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JEFF ADACHI
Public Defender
City and County of San Francisco
TERESA CAFFESE
Chief Attorney
BICKA BARLOW, SBN: 178723
Deputy Public Defender
555 Seventh Street
San Francisco, CA 94103
415-(415) 553-9645; 553-1671
Attorneys for JOHN DAVIS
SUPERIOR COURT OF CALIFORNIA
CITY AND COUNTY OF SAN FRANCISCO
PEOPLE OF THE STATE OF
CALIFORNIA,
Plaintiff,
vs.
JOHN DAVIS,
Defendant.
MCN:
SCN:
DATE:
TIME:
DEPT: 22
NOTICE OF MOTION AND MOTION
TO EXCLUDE “COLD HIT” DNA
EVIDENCE
INTRODUCTION
At trial, the government will seek to introduce scientific evidence, namely DNA
evidence, to prove that Mr. Davis sexually assaulted and killed ??? in 1985.
The evidence in
this case consists of DNA test results using Profiler Plus from two vaginal swabs taken from the
alleged victim. Mr. Davis was identified through a search of the California convicted offender
database, in other words a “cold hit.” No other evidence links Mr. Davis to the alleged crime.
Unlike most criminal cases, a “cold hit” case is initiated by the matching of an DNA
profile obtained from a crime scene or victim to a suspect’s DNA profile which is contained in a
databank, generally a databank consisting of convicted offenders. Prior to the “hit” the suspect
has not been identified and little or no corroborating evidence exists. This is the reverse of the
“confirmation” situation where a suspect is identified via a police investigation and DNA testing
is used to corroborate the already existing evidence that the suspect is the perpetrator.
But as with any other DNA case, the match between a profile found on a piece of
evidence and that of a person found in a databank must be given meaning by the use of statistics.
The statistical question that must always be asked when a DNA match is found is: what is the
probability that this match is a coincidence, and that the suspect is not the perpetrator of the
crime?
In a “cold hit” case, the statistical approach to the question “is this a coincidence?” is
asked differently than in a probable cause case. This difference is due to the fact that in the
databank search case, potentially hundreds of thousands of individuals are searched via the
databank and in a probable cause case, only the defendant is “searched” when his DNA is tested
and compared to an evidence profile. Statistically, these two situations are very different and
the statistical analysis that is scientifically proper changes.1 Even thought the statistical
methods used to analyze the match changes between a cold hit and a probable cause case, both
analyses are asking the same question: what is the probability that the match occurred
coincidentally and the suspect is not the perpetrator.2
The government in Mr. Davis’ case has reported only the RMP and apparently intends to
offer this statistical analysis at trial. It is the defendant’s position that not only is this not the
correct statistical analysis, given that this is a cold hit case, but that no qualified population
geneticist or statistic advocates the use of the RMP as calculated by the San Francisco Crime lab
for a cold hit. The RMP is the probability that a randomly selected individual would have the
same genetic profile as the evidence sample, and is therefore a statistic which is wholly irrelevant
and of no concern to the trier of fact in a case such as this where the match is a result of a data
bank trawl. 3
The primary difference between these two types of matches is the manner in which the
suspect is identified. Virtually everyone agrees that it is not possible to convert one type of case
into the other, for example by simply retesting a sample from the suspect once he or she has
been identified through a “cold hit.” In other words, analyzing a new sample of the suspect’s
DNA after he has been identified as suspect through a database search does not convert the case
into a “probable cause case” for purposes of statistics. Additionally, there is broad scientific
consensus that the statistical significance of these two types of matches must be calculated
differently.
The DNA evidence in this case must be excluded under Kelly because there is no
consensus in the scientific community as to what statistical analysis should be applied to cold hit
DNA evidence. While the type of DNA analysis used in this case, Short Tandem Repeat
Analysis (hereafter STR analysis), has been held admissible by the California Court of Appeals
(See People v. Henderson, (2003) 107 Cal. App.4th 769, People v. Allen, (1999) 72 Cal.
App.4th 1093), there is no California appellate court decision in which a particular method for
calculating the statistical significance of a match in a “cold hit” case has been found to be
generally accepted. The California Court of Appeals in a recent case, People v. Johnson (2006)
139 Cal. App.4th 1135, did not reach the issue and instead held that if the cold hit was merely
being used as an investigative technique and is not being offered as evidence of guilt at trial, a
Kelly hearing is not required. This case will be discussed in further detail in a subsequent
section of this brief.
There exist three distinct groups of well qualified, highly respected experts who disagree
on the scientifically proper statistical analysis for cold hit DNA evidence.
However, they all
agree that the RMP is not the appropriate calculation. Because the results of DNA testing is
meaningless without the appropriate statistical analysis, Kelly requires that the Court exclude the
statistical analysis and underlying DNA evidence.
“The statistical calculation step is the
pivotal element of DNA analysis, for the evidence means nothing without a determination of the
statistical significance of a match of DNA patterns. It is the expression of statistical meaning,
stated in terms of vanishingly small match probabilities, that makes the evidence so compelling.”
People v. Barney (1992) 8 Cal. App. 4th 798, 917. California courts are in complete agreement
that the calculation of the statistical significance of a match is an integral part of the DNA
analysis process and the underlying methodology of arriving at that statistic must be found to be
generally accepted. People v. Axell, (1991) 235 Cal. App.3d 836. In other words, in order for
evidence of a “DNA match” to be admissible, it must be accompanied by a statistic that
represents the chance that the match is coincidental. Accordingly, the evidence in this case is
inadmissible unless the prosecutor can prove that the method used to calculate the chance that
the match is coincidental is generally accepted in the relevant scientific community. Deciding
which statistic should be presented to the jury is not a legal issue; it is a scientific issue properly
left to the “scientific community.” Moreover, presenting a statistic that does not represent the
chance that the match is coincidental given the type of search that took place is highly
misleading. It is noteworthy that the prosecution offers a statistic (a random match probability
without any modification) that has been characterized by many as misleading in the context of a
“cold hit” match, which is what we have in this case.
Because there are no published appellate court decisions in California that address this
particular issue, Mr. Davis submits that the fact that there exists a controversy among scientists
in the relevant community as to how to calculate the statistical significance of a match in a “cold
hit” case requires that the DNA evidence be excluded unless there is some sufficiently
conservative method of calculating the statistical significance as to which there is scientific
agreement.
STATEMENT OF FACTS
On April 25, 2002, the San Francisco Crime Lab issued a report indicating that “a single,
unknown semen donor DNA profile was detected on both the anterior vaginal swab (Item 9C-1)
and the perineal body swab (Item 9G)” taken from the alleged victim. The profile was to be
submitted to the California state “Combined DNA Index System (CODIS) database.” On
September 30, 2002, a supplemental report was issued stating that the lab had electronically
submitted the unknown male profile on June 17, 2002 to CODIS, and on August 30, 2002, the
California Department of Justice, Bureau of Forensic Services, DNA Laboratory “confirmed that
a search of the Convicted Offender DNA Databank revealed a match or ‘cold hit’ between the
evidence swabs (Items 9C-1 and 9G) and a felon identified as John Davis, A08060342, at the
nine AmpFLSTR Profiler Plus DNA markers.” [emphasis original].
On October 28, 2002, a third report was issued reporting results of testing of oral swabs
taken from John Davis and a comparison of the swab results to the previously tested evidence.
