had some alleles in common. In any event, the Cellmark analyst arrived at a single profile — a set of 13 pairs of numbers characterizing the DNA that the rapist inherited from his mother and father. The unnamed analyst believed that the semen had the following profile: D3 (16, 19), DWA (17, 17), FGA (18.2, 22), D8 (14, 14), D21 (29, 30), D18 (13, 17), D5 (12, 13), D13 (11, 11), D7 (10, 12), D16 (9, 11), TH01 (7, 7), TPOX (11, 11), and CSF (8, 10). The analyst’s report included this profile and LJ’s profile (derived from her reference blood sample) as well as at one electropherogram from the mixture.

Meanwhile Williams’ Profile Goes Into the Police Database

During this year of desultory activity, Sandy Williams ran into trouble with the Chicago police:

On August 3, 2000, police arrested the defendant for an unrelated offense and, pursuant to a court order, drew a blood sample from [him]. On August 24, 2000, forensic scientist Karen Kooi [at the ISP lab] performed an analysis on the sample . . . . Kooi extracted a [DNA] profile and entered it into the database at the ISP Crime Lab.

Williams, 939 N.E.2d at 270–71. Which database this was is not clear. It seems unlikely that it was the state’s database for linking known criminals to crime-scene samples, for that database was limited to convicted offenders. Only this year did Illinois pass a law to extend the state database to include certain arrestees.

ISP Uses the Cellmark Profile to Pick Out Williams

The analyst at the police lab who received the materials from Cellmark was Sandra Lambatos. Her testimony is rather fuzzy with regard to how she proceeded. I cannot tell for certain whether she immediately entered the male profile as Cellmark reported it into the unspecified computer database system or whether she waited to do that until she looked more deeply into Cellmark’s report. In any event, the queried the database for the profile that Cellmark reported must have come from the sperm. Bingo! This profile matched the profile of Williams taken after his arrest in August. Every 13-locus profile is exceedingly rare in the general population — perhaps unique to an individual and any identical twins.

Police put Williams in a line-up on April 17, 2001, and LJ identified him as the man who sexually assaulted her over a year earlier.

The ISP Analyst Testifies — and the Cellmark Analyst Does Not

At trial, the state did not call anyone from Cellmark — the laboratory that did the DNA profiling of the vaginal swab. It relied instead on Ms. Lambatos to talk about the results. Lambatos verified that the 13 pairs of numbers that constituted the ISP’s profile of Williams’ reference blood sample were the same as the 13 pairs that constituted the male fraction reported by Cellmark. This established that the database profile with Williams’ name associated with it was, indeed, Sandy Williams’ profile.

At some point before the trial, Lambatos looked at one of the electropherograms from Cellmark — the one following the differential extraction for the mixture in the vaginal sample. She did not take the Cellmark electropherograms from the victim and from the vaginal swab mixture and compare them herself to deduce the rapist profile. Instead, she accepted Cellmark’s report of the victim’s profile as a given and looked only at the mixture electropherogram to infer the rapist profile.

In short, from what I can glean from the testimony, Cellmark's deduction of the male profile is a nontrivial, human inference (although it also could be done with software). But it also looks as if Lambatos made the same deduction using (1) Cellmark's electropherogram of the mixture, and (2) the alleles reported by Cellmark for the reference sample of the victim's blood. With regard to the alleles in (2), getting from the electropherogram for the victim's reference sample of blood to her STR profile could have been quite simple, but there could have been a bit of interpretation there too. In any event, this is not a case of an interpretive analyst starting with two electropherograms of single-source profiles — Lambatos' testimony about forming an "independent opinion" from the Cellmark "data" notwithstanding.

