The Evaluation of Forensic DNA Evidence (1996) / Chapter Skim
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6 DNA Evidence in the Legal System
6 DNA Evidence in the Legal System
Pages 166-211
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From page 166... ...
It describes the most important procedural and evidentiary rules that affect the use of forensic DNA evidence, identifies the questions of scientific fact that have been disputed in court, and reviews legal developments.) All forensic methods for individualization fingerprints, dental impressions, striations on bullets, hair and fiber comparisons, voice spectrograms, neutronactivation analysis, blood-grouping and serum-protein and enzyme typing, as well as DNA profiling demand an ability to match samples with reasonable accuracy with respect to characteristics that can help to differentiate one source from another.
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From page 167... ...
We discuss those general principles and then consider their application to DNA evidence. We also describe pretrial and trial procedures that might help courts to reach decisions on admissibility and to improve the quality and use of the scientific evidence at trial.
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From page 168... ...
The rules of discovery determine the circumstances under which a defendant can compel the production of such records. Because many complex technical, scientific, and statistical issues affect the use of DNA evidence, there will be cases in which defendants will contend that without comprehensive and detailed information, they are unable to prepare for trial adequately.4 Although some courts have ordered liberal discovery, providing access to the documentation and information would broaden the scope of discovery in some junsdictions.
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From page 169... ...
953, 524 N.W.2d 763 (1994) , the Nebraska Supreme Court, in reversing a conviction involving a PCR DQA test on the grounds that the 1992 NRC report indicated lack of general acceptance of calculations that assumed Hardy-Weinberg proportions, noted the absence of testimony from a population geneticist.
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From page 170... ...
Well-qualified experts could assist a court or jury in understanding basic principles of DNA testing, how such procedures such as RFLP- and PCR-based testing work, and the extent and effect of departures from Hardy-Weinberg (HW) proportions and linkage equilibrium (LE)
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From page 171... ...
. are admissible in evidence without antecedent expert testimony that DNA analysis provides a trustworthy and reliable method of identifying characteristics in an individual's genetic material upon a showing that the offered testimony meets the standards of admissibility set forth in the Tennessee Rules of Evidence" (Teen.
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From page 172... ...
267, 489 N.W.2d 192 (1992) ("trial courts may take judicial notice of the reliability of DNA identification testing," but "the prosecutor must establish in each particular case that the generally accepted laboratory procedures were followed")
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From page 173... ...
Labels like "general acceptance," "sound methodology," and "helpfulness" are just that- labels. Cases decided in each jurisdiction help to define the scientific community in which the degree of scientific acceptance is to be ascertained, the extent of disagreement that can be tolerated, the information that may be used to gauge the extent of consensus, and the specific factors other than general acceptance that bear on relevance and helpfulness.~7 The degree of scientific consensus is important to the admissibility of scientific evidence in all jurisdictions, and pretrial hearings in hotly contested cases have lasted months and generated thousands of pages of testimony probing the opinions of experts on various aspects of DNA profiling.
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From page 174... ...
Trends in the Admissibility of DNA Evidence Application of the standards for admitting scientific evidence to the adm~ssibility of DNA profile evidence has produced divergent results. In the United States, the first wave of cnm~nal cases involving DNA identification began in 1986.~8 The focus was on the problems raised in transferring the technology of modern molecular biology from the medical and genetics laboratones, which usually dealt in fresh samples and easily interpretable diallelic probes, to the forensic laboratory, which must handle aged and exposed stains and usually uses more complex, multiallelic genetic systems.
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From page 175... ...
1991) (holding that Cellmark DNA evidence in a rape case had been erroneously admitted in the absence of a showing of the general acceptance of the validity of the product rule, which gave a frequency of 1/(59 million)
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From page 176... ...
1993) (trial courts may take judicial notice of the acceptability of the techniques used in RFLP analysis)
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From page 177... ...
. As of November 10, 1995, 34 cases in which PCR-based DNA testing had been conducted could be retrieved from the Westlaw "allcases" database of court opinions.