The report included the calculation of the random match probability (RMP) for the unknown
male profile in Items 9C-1 and 9G as: 1 in 15 trillion for US Caucasians, 1 in 683 billion for
African Americans, 1 in 63 trillion for California Hispanics, and 1 in 108 trillion for the general
Asian population.
LAW AND ARGUMENT
I.
NEW OR NOVEL SCIENTIFIC EVIDENCE IS INADMISSIBLE UNLESS IT IS
SHOWN TO HAVE GAINED GENERAL ACCEPTANCE IN THE SCIENTIFIC
COMMUNITY BY THE PROPONENT OF THE EVIDENCE
Admissibility of expert testimony based on “a new scientific technique” requires proof of
its reliability – i.e., that the technique is “sufficiently established to have gained general
acceptance in the particular field to which it belongs.” People v. Kelly (1976) 17 Cal.3d 24, 30
[quoting Frye v. United States (D.C. Cir. 1923) 293 F. 1013, 1014, italics omitted][emphasis
added].
Just when a scientific principle or discovery crosses the line
between the experimental and demonstrable stages is difficult to
define. Somewhere in this twilight zone the evidential force of the
principle must be recognized, and while courts will go a long way
in admitting expert testimony deduced from a well-recognized
scientific principle or discovery, the thing from which the
deduction is made must be sufficiently established to have gained
general acceptance in the particular field in which it belongs.
Frye, supra, at1014.
“The goal is not to decide the actual reliability of the new technique, but simply to
determine whether the technique is generally accepted in the relevant scientific community. If
the scientific literature discloses that the technique is deemed unreliable by ‘scientists significant
either in number or expertise . . . ,’ the court may safely conclude there is no general
acceptance.” People v. Barney (1992) 8 Cal. App. 4th 798, 810; People v. Reilly (1987) 196 Cal.
App. 3d 1127, 1134 [quoting People v. Shirley (1982) 31 Cal.3d 18, 56]. “Ideally, resolution of
the general acceptance issue would require consideration of the views of a typical cross-section
of the scientific community, including representatives, if there are such, of those who oppose or
question the new technique."
Kelly, at 37.
In determining the question of general acceptance, courts “must
consider the quality, as well as quantity, of the evidence supporting
or opposing a new scientific technique. Mere numerical majority
support or opposition by persons minimally qualified to state an
authoritative opinion is of little value. . . .” People v. Leahy,
supra, 8 Cal.4th at p. 612, 34 Cal. Rptr.2d 663, 882 P.2d 321.)
People v. Venegas (1998) 18 Cal. 4th 47, 85.
The gate-keeping role of the court requires that the proponent of the evidence make a
substantial showing that the proffered methods are generally accepted as reliable. Kelly created
a tool of “considerable judicial caution” and of an “essentially conservative nature” that is
“deliberately intended to interpose a substantial obstacle to the unrestrained admission of
evidence based upon new scientific principles.” Kelly, supra, 17 Cal.3d at 31. The Court
acknowledged that “(t) here has always existed a considerable lag between advances and
discoveries in scientific fields and their acceptance as evidence in a court proceeding. . . . Lay
jurors tend to give considerable weight to ‘scientific’ evidence when presented by ‘experts’ with
impressive credentials. Exercise of restraint is especially warranted when the identification
technique is offered to identify the perpetrator of a crime.” Kelly, supra at 32. Thus, the
prosecutor must prove that the method used to calculate the significance of the “cold hit” is
generally accepted in the relevant scientific community.
As will be explained below, because
of the controversy surrounding the method for calculating such a statistic, the prosecutor cannot
demonstrate general acceptance as required by Kelly.
A.
THE STATISTICAL INTERPRETATION OF DNA EVIDENCE IS AN
INTEGRAL PART OF THE SCIENCE OF DNA TESTING AND MUST BE
SHOWN TO HAVE BEEN GENERALLY ACCEPTED AS RELIABLE BY
THE SCIENTIFIC COMMUNITY UNDER THE FIRST PRONG OF
KELLY
To make the determination of admissibility of scientific evidence in general, the court
must apply the standard set forth in People v. Kelly (1976) 17 Cal. 3d 24. The Kelly standard
has three “prongs:”
(1) it must be established, usually by expert testimony, that the
scientific methods utilized are generally accepted as reliable by the
relevant scientific community,
(2) the witness furnishing such testimony must be properly
qualified as an expert to give an opinion on the subject, and
(3) the proponent of the evidence must demonstrate that correct
scientific procedures were used in the particular case.
Kelly, 17 Cal.3d at p. 30 [emphasis original].
The statistical interpretation of DNA evidence must, like the DNA testing method itself,
must be shown to be generally accepted as reliable. People v. Venegas (1998) 18 Cal. 4th 47;
Barney, supra, 8 Cal. App. 4th at 817; People v. Axell (1991) 235 Cal. App. 3d 836. “We find
that since a match between two DNA samples means little without data on probability, the
calculation of statistical probability is an integral part of the process and the underlying
method of arriving at that calculation must pass muster under Kelly/Frye.” Axell, supra, 235
Cal. App. 3d at 866-867 [emphasis added].
The statistical calculation step is the pivotal element of DNA analysis, for the evidence
means nothing without a determination of the statistical significance of a match of DNA patterns.
It is the expression of statistical meaning, stated in terms of vanishingly small match
probabilities, that makes the evidence so compelling. Barney, supra, 8 Cal. App. 4th at 817.
A determination that the DNA profile of an evidentiary sample
matches the profile of a suspect establishes that the two profiles are
consistent, but the determination would be of little significance if
the evidentiary profile also matched that of many or most other
human beings. The evidentiary weight of the match with the
suspect is therefore inversely dependent upon the statistical
probability of a similar match with the profile of a person drawn at
random from the relevant population.
Venegas, supra, 18 Cal. 4th at 82 [emphasis added]. Which statistical analysis is scientifically
acceptable and reliable is not a question of weight for the jury. Fundamentally, it is a question
that must be answered by the appropriate scientific community and evaluated by the Court in its
gatekeeping function.
To . . . leave it to jurors to assess the current scientific debate on
statistical calculation as a matter of weight rather than
admissibility, would stand Kelly-Frye on its head. We would be
asking jurors to do what judges carefully avoid – decide the
substantive merits of competing scientific opinion as to the
reliability of a novel method of scientific proof . . . . The result
would be predictable. The jury would simply skip to the bottom
line – the only aspect of the process that is readily understood –
and look at the ultimate expression of match probability, without
competently assessing the reliability of the process by which the
laboratory got to the bottom line. This is an instance in which the
method of scientific proof is so impenetrable that it would . . .
“assume a posture of mystic infallibility in the eyes of a jury.”
Id. at 83-84 [citations omitted][emphasis added].
The California Supreme Court has consistently recognized that mathematical probability
evidence while potentially relevant has the danger of casting “a spell” over jurors. People v.