Appendix
Excerpts from the trial testimony of Sandra Lambatos
identifying Sandy Williams as the rapist

Direct examination [JA 42]

Q  What is your current occupation?
A  I am a stay-at-home mom. [JA 43]
Q  Where did you work before that?
A  At the Illinois state police crime laboratory.
. . . [JA 51]
Q  [O]n the date of November 28th of 2000, was evidence from this case sent to (Cellmark) diagnostic laboratory . . . ?
. . . [JA 52]
Q  What was the evidence that was sent?
A  Vaginal swab and a blood standard from [LJ].
. . . [JA 54]
Q  And does this [shipping manifest] indicate the date that the evidence . . . was sent back . . . from (Cellmark) . . . ?
A  It does.
Q  And what is the date . . . ?
A  April 3d of 2001.
. . . [JA 56]
Q  Was there a computer match generated of the male DNA profile found in semen from the vaginal swabs of [LJ] to a male DNA profile that had been identified as having originated from Sandy Williams?
A  Yes, there was.
Q  Did you compare the semen . . . from the vaginal swabs . . . to the male DNA profile that had been identified by Karen Kooi from the blood of Sandy Williams? [JA 57 ]
A  Yes, I did.
. . .
Q  What was your conclusion?
A  I concluded that Sandy Williams cannot be excluded as a possible source of the semen identified in the vaginal swabs.
Q  In other words, is the semen identified in the vaginal swabs of [LJ] consistent with having originated from Sandy Williams?
A  Yes.
Q  What is probability of this profile occurring in the general population?
. . .
A  This profile would be expected to occur in approximately 1 in 8.7 quadrillion black, 1 in 390 quadrillion white, or 1 in 109 quadrillion Hispanic unrelated individuals.
. . . [JA 58]
Q  In your expert opinion, can you call this a match to Sandy Williams?
A  Yes.

Cross examination [JA 61]

Q  And that report [from Cellmark dated Feb. 15, 2001] included an allele chart, correct?
A  Yes.
. . .
Q  And that included data that you used to run your data bank search.
A  Correct. [JA 62]
Q  You did not interpret the results by [Cellmark], did you?
A  Partially. I did review their data, and I did make my own interpretations. So I looked at what the programs, what they sent to me, and did make my own interpretation, my own opinion.
Q  That would be the vaginal swab with respect to the electropherogram E2, right?
. . .
A  Yes.
Q  You did not receive electropherograms for the E1?
A  I believe all I have in my case file is E2, correct.
Q  And you did not receive electropherograms from the standard of [LJ], did you?
A  No, I did not.
. . . [JA 68]
Q  And you reviewed the electropherograms just for that second fraction from the differential extraction [procedure], correct?
A  Correct.
Q  You did not receive the electropherograms for the first part of the procedure, that first part of the extraction, did you?
A  Correct.
. . . [JA 69]
"[T]hey sent the chart that was in the F1 fraction E1. Also the profile that was in the E2 fraction and the profile that was in [LJ]'s standard, and I had only the electropherograms from the E2 fraction . . . .
Q  But you did not receive their data or their electropherograms?
A.  No, I did not receive electropherograms for those fractions.
Q  You never received any computer data, the electronic data.
A   I myself did not receive that, but that was sent to the laboratory.
Q  You never viewed that?
A  Oh no, I did not.

First posted to The Double Helix Blog, 13 Dec. 2011

Legislation to Implement Kinship Matching in Criminal DNA Databases

A few years ago, “Jeffrey Rosen, a constitutional law professor at George Washington University, warned: ‘I can guarantee if familial searching proceeds, it will create a political firestorm.’” (1) But familial searching, as it is tendentiously called, prompted no huge political protests when California, Colorado, New York, and Virginia adopted it administratively. Now, legislative initiatives to implement it in various states (2, 3) and federally (4) have begun. However, the proposed legislation is timid, usually authorizing the practice only in murder and sexual assault cases and only after traditional investigative methods have failed.