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From page 178... ...
Furthermore, very small frequencies can be obtained by testing at additional loci.35 theory of DNA testing is generally accepted, but whether the PCR technique is generally accepted.") ; State v Grayson, No.
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From page 179... ...
but finding PCR-based evidence admissible after noting that over 30 forensic laboratories were performing DQA testing as of March 1991, that the FBI began using the Cetus kit in 1992, that the British Home Office had adopted DQA as its screening test, and that problems of laboratory error are "either detectable or preventable" when proper techniques and laboratory procedures are used)
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From page 180... ...
In addition to providing the jury with valuable guidance, wide use of this instruction would encourage laboratories to participate in such activities. An instruction might read: '`In evaluating the quality of the DNA evidence, you might wish to consider the laboratory's participation or nonparticipation in the following quality-control activities: (1)
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From page 181... ...
68 (1985~. In Ake, the Supreme Court reversed a conviction because the trial court had refused to appoint an expert to assist the indigent defendant, who was relying on an insanity defense.
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From page 182... ...
Some courts have held that an expert must be provided,49 and others have found no such need.50 Instead of providing a defendant with an expert, a court might appoint an expert to assist the court. As noted in the earlier discussion of expert witnesses, courts have been more inclined to use this procedure to investigate general scientific issues related to DNA profiling than to resolve controversies related to the particulars of the DNA testing in a given case.
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From page 183... ...
2d 855, 857 (1992) (an order that would require defendant to turn over the results of DNA testing even if it did not introduce this evidence at trial denied the defendant the effective assistance of counsel)
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From page 184... ...
With DNA evidence, however, the prosecution will know that the defense wishes to retest because the samples have to be turned over. Nevertheless, it has been suggested that the restriction on discovery should be interpreted as barring the prosecution from calling the defense expert (Contra State v McDaniel, 485 N.W.2d 630 [Iowa 1992]
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From page 185... ...
The 1992 NRC report stated that the probative value of such statistics, when balanced against their potential to mislead a jury, favored admissibility: "laboratory error rates must be continually estimated in blind proficiency testing and must be disclosed to junes" (p 89~. Inasmuch as the purpose of our report is to determine what aspects of the procedures used in connection with forensic DNA testing are scientifically valid, we attempt no such policy judgment.
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From page 186... ...
Some experts discussing DNA evidence in court have questioned the representativeness of convenience samples.65 Most courts have held that the use of convenience samples does not make computations inadmissible, but a few courts have suggested that a database resulting from a convenience sample provides an unacceptable foundation for the probability or frequency estimates being offered.66 Nevertheless, the ideal alternative to convenience sampling some form of random sampling often is impractical, and convenience sampling can produce reasonable estimates in some circumstances. In Chapter 5, we explained why the allele-frequency estimates from existing databases are suitable for computing genotype frequencies.
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From page 187... ...
At the time of the 1992 NRC report, however, little information was available on the extent to which the relative frequencies of VNTR alleles varied among subgroups within the racial groups, and the report described the conflicting views of population geneticists on the validity of simply multiplying allele frequencies. Many courts took the report's description of a "substantial controversy" as proof of a major scientific disagreement.68 Today, the debate is shifting in the direction of accepting the validity of using the assumptions of Hardy-Weinberg proportions and linkage equilibrium to estimate profile frequencies and match probabilities in major racial groups.
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From page 188... ...
Although the data on variations among subpopulations are more limited for these systems than for VNTRs, the experience with VNTRs and other polymorphisms indicates that correcting for population structure should make little difference, and the procedures outlined in Chapter 5 can be expected to give fair estimates of the range of uncertainty in population and subpopulation frequency estimates for discrete allele systems. Ceiling Frequencies in Court Rather than giving a definitive answer to speculations about population structure, the 1992 NRC report assumed that population structure could be a serious threat to estimates of VNTR profile frequencies within the general population or within subpopulations.
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From page 189... ...