Collins, (1968) 68 Cal. 2d 319. “Although we make no appraisal of the proper applications of
mathematical techniques in the proof of facts (see People v. Jordan (1955) 45 Cal.2d 697, 707
[290 P.2d 484]; People v. Trujillo (1948) 32 Cal. 2d 105, 109 [194 P.2d 681]; in a slightly
differing context see Whitus v. Georgia (1967) 385 U.S. 545, 552, fn. 2 [17 L.Ed.2d 599, 604, 87
S.Ct. 643]; Finkelstein, The Application of Statistical Decision Theory to the Jury
Discrimination Cases (1966) 80 Harv.L.Rev. 338, 338-340), we have strong feelings that such
applications, particularly in a criminal case, must be critically examined in view of the
substantial unfairness to a defendant which may result from ill conceived techniques with which
the trier of fact is not technically equipped to cope. (See State v. Sneed, 414 P.2d 858; Note,
supra, Duke L.J. 665.)” Id. at 332.4 If the statistical method does not pass the general
acceptance test of Kelly, all DNA evidence must be excluded. Venegas, supra,18 Cal. 4th 47;
Barney, supra, 8 Cal. App. 4th 798.
Through this motion the defendant requests that the Court exclude the DNA evidence on
the ground that the statistical analysis does not pass the first prong of the Kelly test.
B.
WHEN DETERMINING GENERAL ACCEPTANCE THE COURT MUST
DEFER TO THE APPROPRIATE SCIENTIFIC COMMUNITY, NAMELY
POPULATION GENETICISTS, MATHEMATICIANS AND
STATISTICIANS
The Kelly test requires that the Court consider the opinions of the relevant scientific
community.” Kelly, 17 Cal.3d at 30. In the context of cold hit statistics, the relevant
community consists of population geneticists and statisticians with academic credentials, not
technicians who work in forensic laboratories. Evidence Code § 720 governs an expert witness’
qualifications. The Supreme Court in Kelly explained § 720 stating that while the trial court
has substantial latitude in determining qualifications, “[t]he competency of an expert is relative
to the topic and fields of knowledge about which the person is asked to make a statement. In
considering whether a person qualifies as an expert, the field of expertise must be carefully
distinguished and limited.” [quoting People v. King, (1968) 266 Cal. App. 2d at 437, 445].
The Kelly court found that the expert in that case was not qualified even though he had “an
impressive list of credentials” but these
qualifications are those of a technician and law enforcement
officer, not a scientist. Neither his training under Kersta, his
association with the Tosi study, his limited college study in certain
speech sciences, his membership in organizations promoting the
use of voiceprints, nor his former position as head of the Michigan
State Police Voice Identification Unit, necessarily qualifies Nash
to express an informed opinion on the view of the scientific
community toward voiceprint analysis.
Id. at 39 [emphasis original]. Even the wide experience of Nash did not satisfy the academic
and knowledge requirement of the Supreme Court. Id. Practical experience of a laboratory
technician does not substitute for scientific and academic training for purposes of a Kelly review
of the general acceptance of a technique. DNA testing is an amalgamation of a number of
different disciplines including molecular biology and population genetics People v. Axell (1991)
235 Cal. App.3d 836, as well as statisticians and mathematicians.
The California Supreme Court in Brown used the Kelly analysis when it found that two
technicians who testified regarding serology tests were not qualified to offer opinions on the
general acceptance5 of the method in question there. People v. Brown (1982) 40 Cal. 3d 512,
533. In discussing why the government’s proffer fell short of that expected under Kelly, the
Supreme court discussed the qualifications of the government’s experts:
Springer and Andrus were competent and well-credentialed
forensic technicians, but their identification with law enforcement,
their career interest in acceptance of the tests, and their lack of
formal training and background in the applicable scientific
disciplines made them unqualified to state the view of the relevant
community of impartial scientists.
Id. [emphasis original]. Another factor considered by the court, was that neither expert in
Brown could back up their positions with citations or discussions of the literature. Id. This
Court must hold to the highest standards the qualifications of expert witnesses who testify or
write about issues of general acceptance. The qualified experts must have not only the
educational background but also the intellectual and academic training necessary to understand
the complex issues that relate to the reliability of a particular method.
California courts are not the only ones to hold to this definition for Frye experts. The
District of Columbia Court of Appeals in rejecting the government’s argument that the “relevant
scientific community” was comprised of forensic scientists stated,
It simply is not creditable to argue, and the government does not
do so with much enthusiasm, that general acceptance may be
premised simply on the opinion of forensic scientists. Were it
otherwise, there would have been no need for a month-long Frye
hearing. There is no question but that forensic scientists accept– no
qualifier is necessary – forensic DNA evidence and believe that the
time has come for its use as powerful evidence in criminal trials.
While views of forensic scientists have weight and must be
considered, members of the relevant scientific field will include
those whose scientific background and training are sufficient to
allow them to comprehend and understand the process and form a
judgment about it.
U.S. v. Porter, 618 A.2d 629, 635 (D.C. Ct. App. 192).
In this case, the question of general acceptance for the statistical analysis of cold hit DNA
evidence requires that the Court qualify only those experts with the expertise necessary to assess
the relevant literature. This means those with academic backgrounds in population genetics and
statistical analysis, not crime lab technicians who merely apply the statistical theories and tests
derived by the experts to the test results.
C.
IN REACHING ITS DETERMINATION THE COURT CAN AND
SHOULD CONSIDER SCIENTIFIC LITERATURE TO ASCERTAIN
WHETHER THERE IS A CONTROVERSY IN THE SCIENTIFIC
COMMUNITY
The California Supreme Court in People v. Shirley 31 Cal. 3d 18 (1982) laid out the
procedure that a court should employ in assessing the state of scientific opinion. The Supreme
Court noted that “considerations of judicial economy make it impractical to require those views
to be presented personally by each scientist testifying in open court” because “such a procedure
would be prohibitively expensive, and would be frustrated in any event by the difficulty of
finding local experts qualified to testify.” Id. at 55-56. In the context of Kelly “scientists have
long been permitted to speak to the courts through their published writings in scholarly treatises
and journals.” [citing Kelly, at 35; Huntingdon v. Crowley (1966) 64 Cal.2d 647; People v.
Palmer (1978) 80 Cal. App. 3d 239, 252-254; People v. Law (1974) 40 Cal. App. 3d 69, 75;
United States v. Addison (D.C. Cir.1974) 498 F.2d 741, 744-745.]
Writings from journals and treatises are viewed as “‘evidence,’ not of the actual
reliability of the new scientific technique, but of its acceptance vel non in the scientific
community.” Shirley, supra, at 56.
The courts also are not required to “pick and choose”
among the writings because the scientific literature can be so vast that no court could possibly
absorb it all. Id. Kelly does not demand judicial absorption of all the relevant literature, nor
does it require a decision once and for all whether a particular kind of scientific evidence is
reliable. People v. Brown (1982) 40 Cal. 3d 512, 533. The burden falls on the proponent of
the new technique to “show a scientific consensus supporting its use; if a fair overview of the
literature discloses that scientists significant either in number or expertise publicly oppose [the
technique] as unreliable, the court may safely conclude there is no such consensus at the present
time.” Id. [emphasis added].
In this case, the defendant has presented not only literature from the relevant scientific
communities, but also the affidavit of a number of experts on all sides of the debate to support
the contention that the debate is ongoing (Exhibits ??-??) and testimony through declarations of a
number of experts in Washington D.C in the case of U.S. v. Jenkins, Superior Court of the
District of Columbia, Case No. F320-00 (Exhibit ??). All of these experts are eminently
qualified as shown by their CV’s and stated qualifications under oath. All come from the
community of relevant scientists, human geneticists, population geneticists, and statisticians.