References

1. Maura Dolan & Jason Felch, California Takes Lead on DNA Crime-fighting Technique: The State Will Search its Database for Relatives of Unidentified Suspects in Hopes of Developing Leads, Los Angeles Times, Apr. 26, 2008

2. Mike Cook, DNA — It’s All in the Family, Minnesota House of Representatives Session Weekly: News from the House, Apr. 8, 2011

3. Mark Scolforo, DNA Proposal Has Foes: Pa. Bill to Expand its Collection Opposed by the ACLU, Phil. Inquirer, Oct. 2, 2011

4. Press Release, Schiff's Familial DNA Language Passes as Part of Conference Report, Nov. 21, 2011

Cross-posted from The Double Helix Law Blog

New Mexico Supreme Court Proposes Rules on Lab Reports

The New Mexico Supreme Court is soliciting comments on "three representative proposals that have been suggested . . . to address the admission of state laboratory forensic analyses in light of Bullcoming." [1] The state court is referring to the U.S. Supreme Court's somewhat unenlightening opinion in Bullcoming v. New Mexico, 131 S. Ct. 2705 (2011), discussed here some months ago. Inasmuch the the rules are a response to a Confrontation Clause decision that applies only in criminal cases, I shall assume that these rules also would apply solely in criminal cases.

The first proposal covers all "forensic scientific evidence including blood and breath alcohol test reports, controlled substance chemical analysis reports." It orders the parties to confer about stipulating to a waiver of the defendant's right to be presented with a laboratory analyst to cross-examine on the laboratory's findings and of the state's right to present a live witness along with the reports. [1]

Of course, the parties can initiate such such discussions now, and they need not stipulate to anything anyway. The proposed rule states that "The report or print-out ... shall not be admitted at trial without the testimony of necessary witnesses unless the defendant stipulates in writing," but that adds nothing of substance to the status quo (or to the rest of the rule).

But what happens if one or more of the parties do not event want to talk about a stipulation. The rule says that "[i]f either party cannot obtain the opposition’s position regarding a proposed stipulated order, that party may file a motion requesting a hearing to determine the opposition’s position regarding the need for testimony ... ." Maybe the judge can induce the parties to take an irrevocable position well before the trial, as the rule seems to contemplate. This might help the lab schedule its staff time, but is all this judicial rule-making worth the effort to achieve this convenience?

Alternative Rule 2 abolishes the hearsay rule as applied to "a written report of the conduct and results of a chemical analysis of breath or blood for determining blood alcohol concentration." Of course, many jurisdictions have statutes to this effect, and others apply the business records exception to reach the same result. The (new?) hearsay exception makes no difference in New Mexico criminal cases as long as the U.S. Supreme Court adheres to the interpretation of the Confrontation Clause articulated in Crawford v. Washington, 541 U.S. 36 (2004).

Alternative 3 requires the prosecution to serve on the defendant "[a] copy of a report of the methods and findings of any examination conducted by an employee of any governmental laboratory ... no later than ninety (90) days before trial" and to give notice to the defense at the same time if it intends to introduce the report into evidence. If New Mexico prosecutors do not already provide timely disclosure of reports or if this provision requires that more complete reports be prepared than is currently the practice, it would be a significant improvement.

Yet, the rule also imposes a burden on the defense to object in writing before trial. In this regard, it reads as follows: "If the defendant does not file a written objection with the court to the use of the laboratory report and certificate within the time allowed by this subparagraph, then the report and certificate are admissible in evidence."

The thinking seems to be that the defense ordinarily should not have to object before trial. However, if the prosecution affirmatively notifies defense counsel that it does not plan to present a necessary witness, then the defense should be forced to make a pretrial demand for confrontation (or lose that right). The rule would require the defendant to give notice at least 30 days before trial. [3] A dictum in Melendez-Diaz v. Massachusetts, 129 S. Ct. 2527 (2009), approves of such notice-and-demand rules. [4]

Interestingly, none of the proposed rules go so far as to place the burden on the defendant to give notice in all cases in which it learns that a laboratory report exists. The state rules committee's commentary to the first alternative rule states: "This rule applies in lieu of a notice and demand rule, which the committee rejected, and is meant to ensure that the waiver is not made by accident or lack of knowledge. The defendant may waive this right by stipulated order, but the waiver shall be made willingly, knowingly, and intelligently."