The interim ceiling principle does not purport to measure the frequency of an incriminating profile in the reference population, but rather an upper limit of the random-match probability that is unrelated to the reference population. In late 1994, one of the authors of that report, and an early advocate of the ceiling procedure, expressed his belief that the committee intended to offer ceiling frequencies as a supplement rather than as a necessary substitute for estimates derived from data on the population or subpopulation of interest (Lander and Budowle 1994)
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From page 190... ...
In time, however, the courts began to assimilate this literature. Although a few courts interpreted the criticism as "precluding the admissibility of DNA evidence" under the general-acceptance standard,' most have recognized that much of the criticism amounted to claims that there was no need for subpopulation studies and ceiling frequencies in the first place or that the recommended procedure for estimating an upper bound was unnecessarily cautious in its details.77 In Estate v Sivri, 646 A.2d 169 (Cone.
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From page 191... ...
1995) (ceiling frequency properly admitted with other estimates against American Indians under relevancy standard when the trial court found that the interim ceiling method satisfied Daubert)
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From page 192... ...
EXPLAINING THE MEANING OF A MATCH Once two samples are found to have similar profiles, the question arises as to what, if anything, the trier of fact may be told about the significance of this finding. Before forensic experts can conclude that DNA testing has the power to help identify the source of an evidence sample, it must be shown that the DNA characteristics vary among people.
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From page 193... ...
," but "whether, and if so, to what extent we will allow DNA evidence without the accompanying statistical evidence in other criminal cases will be decided in a future case.") ; Rivera v State, 840 P.2d 933 (Wyo.
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From page 194... ...
. There might already be cases in which it is defensible for an expert to assert that, assuming that there has been no sample mishandling or laboratory error, the profile's probable uniqueness means that the two DNA samples come from the same person.84 (suggesting that the better practice is not to refer to probability estimates when introducing DNA results)
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From page 195... ...
2d 856, 865 n.l3 (1995~. At least one state supreme court has endorsed that more modest approach as a substitute to the presentation of more debatable numerical estimates.85 Although different jurors might interpret the same words differently, the formulas provided in Chapters 4 and 5 produce frequency estimates for profiles of three or more loci that almost always can be conservatively described as "rare." Quantitative Assessments: Frequencies and Match Probabilities Except for strong claims of uniqueness, purely qualitative presentations suffer from ambiguity.
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From page 196... ...
When the numbers have been presented as estimating the frequency of a profile or the probability of a random match and have not been m~scharacter~zed as the probability that the defendant is not the source of the incriminating DNA, the argument that numbers will overwhelm the jury rarely has prevailed. Only one jurisdiction has routinely excluded quantitatively framed testimony of probabilities or population frequencies in criminal cases for fear of unduly influencing lay jurors,89 and the supreme court of that state carved out an exception to the exclusionary rule for ceiling calculations of DNA profile frequencies (State v Bloom, 516 N.W.2d 159 [Minn.
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From page 197... ...
To the contrary, several studies with mock jurors suggest that decision-makers generally make smaller adjustments in their judgments in response to probability evidence than the statistical evidence warrants.90 Nonetheless, the extremely low random-match probabilities associated with much DNA evidence might cause jurors to perceive the evidence as different in quality, as well as quantity. Virtually no studies of juror reactions have assessed the impact of probabilities as extreme as those in Commonwealth v Curnin.9i Courts that are especially concerned that small estimates of the match probability might produce an unwanted sense of certainty and lead a jury to disregard other evidence might wish to adopt procedures to reduce this risk.
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From page 198... ...
. 93As regards the transposition fallacy, such an instruction might be framed along these lines: "In evaluating the expert testimony on the DNA evidence, you were presented with a number indicating the probability that another individual drawn at random from the [specify]
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From page 199... ...