This Court’s role is only to determine if there is a consensus in the relevant scientific
community. The Court cannot assume the role of scientist and pick that method which “seems”
to be the best or most appropriate, or even the most conservative. The Court’s role is to simply
answer the following question: has the relevant scientific community of population geneticists
and statisticians come to a consensus as to how a statistic should be calculated for a cold hit case.
If the answer to that question is no and there is disagreement between groups of qualified
scientists, the Court must exclude the statistical analysis and the underlying DNA evidence under
the first prong of Kelly.
II.
THE STATISTICAL EVIDENCE IS INADMISSIBLE BECAUSE THERE IS A
SPLIT IN THE SCIENTIFIC COMMUNITY REGARDING HOW TO
CALCULATE THE PROBABILITY OF A COINCIDENTAL MATCH IN THE
CASE OF A DATABANK SEARCH/COLDHIT CASE
There are essentially two types of DNA cases. The first, usually referred to as a
“confirmation” or “traditional” case arises when DNA testing is performed on a suspect that is
already strongly linked to the crime by direct or circumstantial evidence. A “random match
probability” (RMP) is calculated to assess the probative nature of the DNA “match.” This
figure estimates the chance that a random person drawn from the population would have the
profile found in the evidence sample.
This statistic is offered to respond to the defense
argument that the two DNA profiles coincidentally match and is therefore the relevant question
that needs to be answered Since only one person, the pre-existing suspect, was looked at, that
estimate accurately frames the chance of a coincidental match. The appellate court decisions
upholding the use of the random match probability have exclusively dealt with probable cause
cases, with the exception of Johnson, discussed below.
With the advent of large known offender databanks, a second type of DNA case is now
starting to emerge, the “cold hit” or “databank”6 case. This case arises when, with no suspect
linked to the crime, law enforcement chooses to “trawl” through a databank of known
offenders, and they find someone in that databank who has the same profile as is found in the
evidence.7 Because the DNA match in this situation was found by trawling through a data bank
looking for a profile that matched, a random match probability is not the appropriate statistic to
describe the significance of the match and on this point the scientific community generally
agrees. However, how the statistical significance of the match should be calculated is a matter
of controversy among those in the relevant scientific community.
There is virtually no dispute that “cold hit” or “data bank” case must be treated
differently than a “confirmation” case. This is due to the fundamental difference in how a
coincidental match can occur. The “random match probability”8 that the prosecutor seeks to put
before the jury in this case answers the question, “What is the chance that an unrelated9
randomly selected person would happen to match the evidence profile or one of the profiles in
the mixed sample?” This is an appropriate question in a “confirmation case” because, for the
defendant to be innocent, the prosecutor's suspect just happened to also match the evidence DNA
profile. This question is irrelevant, however, when the very reason the prosecutor suspects the
person is because he was found to match the evidence profile after authorities had trawled
through thousands of profiles looking for one that matched. No amount of subsequent retesting
can ever convert this a “confirmation” case. The initial identification by the database search and
the subsequent retesting of the suspects DNA and match to the crime scene profile are not
independent events.
Consider for example a case where an eyewitness describes her attacker but the attack left
her blind and unable to engage in an identification procedure. A suspect is nonetheless
developed based upon, for example, his suspicious behavior and the fact that he was stopped
near the crime scene. If the victim describes her attacker very precisely, naming eight visible
characteristics (e.g., male, 5'10" tall, 160 pounds, 30s, black, facial hair, no glasses, and dark
clothing), and the suspect matches every one of them, then the probative value of this match is
great. If the circumstances making him the suspect were just unfortunate coincidences and he is
innocent, one would ask, “But what are the chances a randomly selected person just happens to
share the eight distinctive visible characteristics as the assailant?” This is the question that is
relevant in a “confirmation” case.
Assume instead that no leads were developed after the attack. The police start looking
through DMV photographs and descriptions. Eventually, after looking through many
thousands, they come across a person who matches all eight characteristics. In this instance
clearly it would not be appropriate to ask “What are the chances a randomly selected person
would just happen to share the eight distinctive visible characteristics as the assailant” because
the new suspect was not randomly selected. Rather, he was selected for the very reason he
shared those traits. Common sense is consistent with the lesson of statistics that if you look
through thousands of samples for something rare, you are more likely to come across it than if
you look at only one. “[T]he argument is essentially the same as for a lottery; if P is the
probability of winning with one ticket, the probability of winning with N tickets is NP.”
Newton E. Morton, The Forensic DNA Endgame, 37 Jurimetrics J. 477, 499 (1997) (attached).10
In other words, the chance that one random individual will win the lottery is very rare but the
chance that one of the many individuals who purchase a lottery ticket will win is not so rare.
Scientists acknowledge this difference between a “cold hit” case and a “confirmation”
case. “[T]he more extensive was your search, the less impressed I am. The reason is that
among many comparisons, it is unsurprising that one of them will, “by chance,” display results
that, on their own, might be regarded as significant. In this setting, the effect of the search is to
weaken evidential strength. This phenomenon is widely understood by scientists. Indeed,
conveying the idea to the general public is often regarded as a major challenge in advancing the
public understanding of science.” David J. Balding, Errors and Misunderstandings in the
Second NRC Report, 37 Jurimetrics J. 469, 471 (1997) (attached).
Two committees of The National Research Council of the National Academy of
Sciences11 issued two widely read reports that deal with DNA evidence, including DNA
evidence derived from “cold hits.” The members of both National Research Council
committees have been described as “a distinguished cross section of the scientific community.”
People v.Venegas, 18 Cal. 4th 47,89. As to the stark difference between “confirmation” and
“cold hit” cases, both NRC reports agree. “The distinction between finding a match between an
evidence sample and a suspect sample and finding a match between an evidence sample and one
of many entries in a DNA profile database is important. The chance of finding a match in the
second case is considerably higher.” National Research Council, DNA Technology in Forensic
Science 124 (Nat'l Acad. Press 1992) (“NRC I (1992)”) (selected portions attached). "There is
an important difference between the ‘confirmation’ case and one in which the suspect is initially
identified by searching a database to find a DNA profile matching that left at a crime scene. In
the latter case, the calculation of a match probability or likelihood ratio should take into account
the search process.” National Research Council, The Evaluation of Forensic DNA Evidence
134 (Nat'l Acad. Press 1996) (“NRC II (1996)”) (selected portions attached). See NRC II
(1996) at p.161 (“If the suspect is identified through a DNA database search, the interpretation of
the match probability and likelihood ratio . . . should be modified.”).
The DNA Advisory Board, which was led by the FBI, also agrees that “cold hit” cases
are qualitatively different than “confirmation” cases and, because “the probability of identifying
a DNA profile by chance increases with the size of the database[,] this chance event must be
taken into account when evaluating the value of the matching profile found by a database
search.” DNA Advisory Board, Statistical and Population Genetics Issues Affecting the
Evaluation of the Frequency of Occurrence of DNA Profiles Calculated from Pertinent
Population Database(s), 2 Forensic Sci. Communications 1, 5 (July 2000) (attached).