Notes

1. Myrna Raeder initiated a discussion of these rules on Roger Park's discussion list for law professors. The rules are available at http://nmsupremecourt.nmcourts.gov/rules/pdfs/proposed_alt._rules_1,_2,_3.pdf

2. The proposed rule states that "[t]he parties shall confer and either party may file a stipulated order to admit a report or print-out of results ... or to limit the witnesses required to appear at trial."

3. This proposed rule include the statement that "[i]f the defendant does not file a written objection with the court to the use of the laboratory report and certificate within the time allowed by this subparagraph, then the report and certificate are admissible in evidence ... ."

4. See http://federalevidence.com/node/1228; http://www.scotusblog.com/case-files/cases/briscoe-v-virginia-2/.

DNA Identification Technology: Fast and Furious

Today’s talks at the International Symposium on Human Identification indicated some directions in which DNA-based identification technology will move in the near future. For example, one company reported a way to type 26 different STRs simultaneously. Is that enough to justify testimony of global individualization (with the exception of identical twins)?

The Departments of Defense, Homeland Security, and Justice are seeking self-contained devices for rapid STR profiling and interpretation, and several companies claim to be on the verge of delivering them. “Rapid” means an hour or so, and the hope is that these microfluidic devices will permit on-the-spot (or at-the-police-station) results for investigations as well as DNA database queries and entries. One company promises a functioning product in April 2012. Another refers to an existing instrument “compact enough to be used in an office setting, airport security area, mobile van, or field-forward military site.”

None of these has been fully validated. The FBI is figuring on widespread implementation at local police stations in 4-7 years, but police in Palm Bay, Florida, have posted videos on YouTube to advertise their success with a microfluidic device in “Operation Rapid Hit.”

Finally, companies are supplying police with phenotype and ancestry data, including probable eye and hair color. For the future, the most impressive -- and disquieting -- approach uses “next-generation sequencing” to extract all the usual STRs, together with phenotypically and medically informative data in one fell swoop.

Indeed, sequencing the oral bacteria that we host is possible. A speaker described one individual whose microbiome included a bacterium used in the industrial production of yogurt and cheese. Just imagine the APB: “The suspect is a white male with brown hair (probability = 0.45) and blue eyes (probability = 0.95) who likes yogurt.”

An Odd Set of Odds in Kinship Matching with DNA Databases

The 22d International Symposium on the Future of Human Identification began yesterday with a set of workshops. One was on "familial searching." The phrase refers to trawling the profiles in a DNA database for certain types of partial matches to a DNA profile from a crime-scene sample.

Partial matches that are useful in generating investigative leads to family members arise much more often when a particular kind of relative (say, a full sibling) is the source of the crime-scene sample than when an individual who is not closely related to the database inhabitant is the source. The ratio of the probability of the partial match under the former condition (a given genetic relationship) to the latter (unrelated individuals) is a likelihood ratio (LR). The LR (or, technically, its logarithm) for siblingship expresses the weight of the evidence in favor of the hypothesis that the source is full sibling as opposed to an unrelated individual.

After explaining the this idea, the first speaker presented the following formula:

"Odds" = LR_autosomal x LR_Y-STR x 1/N         (1)

She attributed this formula to the California state DNA laboratory that does familial searching in that state. In this equation, N is the size of the database, LR_autosomal is the likelihood ratio for the partial match at a set of autosomal STR loci, and LR_Y-STR is the likelihood ratio for the matching Y-STR haplotype.

She described this as a Bayesian computation that could lead to statements in court such as "there is a 98% probability" that the person whose DNA was found at the crime scene is a brother of Joe Smith, a convicted offender whose DNA profile is in a DNA database.

