On cross-examination, ISP analyst Karen Kooi Abbinanti, who examined the blood sample that Williams gave under court order in another case, testified to William’s STR profile. Because ISP analyst Sandra Lambatos, who provided the state’s only evidence of a DNA match, testified that “there [was] 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,” I presumed that the Cellmark report listed this profile as coming from the male fraction of DNA in the vaginal swab. Indeed, Lambatos testified that the “allele chart” in the Cellmark report “included data that [she] used to run [her] data bank search.” Joint Appendix at 61. Thus, I wrote that
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 . . . .Now that the report is lodged, it is clear that this singular profile is not what the anonymous Cellmark analyst and Cellmark’s two laboratory directors, Robin Cotton and Jennifer Reynolds, signed off on. Their table, which was Lambatos’s “data,” has the entry of (10, 12, 13) instead of (12, 13) for the D5S818 locus. Had Ms. Lambatos used this tri-allelic genotype, Williams would have been excluded! (Tri-allelic, single locus profiles are rare, but they are not unheard of. For example, one paper reports three cases of tri-allelic patterns observed during routine forensic casework on 5964 Belgian residents , and the D5S818 (10, 12, 13) profile has been observed .)
Ms. Lambatos, however, testified on cross-examination that the Cellmark report’s “deduced male donor profile” (to quote the report itself) was not actually a deduced profile, but only a list of deduced alleles. Joint Appendix at 71. Interpreting it in this fashion (which may well be the correct understanding what the unknown analyst meant to write), she searched the unspecified database for certain two-allele subsets of the three alleles— namely, (13, 13), (10, 13), and (12, 13). Id. This made sense because, if Cellmark had correctly identified the victim’s profile — something that Lambatos did not check — then the rapist rather than the victim had to be the source of the 13-repeat allele.
The circumscribed nature of Ms. Lambatos’s testimony on direct examination about the “DNA match” is worthy of comment. Full disclosure would have required a scientist to reveal that other male profiles than just Williams’ profile were “consistent with” the vaginal-swab mixture and could have been picked out of a database in her trawl. Instead, Ms. Lambatos acquiesced in or suggested confining her testimony to Williams’s matching profile and the random-match probability associated with that one profile. In other words, she chose not to acknowledge possibilities that were inconsistent with the state’s theory. Does such selectivity contravene the professional responsibility of forensic scientists to “[a]ttempt to qualify their responses while testifying when asked a question with the requirement that a simple ‘yes’ or ‘no’ answer be given, if answering ‘yes’ or ‘no’ would be misleading to the judge or the jury”? 
The answer, I think, depends on how misleading Ms. Labatos’s answers on direct examination were. This was not a case of a single profile that probably could exclude everybody except for a twin brother. The analysts were unable to distinguish between Sandy Williams and other males with similar, but not identical profiles, as possible sources of the male DNA. By not disclosing this fact, Ms. Lambatos and the prosecutor made the DNA “match” sound especially compelling. The prosecutor asked about “the male DNA profile found in the semen.” Ms. Labatos made no effort to correct or clarify even though she firmly believed that Cellmark was reporting at least three different male profiles for the semen (and that Williams was, of course, a match to only one of them). Hammered with Ms. Lambatos’s figures for the Williams’ profile frequency, a judge surely would think that only Williams or a mythical twin could have been the rapist. In contrast, a judge who understood that Cellmark's tests also pointed to men with other DNA profiles might have been more willing to entertain some doubt.
The counterargument is that the probative value of the evidence for the ambiguous profile is essentially the same as the probative value of the evidence for the unambiguous profile that Ms. Lambatos was asked about. Assuming that the vaginal swab DNA is a mixture of the victim’s DNA and one man’s DNA, and assuming that the laboratory called all the alleles correctly, the likelihood ratio for the hypotheses of Williams versus that of a random, unrelated man is 1/[p(10,12,13) + p(13,13) + p(10,13) + p(12,13)], where p is the random-match probability for the full genotype, including the alleles shown in parentheses. Ms. Lambatos computed the probability p(12,13) as falling in the quadrillionths. Although I have not consulted allele frequency tables, it is a safe bet that similarly small probabilities would pertain to the profiles with the (13,13) and (10,13) genotypes. The random-match probability for the profile with the tri-allelic pattern would be even smaller. (When asked by the defense, Ms. Lambatos testified that a tri-allelic male was not a real possibility.) Therefore, I would predict that the correct computation would not differ from the number given to the judge by more than an order of magnitude. Hence, the witness’ failure to clarify or correct the prosecutor in her questioning affected the probative value of the evidence minimally.
Nevertheless, for the expert to present such oversimplified testimony without any qualification seems problematic to me. When confronted with the omissions on cross-examination, the expert owned up to them, but did she not ask the prosecutor to present the expert's reasoning accurately in the first place? And if she did, why did the prosecutor not do it?
1. G. Mertensemail, S. Rand, E. Jehaes et al., Observation of Tri-allelic Patterns in Autosomal STRs During Routine Casework, 2 Forensic Sci. Int’l: Genetics Supplement Series 38-40 (2009).
2. NIST STR-base, Tri-Allelic Patterns, June 2, 2011, http://www.cstl.nist.gov/strbase/var_D5S818.htm#Tri
3. American Society of Crime Laboratory Directors Laboratory Accreditation Board, ASCLD/LAB Guiding Principles of Professional Responsibility for Crime Laboratories and Forensic Scientists, Principle 19, Version 1.1, 2009.
Cross posted from The Double Helix Law Blog, 15 Dec. 2011