The reason that scientists pay particular attention to “cold hits” is because, as in the
blinded-witness example above, when an individual is identified through a databank trawl, there
is the very real danger that an innocent person, who coincidentally shares an identical profile at
the evidence loci, will be caught up in the net. There have been too many examples of two
persons sharing genetic profiles on low-number loci comparisons to fail to account for the
unfortunate reality that database searches can lead to false matches and can generate inflated
probability statistics. See, e.g., Kathryn Troyer et al., A Nine STR Locus Match Between Two
Apparently Unrelated Individuals Using AmpF/STR Profiler Plus and Cofiler, presented at 12th
International Symposium on DNA Identification (Phoenix, Ariz. 2001) (attached)12 (reporting
9-loci DNA profile match between two unrelated individuals (one white, one black) in state
DNA database); Department of Justice, National Institute of Justice, The Future of Forensic
DNA Testing 25 n.13 (Nov. 2000) (reporting ten 6-loci DNA profile matches in New Zealand
database of 10,907 records in which eight of matches were brothers and two of matches were
unrelated persons); Richard Willing, Mismatch Calls DNA Tests into Question, USA Today
(Feb. 8, 2000), (reporting instance of false match involving 6-location DNA search in UK
database) (attached); The Age, “DNA Testing and Human Error” (Dec. 11, 2003) (attached)
(10-loci match was either coincidental as opined by Australian crime lab director or result of
contamination).
Notwithstanding that virtually all experts agree that the statistics are different in “cold
hit” cases, there remains the debate about what method should be used to take into account
“ascertainment bias.” In other words, how should the statistic be calculated to take into
consideration the fact that the match was found by searching a database. In this regard there is a
true scientific debate, where esteemed members of the relevant scientific community strongly
disagree. There are at least three different methods that have been proposed in the literature for
calculating the statistical significance of a “cold hit.”
The first two, the NRC I and NRC II methods, are founded on the same assumptions: a
databank search and match is less probative than a match obtained in a probable cause case
because it is more likely that a match in a databank will be coincidental and the persons
“matched” will no be the perpetrator. The third method, the DMP or Bayesian approach, is
based on a wholly different assumption: a databank search and match is more probative than a
probable cause match because the you have excluded all but one person during the search.
A.
THE NRC I APPROACH: Proposed by the NRC I Committee as Well as a
Number of Well-Respected Scientists Today is that the Loci Used in the
Database Search not be used in the Statistical Calculation and that a
Random Match Probability for Other Independent Loci be Calculated
The first method to deal with the question of statistics in databank match cases was
promulgated in 1992 by the first body of experts appointed to the Committee on DNA Science
by the National Research Council. This panel concluded that selection bias, trawling through a
databank, invalidates the use of searched profiles at trial:
The distinction between finding a match between an evidence
sample and a suspect sample and finding a match between an
evidence sample and one of many entries in a DNA profile
databank is important. The chance of finding a match in the
second case is considerably higher, because one does not start with
a single hypothesis to test (i.e., that the evidence was left by a
particular suspect) but instead fishes through the databank, trying
out many hypotheses. . . .
When a match is obtained between an evidence sample and a
databank entry, the match should be confirmed by testing with
additional loci. The initial match should be used as probable
cause to obtain a blood sample from the suspect, but only the
statistical frequency associated with the additional loci should be
presented at trial (to prevent the selection bias that is inherent in
searching a databank). Forensic DNA typing laboratories should
recognize that they will require additional loci beyond those used
in the databank to prove a case against a suspect.
NRC I at 124.
The primary criticism leveled against this approach, and recognized by the NRC I
committee itself, is that, out of concern that the database match was purely coincidental such that
an innocent person might get charged with an offense, NRC I elects to not use all the information
available to determine identity. Rather, some loci are used solely for the database trawl and are
subsequently discarded if the analyst goes on to compare the “cold hit” person’s genetic profile
to that of the crime scene evidence. So, for example, if the database search that generated the
“cold hit” used 10 particular loci, those same 10 loci could not be used for the subsequent
comparison between the “cold hit” suspect’s profile and the crime scene evidence profile.
Rather, the forensic laboratory should use additional, different loci in making its comparison.
Depending on the number and specifics of the additional loci used, this procedure could reduce
the random match probability calculations by a number of magnitudes.
If the second test indicates a match, “only the statistical frequency associated with the
additional loci should be presented at trial [to prevent the selection bias that is inherent in
searching a databank].” Id. at 124. The “selection bias” or “ascertainment bias”13 mentioned
by both the NRC I and NRC II, infra, refers to the fact that as the databank searched increases in
size, it become more likely that a coincidental match to a person who is not the perpetrator will
be made.
All DNA forensic laboratories have available to them the core 13 CODIS loci. Both
Applied Biosystems and Promega have developed additional loci that may be used to generate a
DNA profile.14 These are examples of the STR loci commercially available at this time. Given
the abundant number of genetic markers available at the present time, NRC I approach currently
enjoys a number of followers.
The NRC I approach is endorsed by eminent scholars. N.E. Morton, The Forensic DNA
Endgame, 37 Jurimetrics J. 447 (1997) 487-492 (Exhibit B); Richard Lempert, After the DNA
Wars: Skirmishing with NRC II, 37 Jurimetrics J. 439, (1997) 461-462 (Exhibit C). It is a
practical solution because other DNA kits can be easily used to test other loci and verify or
dispel the conclusion from the databank trawl.15 This approach is also the most conservative
approach to the question of statistical analysis.
Newton E. Morton, Professor, Human Genetics, University of Southhampton opined in a
1997 peer-reviewed article in Jurimetrics: “As recognized by NRC I, the best solution is to
confirm a match by a panel of markers independent of the ones used in trawling in the database.
This evidence is free of ascertainment bias, and the corresponding likelihood ratio can be
presented as evidence without indicating that the defendant was identified through a criminal
database.” Morton, supra, at 489.
Writing separately in that volume of Jurimetrics, Dr.
Richard Lempert, who served on the committee that produced NRC I, affirmed his belief that
NRC I best presented the manner in which to report “cold hit” DNA statistic. Richard
Lempert, After the DNA Wars: Skirmishing With NRC II, 37 Jurimetrics J. 439, 461-62 (1997)
(attached). See also Aidan Sudbury, Comment to David J. Balding & Peter Donnelly, Inference
in Forensic Identification, 158 J. Royal Stat. Socy. A, Par 1, 21, 48-49 (1995) (attached)
(agreeing in substance with NRC I). See also James F. Crow, The 1996 NRC Report: Another
Look, (attached) (noting that Newton Morton advocates going back to the NRC I method to “put
an end to the controversy.”)
B.
THE NRC II APPROACH: Proposed by the Committee as Well as a Number
of Well-Respected Scientists Today that a Random Match Probability be
Calculated Using the Same Loci that Were Used to Conduct the Search but
that the Number be Modified by Multiplying it by the Size of the Database
In 1996, a different approach was suggested by a second Committee formed by the
National Research Council. This group recommended the question be assessed in terms of the
size of the databank. “When the suspect is found by a search of DNA databanks, the
random-match probability should be multiplied by N, the number of persons in the databank.”
Statistical and Population Genetics Issues Affecting the Evaluation of the Frequency of
Occurrence of DNA profiles Calculated from Pertinent Population Databank(s), (Exhibit A,
supra); Recommendation 5.1, The Evaluation of Forensic DNA Evidence, National Research
Council (1996), (NRC II) p. 40, 161. The NRC II committee and other scholars believe that a
“cold hit” match is less probative than a single DNA match since the chance of finding a match
to a particular evidence DNA profile increases as it is compared to more people. Thus, for
example, finding that single person chosen at random is left-handed is somewhat unusual but
finding at least one left-handed person in a sample of 1000 is not at all surprising. The NRC II
committee offered a different method to account for the risk of a coincidental match in a large
database. Their proposal was to “(m)ultiply the match probability by the size of the database
searched. This is the procedure we recommend.” Id. at 32.