There are three interesting things to note about these suggestions. To begin with, it is not clear why such a statement would be introduced in a trial. By the time the suspect has become a defendant, a new sample of his DNA should have been tested to establish a full match to the crime-scene sample. At that point, why would the judge or jury care whether defendant is related to a database inhabitant. The relevance of the DNA evidence lies in the full match to the crime-scene sample, and the jury need not consider whether the defendant is a relative of someone not involved in the alleged crime. (One might ask whether the trawl through the database somehow degrades the probative value of the full match, but, if anything, it increases it. [1])

The issue could arise, however, if police were to seek a court order or search warrant to collect a DNA sample from the suspect. At that point, they would need to describe the significance of the partial match to the convicted offender.

This possibility brings us to the second noteworthy point about equation (1). The "odds" (or the corresponding probability) are not the way to present the weight of the partial match. Consider the prior probability of a match in a small database, say, of size N=2. Prior to considering the partial match, why would one think that the probability of a database inhabitant being the sibling of the criminal who resides outside the database is 1/N = 1/2? It is quite improbable that the database of two people includes a relative of every criminal who leaves DNA at a crime-scene. The a priori probability for a small database must be closer to 0 than 1/N.

That the prior probability is less than 1/N is a general result. The only exception occurs when it is absolutely certain that a sibling of the perpetrator is in the database. On that assumption, prior odds of 1 to N-1 are not unreasonable. But that assumption is entirely artificial, and to advise a magistrate that the posterior odds have the value computed according to (1) would be to overstate the implications of the partial match.

The third thing to note about dividing by N is that it accomplishes nothing in producing a viable list of partially matching profiles in a DNA database trawl. The straightforward approach is to produce a short list of candidates in the database whose first-degree relatives might be the source of the crime-scene sample. The minimum value of LR_autosomal x LR_Y-STR should be large enough to keep the two conditional error probabilities (including a candidate when there is no relationship, and not including a candidate when there is a relationship) small. This threshold value does not depend on N. (A later speaker made this observation.)

Equation (1), it seems, is useless. Instead, the magistrate should be told the value of the LR and how often such large LRs would occur when a crime-scene sample comes from a relative versus how often it would occur when it comes from an related person.

Reference

1. David H. Kaye, 2009, Rounding Up the Usual Suspects: A Legal and Logical Analysis of DNA Database Trawls, North Carolina Law Review, 87(2), 425-503.

Prometheus Unbound: Releasing the New Edition of the FJC Reference Manual on Scientific Evidence

Two days ago, the National Academy of Sciences released a third edition of the Federal Judicial Center’s Reference Manual on Scientific Evidence. I listened to the webcast of the unveiling as an insider and an outsider. An insider in that I co-authored two of the chapters. An outsider in that until the Manual eventually emerged, I seemed poised on the exterior side of the event horizon of a singularity into which drafts disappeared and time slowed.

Wednesday's unveiling included remarks from the two co-chairs of the NAS committee assembled to commission and supervise the writing of the manual—a group of five judges and five science professionals (a physician, a toxicologist, an engineer, a statistician, and an epidemiologist). Judge Gladys Kessler explained that in 1993, Daubert v. Merrell Dow Pharmaceuticals established “the gatekeeping role” of federal judges.

Certainly, the majority opinion, penned by Justice Blackmun in Daubert, is famous for this metaphor, but if federal judges were not gatekeepers before 1993, waht were they? Surely not sheep. The Daubert opinion draws heavily on prior case law regarding the Federal Rules of Evidence, substituting for the previously dominant “austere” requirement of general acceptance in the scientific community a multifaceted inquiry into “evidential reliability.” Under either legal standard, however, judges are gatekeepers.

Although Judge Kessler correctly suggested that Daubert’s reliability standard (borrowed from earlier court of appeals cases) goes beyond mere relevancy, the same can be said for the general-acceptance standard (announced in a court of appeals case in 1923) that it displaced. Thus, the notion that judges were not gatekeepers for scientific evidence until 1993 always has struck me as odd. (For more on this legal history and the meaning of Daubert, see Kaye et al. (2011).)