But efforts should be made to fill the glaring gap in empirical studies of such matters. Because of the potential power and probative value of DNA evidence, it is important to learn more about juror and judicial response to this evidence in the face of strong and weal: nonstatistical evidence.95 Quantitative Assessments: Likelihood Ratios and Posterior Odds Small values of the probability of a random match undermine the hypothesis (which we may abbreviate as SC)
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From page 200... ...
As with match probabilities, qualitative as well as overtly quantitative presentations can be devised (see Evett 1991, p 201, proposing "a verbal convention, which maps from ranges of the likelihood ratio to selected phrases," such as "strong evidence" or "weak evidenced. Although LRs are rarely introduced in criminal cases,97 we believe that they are appropriate for explaining the significance of data and that existing statistical knowledge is sufficient to permit their computation.
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From page 201... ...
The prosecution relied on a Bayesian analysis of this type in State v Klindt, 389 N.W.2d 670 (Iowa 1986) , a gruesome cha~nsaw-murder case decided before the emergence of DNA testing.
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From page 202... ...
Rather, the expert presents the jury with a table or graph showing how the posterior probability changes as a function of the prior probability.~°i Although the var~able-pnor-odds implementation of Bayes's rule has garnered the most support among legal scholars and is used in some civil cases, very few courts have considered its merits in criminal cases.~02 How much it would contribute to jury comprehension remains an open question, especially considering the fact that for most DNA evidence, computed values of the likelihood ratio (conditioned on the assumption that the reported match is a true match) would swamp any plausible prior probability and result in a graph or table that would show a posterior probability approaching 1 except for very tiny prior probabilities.
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From page 203... ...
The small amount of research on reactions to probabilistic evidence suggests that methods of presentation may strongly affect reactions to DNA evidence. Unexamined are the effects of testimony about extreme probabilities or laboratory error when DNA evidence is presented by expert witnesses who are subjected to cross-examination.
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From page 204... ...
Before making use of evidence derived from scientific advances, courts must scrutinize the proposed testimony to determine its suitability for use at trial, and controversy within the scientific community often is regarded as grounds for the exclusion of the scientific evidence. Although some controversies that have come to closure in the scientific literature continue to limit the presentation of DNA evidence in some jurisdictions, courts are making more use of the ongoing research into the population genetics of DNA profiles.
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From page 205... ...
TABLE 6.1 Leading Cases and Statutes on Admissibility of Inclusionary DNA Evidence by Jurisdiction, as of June 1995 DC Cir.
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From page 206... ...
915, 629 N.E.2d 634 (1994) (VNTR product-rule estimate inadmissible under Frye; remanded fc~r Frye hearing on admissibility of ceiling estimates)
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From page 207... ...
State v Bloom, 516 N.W.2d 159 (1994) (VNTR ceiling estimates admissible under Frye)
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From page 208... ...
ceiling estimates admissible under relevance standard) , review allowed, 319 Or.
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From page 209... ...
1995) (VNTR product-rule and interim-ceiling estimates admissible under relevance standard)
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From page 210... ...
210 THE EVALUATION OF FORENSIC DNA EVIDENCE TABLE 6.2 Admissibility of Inclusionary DNA Evidence by Junsdichon, as of June 1995 Il03 II m IV v Standard Of Jurisdiction Admissibility DNA Test VI Opinion As Only Fact Of Method Of To Source Match Computation Admissible? Admissible?
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From page 211... ...
NC R RFLP Product Yes Ohio R RFLP Product Yes PCR Yes Okla D RFLP Product Yes Ore R RFLP Product Yes PCR Yes Pa F RFLP None Yes SC F,R RFLP Product Yes SD F RFLP Product Yes Tenn F,D,R,S RFLP Product Yes Tex R RFLP Product Yes DQA, PCR Vt D RFLP Product No Yes Ceiling Va R,S RFLP Product Yes DQA Product Yes Wash F RFLP Product No No Yes PCR Wisc R RFLP Product Yes Yes Ceiling Wyo R RFLP Product Yes Ceiling i03 I Federal or state jurisdiction in which at least one court opinion on the admissibility of DNA test results that incriminated the defendant was reported.
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