The formula that the NRC II committee espouses, and which is supported by Professor
Stockmarr, is:
likelihood ratio = 1/(Np)
where N is the number of profiles in the databank and p is the random match probability as
calculated in a “probable cause case.” Id.; Likelihood Ratios for Evaluating DNA Evidence When
the Suspect is Found Through a Databank Search, A. Stockmarr, Biometrics (1999) 55:671-677
(Exhibit D).
This approach results in a number more conservative than the RMP.
The proponents of this method believe that the NRC I approach (infra described in
Section A) is too conservative. This committee described the NRC I proposal as “a sound
procedure,” but “it wastes information, and if too many loci are used for identification of the
suspect, not enough might be left for an adequate subsequent analysis.” Id. at 32-33. Their
alternative method differs in three ways: 1) no testing is performed at additional loci; 2) genetic
markers used in the original databank search are included in the statistical calculations; and 3)
the size of the databank being searched (N) is taken into consideration. This group of scientists
believes that the larger the searched databank, the less reliable is the DNA identification of a
suspect identified from the databank. In other words, the greater the size of the databank, the
greater the chance that a profile from the data base will coincidentally match the profile from the
evidence sample. This is true until the size of the databank approaches the entire population.
The proponents of this method acknowledge, “as databases grow large enough to be a substantial
fraction of the population, a more complicated calculation is required. Id. at 32-33.
However, this approach was immediately and virulently attacked by a third group of
scholars, who take the polar opposite position and believe that the larger the searched database,
the more reliable is the DNA identification of a suspect identified from the databank.
Notwithstanding those attacks, statisticians and other supporters of NRC II have stood their
ground and reaffirmed the correctness of their position and the incorrectness of that of their
opponents. See, e.g., Anders Stockmarr, Likelihood Ratios for Evaluating DNA Evidence
When the Suspect is Found Through a Database Search, 55 Biometrics 671 (Sept. 1999)
(attached); Anders Stockmarr, Reply to Comment on Stockmarr's "Likelihood Rations for
Evaluating DNA Evidence When the Suspect is Found Through a Database Search, 57
Biometrics 978 (Sept. 2001) (attached); Anders Stockmarr, Reply to Ewett Letter to the Editor of
Biometrics, 56 Biometrics 1275 (Dec. 2000) (attached); Bernie Devlin, The Evidentiary Value of
a DNA Database Search, 56 Biometrics 1276 (Dec. 2000) (attached).
C.
The Databank Match Probability (DMP): A Third Set of Scientists Reject the
Proposals of the NRC I and NRC II Committee Members and Propose that a
Likelihood Ratio Approach be Used. These Scientists Contend that the
Evidence is Actually Stronger, Not Weaker, When a Match is Discovered as a
Result of a Database Search
The third approach comes from statisticians who argue the opposite position from the
NRC I and NRC II, that the value of a match in a “cold hit” case is, in fact, more probative than
one in a “confirmation” case. Statistical and Population Genetics Issues Affecting the Evaluation
of the Frequency of Occurrence of DNA profiles Calculated from Pertinent Population
Databank(s), DNA Advisory Board, Forensic Science Communications (July 2000) Vol. 2 No. 3
(Exhibit A, supra); Errors and Misunderstandings in the Second NRC Report, D. J. Balding,
Jurimetrics (Summer 1997) 37:469-476 (Exhibit E); Evaluating DNA Profile Evidence When the
Suspect is Identified Through a Databank Search, D. J. Balding and P. Donnelly, Journal of
Forensic Sciences (1996) 41:603-607 (Exhibit F); Interpreting DNA Evidence, I. W. Evett and
B.S. Weir, Sinaur Press (1998) pp. 219-222 (Exhibit G); Weight-of-Evidence for Forensic DNA
Profiles, D. J. Balding, Wiley Press at 35-36; 154-55 (2005) (Exhibit H).
This approach has been called the databank match probability (DMP). These
statisticians who apply the “Bayesian” theory, argue that the fact of a databank search means
that, not only has the defendant been found to match the evidence, but all others in the databank
have been found not to match. In other words, with a “confirmation” case, you have one match
and no exclusions, but with a “cold hit” case, you have one match and, depending on the size of
the databank, potentially thousands of exclusions.
To these scientists, in “confirmation” cases
where only a single match is found during the course of DNA testing, there is at least still a
formal possibility that one or more untested people may also match the evidence, and that
possibility becomes increasingly less likely as the database used for a cold hit becomes larger.
These scientists reject the “classical statistics” orientation of NRC I and NRC II:
[Classical statisticians] grew up to facilitate objective inferences
from data. Classical statisticians try to avoid subjective
judgments, seeking instead to determine what conclusion can be
drawn solely on the basis of frequency of observation. [Our]
Bayesian approach - updating the odds assigned to a given
proposition in light of evidence subsequently received - is thus
unacceptable to classical statisticians because it depends on the
subjective assignment of odds in the absence of objectively
measurable data. . . .
It is considered bad science to trawl data, looking for surprising
results and then proclaiming that the data proves a proposition that
would likely lead to such results.
Peter Donnelly & Richard D. Friedman, DNA Database Searches and the Legal Consumption of
Scientific Evidence, 97 Mich. L. Rev. 931, 966-68 (Feb. 1999) (attached). These scientists
reject the random match probability calculation, whether corrected or not. Instead, they rely
solely upon the determination of a likelihood ratio. Their form of statistical analysis, known as
“Bayesian,” requires making statistical assumptions as to the prior odds of guilt for a chosen
individual.
Their method differs from NRC I's approach in three ways: 1) no testing is performed at
additional loci; 2) genetic markers used in the original database search are included in the
statistical calculations; and 3) the size of the database being searched (N) is taken into
consideration. Their method differs from the NRC II's approach in one, extremely fundamental
way. To this group of experts, the effect of the database size on the significance of a match is
precisely the opposite – large databases generate the most damning statistics for a defendant
while, in the NRC II approach, the larger the database the less damning the statistics become to a
defendant. This method, unlike the NRC II does not break down as the size of database
approaches the number of persons in the entire world. Thus, the second and third approaches
are diametrically opposed in scientific theory and in practical consequences with respect to
implications of the size of the database that is searched.
D.
THERE IS A CURRENTLY IRRECONCILABLE SPLIT IN THE
RELEVANT SCIENTIFIC COMMUNITY AS TO WHAT METHOD
SHOULD BE USED FOR COLD HIT STATISTICAL ANALYSIS
“Both camps appear to present rigorous arguments to support their positions.” Statistical
and Population Genetics Issues Affecting the Evaluation of the Frequency of Occurrence of DNA
profiles Calculated from Pertinent Population Databank(s), DNA Advisory Board, Forensic
Science Communications (July 2000) Vol. 2 No. 3 (Exhibit A, supra). The above cited DNA
Advisory Board article sought to decide which treatment was more proper for the courtroom.