Dr. Jerome Kassirer fielded a number of questions from the virtual and physical audiences. One came from a forensic scientist or analyst in Florida who wanted to know if there were any “practicing forensic scientists” on the editorial committee. The response, that the Manual relied on the very detailed 2009 NRC report on forensic science in its treatment of the forensic sciences, missed the subtext of the question. The practicing forensic science community has been bashing the 2009 report for not accurately depicting the knowledge base of forensic identification techniques (other than DNA evidence). The criticism often takes the form of complaints that the committee lacked enough forensic scientists. (For a rejoinder from the co-chair of that committee, see Edwards (2010).)

Another question was why there was no chapter on digital forensics. The answer referred to the failure of the designated author to produce a manuscript that the committee thought would be useful or intelligible to judges. This probably was not the only chapter to fall by the wayside. Indeed, the problems encountered with such chapters may be part of a more complete answer than the one given to the interlocutor who asked whether an 11-year gap between editions was not a bit much.

A final question to which I alerted included a little speech about the importance of Bayesian inference. The questioner wanted to know why the Manual did not mention Bayes’ rule. Evidently, the questioner was not updating his prior beliefs with any data, for the chapters on statistics, DNA evidence, and medicine have substantial discussions of Bayes' rule. A better question would have been why there is not more discussion in the epidemiology chapter or why the presentation in the medicine chapter is so garbled. But that is a subject for another day.

References

Harry T. Edwards, 2010, The National Academy of Sciences Report on Forensic Sciences: What it Means for the Bench and Bar, Presentation at the Superior Court of the District of Columbia Conference on The Role of the Court in an Age of Developing Science & Technology, Washington, D.C., May 6, available at http://www.fd.org/pdf_lib/The NAS Report on Forensic Science.pdf.

David H. Kaye et al., 2011, The New Wigmore, A Treatise on Evidence: Expert Evidence, 2d ed., New York, NY: Aspen Pub.

National Research Council Committee on the Development of the Third Edition of the
Reference Manual on Scientific Evidence ed., 2011, Reference Manual on Scientific Evidence, Washington DC: National Academies Press.

"The first experimental study exploring DNA interpretation"

A recent study, entitled “Subjectivity and Bias in Forensic DNA Mixture Interpretation,” proudly presents itself as “the first experimental study exploring DNA interpretation.” The researchers are to be commended for seeking to examine, on at least a limited basis, the variations in the judgments of analysts about a complex mixture of DNA.

In addition to documenting such variation, they suggest that their experiment shows that motivational or contextual bias caused analysts in an unnamed case in Georgia to include one suspect in a rape case as a possible contributor to the DNA mixture. This claim merits scrutiny. The experiment is not properly designed to investigate causation, and the investigators' causal inference lacks the foundation of a controlled experiment. To put it unkindly, if an experiment is an intervention that at least attempts to control for potentially confounding variables so as to permit a secure inference of causation, then this is no experiment.

In the study, Itiel Dror, a cognitive neuroscientist and Honorary Research Associate at University College London, and Greg Hampikian, a Professor of Biology and Criminal Justice at Boise State University, presented electropherograms to 17 “expert DNA analysts ... in an accredited government laboratory in North America.” The electropherograms came from a complex mixture of DNA from at least four or five people recovered in a gang rape in Georgia. The article does not state how many analysts worked on the case, whether they worked together or separately, the exact information that they received, or whether they peeked at the suspects’ profiles before determining the alleles that were present in the mixture. They imply that the analysts were told that unless they could corroborate the accusations, no prosecution could succeed. Reasonably enough, Dror and Hampikian postulate that such information could bias an individual performing a highly subjective task.

In the actual case, one man pled guilty and accused three others of participating. The three men denied the accusation. The Georgia laboratory found that one of the three could not be excluded. Contrary to the expectations or desires of the police, the analysts either excluded the other two suspects or were unable to reach a conclusion as to them.

The 17 independent analysts shown the electropherograms from the case split on the interpretation of the complex mixture data. The study does not state the analysts’ conclusions for suspects 1 and 2. Presumably, they were consistent with one another and with the Georgia laboratory’s findings. With regard to suspect 3, however, “One examiner concluded that the suspect ‘cannot be excluded’, 4 examiners concluded ‘inconclusive’, and 12 examiners concluded ‘exclude.’”

From these outcomes, the researchers draw two main conclusions. The first is that “even using the ‘gold standard’ DNA, different examiners reach conflicting conclusions based on identical evidentiary data.”

That complex mixture analysis is unreliable (in the technical sense of being subject to considerable inter-examiner variation) is not news to forensic scientists and lawyers. Although the article implies that the NRC report on forensic science presents all DNA analysis as highly objective, the report refers to “interpretational ambiguities,” “the chance of misinterpretation,” and “inexperience in interpreting mixtures” as potential problems (NRC Report 2009, 132). The Federal Judicial Center’s Reference Manual on Scientific Evidence (Kaye & Sensabaugh 2011) explains that “A good deal of judgment can go into the determination of which peaks are real, which are artifacts, which are ‘masked,’ and which are absent for some other reason.” In The Double Helix and the Law of Evidence (2010, 208), I wrote that “As concurrently conducted ... , most mixture analyses involving partial or ambiguous profiles entail considerable subjectivity.” In 2003, Bill Thompson and his colleagues emphasized the risk of misinterpretation in an article for the defense bar.

These concerns about ambiguity and subjectivity have not escaped the attention of the courts. Supreme Court Justice Samuel Alito, quoting a law review article and a book for litigators, wrote that

[F]orensic samples often constitute a mixture of multiple persons, such that it is not clear whose profile is whose, or even how many profiles are in the sample at all. All of these factors make DNA testing in the forensic context far more subjective than simply reporting test results … .

and that

STR analyses are plagued by issues of suboptimal samples, equipment malfunctions and human error, just as any other type of forensic DNA test.

District Attorney’s Office for Third Judicial Dist. v. Osborne, 557 U.S. __ (2009) (Alito, J., concurring). Dror and Hampikian even quote DNA expert Peter Gill as saying that “If you show 10 colleagues a mixture, you will probably end up with 10 different answers.” Learning that 17 examiners were unanimous as to the presence of two profiles in a complex mixtures and that they disagreed as to a third supports the widespread recognition that complex mixtures are open to interpretation, and it adds some more information about just how frequently analysts might differ in evaluating one set of electropherograms.

The second conclusion that the authors draw is that in the Georgia case “the extraneous context appears to have influenced the interpretation of the DNA mixture.” This conclusion may well be true; however, it is all but impossible to draw on the basis of this “first experimental study studying DNA interpretation.” As noted at the outset, the “experimental study” has no treatment group. The study resembles collaborative exercises in DNA interpretation that have been done over the years. A true experiment—or at least a controlled one—would have included some analysts exposed to potentially biasing extraneous information. Their decisions could have been compared to those of the unexposed analysts.

Instead of controlling for confounding variables, the researchers compare the outcomes in their survey of analysts’ performance on an abstract exercise to the outcomes for one or two analysts in the original case. This approach does not permit them to exclude even the most obvious rival hypotheses. Perhaps it was not information about the police theory of the case and the prosecution's needs, but a difference in the labs' protocols that caused the difference. Perhaps the examiners outside of Georgia, who knew they were being studied, were more cautious in their judgments. Or, perhaps the police pressure, desires, or expectations really did have the hypothesized effect in Georgia. The study cannot distinguish among these and other possibilities.