Id. However, it is not for them, nor for us, to try to fathom the correct or best approach or
decide which camp to endorse. Indeed, the debate still ongoing as illustrated by the recent
publication of Dr. D.J. Balding, Weight-of-Evidence for Forensic DNA Profiles, D. J. Balding,
Wiley Press at 35-36; 154-55 (2005) (Exhibit H). Dr. Balding still espouses his position of
taking a Bayesian approach to the databank search question, id. at 35-36, and he roundly
criticizes the NRC II approach to the question stating unequivocally:
Perhaps, the most astonishing feature [of the NRC II report] is the
serious error on the issue of a defendant identified by a databank
search. . . . The report recommends that the weight of DNA
evidence in this setting be measured by the profile frequency
multiplied by the size of the database, indicating much weaker
evidence than in a no-search case. [¶] The rationale for this
recommendation is clearly flawed.
Id. at 154 [emphasis added].
There is no question that these experts strongly disagree with one another. Dr. Balding
refers his readers to Dr. Stockmarr’s 1999 Biometrics article to observe the “flaws” in NRC II's
reasoning and calls Dr. Devlin’s support of NRC II “flawed” with reasoning that has “no bearing
on the issues’ and with an approach that “makes little sense.” Balding, The DNA Database
Search Controversy, supra, at 243.
Even the FBI recognizes the dispute noting that, with neither camp’s view clearly
superior or inferior to that of the other, “we are left with an interesting dilemma.” DAB,
Statistical and Population Genetics Issues, supra, at 7. In discussing the battle between the
NRC II camp and the Balding/Donnelly camp, the DNA Advisory Board wrote “(b)oth camps
appear to present rigorous arguments to support their positions. Indeed, the proper treatment
superficially appears to rest in the details of arcane mathematics.” Id. at 6-7.
Under this set of circumstances the Court cannot count heads or pick and choose among
the alternatives by trying to determine which is the “best” or most conservative approach.
[T]he point is not whether there are more supporters than
detractors, or whether … the supporters are right and the detractors
are wrong. The point is that there is disagreement between [ ]
groups, each significant in both number and expertise.
Barney, supra, 8 Cal. App. 4th at 819 [emphasis added]. The task is not to choose sides in this
dispute over the reliability of the statistical calculation process. Id. It must be left to the
scientific community, not prosecution experts or the court, to determine acceptance within its
own community.
“It is the task of scientists – not judges, and not jurors – to assess reliability.”
Barney, supra, 8 Cal. App. 4th at 818. “The requirement of general acceptance in the scientific
community assures that those most qualified to assess the general validity of a scientific method
will have the determinative voice.” Kelly, supra, 17 Cal. 3d at 31 [emphasis original]. “Once
we discern a lack of general scientific acceptance – which in this instance is palpable – we have
no choice but to exclude the ‘bottom line’ expression of statistical significance in its current
form.” Barney, supra, 8 Cal. App. 4th at 819.
At this time, there is a three way split in the scientific community of population
geneticists and scientists as to the method that should be used to generate a statistical weight for
DNA evidence in a case such as this. The debate is ongoing and furious. Given this split, and
given that there is no generally accepted method, the Court must not act as a scientist and
determine which analysis is correct. Rather, the Court must step aside and allow the scientific
debate to work its way to a conclusion based on further scientific discussions and research. At
this time, because there is no one generally accepted method for statistical analysis of databank
searches, the DNA evidence is inadmissible. Without a statistical analysis, the DNA match has
no meaning and cannot go before the jury.
In this case, the prosecutor offers either a random match probability alone. No where in
the peer reviewed literature is there any suggestion this statistic is appropriate when the DNA
match is derived from a “cold hit.” A random match probability does not represent the answer
to the only relevant question in a cold hit case, what is the chance that the cold hit is
coincidental.
III.
THE HOLDING IN JOHNSON IS SCIENTIFICALLY WRONG, BASED ON AN
INCOMPLETE RECORD AND INAPPLICABLE TO THIS CASE
On May 25, 2006, the 5th District Court of Appeals issued an opinion in People v.
Johnson, (2006)139 Cal. App.4th 1135"
(W)e hold that a “cold hit” from a DNA database is not subject to
the Kelly-Frye standard of admissibility, at least when, as here, it is
used merely to identify a possible suspect.
This holding is scientifically untenable because all scientists agree that one cannot
eliminate the impact of the search process (“ascertainment bias”) on the relevant statistic by
simply ignoring it. In other words, simply retesting the DNA of the individual who was
identified as a result of a cold hit does not eliminate the ascertainment bias caused by the
databank search process. However, it is not difficult understand why the Johnson court may
have had difficulty with these issues. This issue of whether there is a generally accepted method
for calculating the statistical significance of a “cold hit” match was not addressed, litigated or
developed in the trial court. Absence of a well-developed record below makes a reviewing
court’s task more difficult and the defendant suggests more prone to error.16
The only witness who testified at the trial court DNA admissibility hearing in Johnson
was a criminalist, Maosheng Ma. From her testimony it is evident that Ms. Ma had only a
rudimentary understanding of statistics. For example, Ms. Ma stated that in calculating a
statistic she relied on the “National Research Council’s book-NRC II, a DNA analyst’s Bible.”
Yet it is clear from her testimony that she was unaware that the NRC II unequivocally states that
“(i)f the only reason that the person becomes a suspect is that his DNA profile turned up in a data
base, the calculation [referring to the random match probability] must be modified.” (The
Evaluation of Forensic DNA Evidence, (1996) National Research Council, page 32.) (Emphasis
added).
While the Johnson court clearly understood that “(t)he computation step of DNA analysis
falls under the Kelly-Frye umbrella” Johnson supra, at 593, the entire premise of its ruling is
based on the misconception that if evidence of the data bank search is not presented in court the
impact of the data bank search on the probability that the match in the case is coincidental has
somehow been eliminated. “(T)he database search merely provides law enforcement with an
investigative tool, not evidence of guilt.” Id.
The Johnson court presents a number of different examples to demonstrate that the
manner by which an individual becomes a suspect is irrelevant. While it may very well be true
that the manner by which someone becomes a suspect is sometimes legally irrelevant, it is never
statistically irrelevant. All the examples cited by the court fail to prove its point because in none
of the cited examples is a statistic being offered to describe the significance of the actual
evidence presented as in the case of DNA cold hit evidence. One of the examples relied on by
the court is the case of People v. Farnam (2002) 28 Cal.4th 107. In Farnam over a defense
objection, the prosecution was permitted to present evidence that Mr. Farnam’s fingerprints were
run through a computerized system called AFIS and that his prints were identified as a possible
match to the prints from the crime scene. A print examiner subsequently confirmed the match
and testified that the prints from the crime scene matched those of Mr. Farnam.
At first blush, this appears to be a compelling analogy. However, the analogy breaks
down because no statistic was being offered by the print examiner to explain the statistical
significance of match. For better or worse, print experts are not required to give a statistic; print
examiners are legally permitted to testify that the prints match to the exclusion of all other prints.
See, People v. Reza (1984) 152 Cal. App.3d 647. This is simply is not so with DNA evidence
and thus, the situations are in no way analogous. The same is true for each example cited by the
court. This underscores the difficulty for the Johnson court in trying to address this very
sophisticated statistical issue, where no one with expertise in statistics testified in the trial court,
the experts in the field disagree and the record below was never developed.
IV.