In addition, the difference in outcomes between the Georgia group and the subjects in the study seems to be within the range of unbiased inter-examiner variability. How can one conclude that the Georgia analysts would not have included suspect 3 if they had not received the extraneous information and had followed the same protocol as the other 17? If the variability due to ordinary subjectivity in the process is such that 1 time out 17 an analyst will include the reference profile in question, then the probability that a Georgia analyst would do so is 0.059. I am not a firm believer in hypothesis testing at the 0.05 level, but I cannot help thinking that even under the hypothesis that the Georgia group was not affected to the slightest degree by the extraneous information, the chance that the result would have been the same is not negligible.

In raising these concerns, I certainly am not claiming that an expectation or motivation effect arising from information about the nature of the crime and the need for incriminating evidence played no role in the Georgia case. But the research reported in the paper does not go very far to establish that it was a significant factor and that it was the cause of the disparity between the Georgia analysts and the 17 others.

The authors’ caveat that “it is always hard to draw scientific conclusions when dealing with methodologies involving real casework” is not responsive to these criticisms. The problem lies with conclusions that outstrip the data when it would have been straightforward to collect better data. The sample size here does not give a reliable estimate of variability in judgments of examiners working in the same laboratory. The sampling plan ignores the possibility of greater variability across laboratories. Perhaps the confined scope of the study reflects a lack of funding or an unwillingness of forensic analysts to cooperate in research because of the pressure of their caseloads or for other reasons -- a complaint aired in Mnookin et al. (2011). Inasmuch as the researchers do not explain how they chose their small sample, it is hard to know.

Beyond the ubiquitous issue of sample size, the subjects in the "experiment" were not assigned to treatment and control groups. No analysts were given the same extraneous information that the Georgia ones had. Of course, extraneous information presented in an experiment could be less influential than it would be in practice. External validity is always a problem with experiments. But there is reason to believe that a controlled experiment could detect an effect in a case like this. Bill Thompson (2009) reported anecdotes suggesting that even outside of actual casework, different DNA examiners presented with electropherograms of mixtures can be induced to reach different conclusions when given extraneous and unnecessary information about the case. That the effect might be less in simulated conditions does not mean that it is undetectable in a controlled experiment.

Convincing scientific knowledge flows from a combination of well designed experimental and observational studies. Dr. Dror's work on fingerprint comparisons (e.g., Dror 2006) has contributed to a better understanding of the effect of examiner expectations in that task. Experiments designed to detect the impact of potentially biasing information on interpretation of DNA profiles in a controlled setting also would be worth undertaking.

References

Itiel E. Dror & Greg Hampikian (2011), Subjectivity and Bias in Forensic DNA Mixture Interpretation, Sci. & Justice, doi:10.1016/j.scijus.2011.08.004, http://www.scienceandjusticejournal.com/article/S1355-0306%2811%2900096-7/abstract

Itiel E. Dror, David Charlton & Ailsa E. Peron (2006), Contextual Information Renders Experts Vulnerable to Making Erroneous Identifications, Forensic Science International, 156(1): 74-78

David H. Kaye (2010), The Double Helix and the Law of Evidence

David H. Kaye & George Sensabaugh (2011), Reference Guide on DNA Identification Evidence, in Reference Manual on Scientific Evidence, 3d ed.

Jennifer L. Mnookin et al. (2011), The Need for a Research Culture in the Forensic Sciences, UCLA Law Review, 58(3): 725-779

National Research Council Committee on Identifying the Needs of the Forensic Sciences Community (2009), Strengthening Forensic Science in the United States: A Path Forward, Wash DC: National Academy Press

William C. Thompson, Simon Ford, Travis Doom, Michael Raymer, Dan E. Krane, Evaluating Forensic DNA Evidence: Essential Elements of a Competent Defense Review, The Champion, Apr. 2003, at 16

William C. Thompson (2009), Painting the Target Around the Matching Profile: the Texas Sharpshooter Fallacy in Forensic DNA Interpretation, Law, Probability & Risk 8: 257-276

Cross-posted at the Double Helix Law blog