THE HOLDINGS IN JOHNSON AND JENKINS ARE INCONSISTENT WITH
CALIFORNIA LAW
In United States v. Jenkins the District of Columbia Court of Appeals held that the issue
of which particular statistical calculation reliably states the significance of a “cold hit” match is
“a question of law, not of science.” The Court of Appeals in ruling in Johnson, supra, is
actually inconsistent with Jenkins. Johnson seems to recognize that the “computation step of
DNA analysis” is subject to Kelly but holds that the statistic offered in a “cold hit” case is not
subject to Kelly when the cold hit is merely used to identify a suspect. In other words, the court
appears to conclude that a Kelly hearing would be required if evidence of the databank search
will be offered as evidence at trial. However, at a later point in the opinion, the court states that
it agrees with the Jenkins court. “We also agree with the Jenkins court's conclusion that no new
methodology is involved in “cold hit” cases; hence, Kelly is not implicated.” Johnson at 601.
The Kelly line of cases, however, is clear and is inconsistent with the holdings in both
Jenkins and Johnson. Kelly and the cases construing Kelly hold that it is the province of
scientists, not courts, to determine whether the prosecution has chosen and correctly applied a
statistical formula that accurately estimates the chance of a coincidental match in a particular
situation. The California Supreme Court has rejected the argument that objections to the
prosecution’s statistical formula merely go to its weight, and has held instead that, because the
determination of statistical probability is an integral part of the process, the underlying method of
arriving at the calculation must satisfy Frye. “To . . . leave it to jurors to assess the current
scientific debate on statistical calculation as a matter of weight rather than admissibility, would
stand Kelly-Frye on its head.” People v. Venegas (1998) 18 Cal.4th 47, 83. If the rule were
otherwise, the jury would be required to hear and consider the competing views of scientists
regarding the various scientific matters which are relevant to the validity of the probability
estimate. It would be up to the jury to determine what is ultimately a scientific question: what is
the meaning of the match.
California has squarely rejected any such proposition. Instead of declaring themselves
competent to delve into the realm of statistics and determine whether a given formula accurately
estimates the chance of a coincidental match or putting this scientific question into the lap of
juries, California courts have looked to scientists, and have precluded the use of a particular
formula in the absence of scientific consensus that it appropriately reflects the relevant
probability. Venegas, 18 Cal. 4th at 83-84. Johnson is simply wrong because the fact that the
suspect was identified through a databank search must be accounted for in any calculation used
to obtain a statistic that represents the chance that the match is coincidental. Again, all scientists
agree with this proposition. See The Evaluation of Forensic DNA Evidence, (1996) National
Research Council, at 32. [“If the only reason that the person becomes a suspect is that his DNA
profile turned up in a database, the calculation must be modified.”]
A jury would have no way of understanding how the search process itself affects the
probability that the match is coincidental if told that they are to consider the fact that the match
was achieved by virtue of a database search.17
The Jenkins decision is also inconsistent with California law to the extent it holds that a
probability that represents something other than the chance that the match is coincidental is a
relevant consideration for the jury. In a cold hit case, the random match probability or
frequency does not represent the probability that the match is coincidental. (The Evaluation of
Forensic DNA Evidence, (1996) National Research Council, Recommendation 5.1) California
cases hold that DNA match evidence is not probative without an estimate of the chance that the
match is merely coincidental. 18 “[T]he evidence means nothing without a determination of the
statistical significance of a match of DNA patterns.” People v. Barney (1992) 8 Cal. App.4th
798, 817; accord, People v. Axell (1991) 235 Cal. App.3d 861, 866.
DNA profiles are consistent with each other either because they in fact came from the
same person, or else because the match is a mere coincidence. A false match could also be
caused by laboratory error.19 Because jurors lack the scientific knowledge to independently
assess this likelihood of such a coincidence, DNA match evidence is probative only if
accompanied by an accurate statistical estimate of the probability of a coincidental match.
“[T]he question the jury needs answered” is “how likely is it that a match would be reported if
the evidence DNA was not the suspect’s.” Lempert, After the DNA Wars: Skirmishing With
NRC II (1997) 37 Jurimetrics 439, 442, (reprinted in ARJN at p. 272).
In other words, the only legally relevant statistic is a statistic which represents the chance
that the DNA match is a coincidence. In a cold hit case this statistic is not represented by a
random match probability.
V.
THE DEFENDANT IS ENTITLED TO A HEARING UNDER EVIDENCE CODE
SECTIONS 801(b), 352, AND THE THIRD PRONG OF KELLY
The defendant is entitled to a hearing on the admissibility of random match probability
offered by the prosecution to describe the significance of the cold hit match in this case. He is
entitled to a hearing under Evidence Code Sections 801(b), 352 as well as People v. Kelly
(1976) 17 Cal.3d 24, because the method for calculating the statistic is not generally accepted
and generally accepted procedures were not followed in this case.
An expert opinion is inadmissible if it is unreliable and not based on the type of
information relied upon by experts in the field. (Evidence Code Section 801(b)). An expert’s
opinion, even if uncontradicted, may be rejected if the reasons given for it are unsound. Kelley
v. Trunk (1998) 66 Cal. App.4th 519. Expert testimony is inadmissible if it is not the type of
evidence that is reasonably relied upon by experts in the particular field in forming their
opinions. People v. Gardeley (1996) 14 Cal.4th 605, [modified on denial of rehearing, certiorari
denied 118 S.Ct. 148, 522 U.S. 854, 139 L.Ed.2d 94]. In addition, any material that forms the
basis of an expert’s opinion testimony must be reliable. People v. Boyette (2002) 29 Cal.4th
381, [rehearing denied].
Evidence Code Section 352 provides for the exclusion of relevant evidence “if its
probative value is substantially outweighed by the probability that its admission will (a)
necessitate undue consumption of time or (b) create substantial danger of undue prejudice, of
confusing the issues, or of misleading the jury.” “(E)xpert testimony may be assigned
talismanic significance in the eyes of lay jurors, and, therefore, the district courts must take care
to weigh the value of such evidence against its potential to mislead or confuse.” Cook ex rel.
Estate of Tessier v. Sheriff of Monroe County, Fla.(2005) 402 F.3d 1092.
The prosecution is offering a random match probability in this case to describe the
significance of the DNA match obtained as a result of a data base search or “cold hit.” The
defendant submits that the random match probability does not reliably state the significance of
a match in a cold hit case. It is misleading and creates a substantial danger of undue prejudice
and confusion. Additionally, the defendant submits that there is no general consensus in the
scientific community regarding which formula reliably describes the chance that the match is
coincidental when the match is a result of a data base search.
In the context of a database search case, random match probabilities are irrelevant and
thus, have no probative value. Such a statistic is misleading in that it tells the trier of fact
nothing about the likelihood that the DNA match is coincidental. Additionally, the random
match probability, the data base match probability and the Bayesian calculations are all based on
the assumption that the data base samples are composed of a “random sampling.” All experts
would agree that the convicted offender databank is anything but a random sampling. Using a
calculation that assumes a random sampling for a match that is derived from a non-random data
base is entirely misleading.
CONCLUSION
In this case, the conclusion that the profile of Mr. Davis matches that of the perpetrator is
based on a search of computerized databanks of convicted felons. At the time of his arrest and
to this date, there was no evidence of Mr. Davis’ guilt absent the results of DNA testing. DNA
testing in this case was not used to confirm other evidence, it is the only evidence. This is a
classic coldhit case.
The government cannot present evidence of the statistical significance of the similarity in
profiles unless and until the scientific community generally accepts one method over the other.
Because there is no general acceptance as to which of the three methods is “the” proper scientific
method, the Court cannot admit the evidence at all. The defendant respectfully asks the court to
exclude such evidence.