Explaining and trusting expert evidence: What is a ‘sufficiently reliable scientific basis’?

AuthorTony Ward
Published date01 July 2020
Date01 July 2020
Subject MatterArticles
Explaining and trusting expert
evidence: What is a ‘sufficiently
reliable scientific basis’?
Tony Ward
Northumbria University, UK
Through a series of judicial decisions and Practice Directions, the English courts have devel-
oped a rule that expert evidence must have ‘a sufficiently reliable scientific basis to be
admitted’. There is a dearth of case-law as to what degree of reliability is ‘sufficient’. This article
argues that the test should be interpreted as analogous to one developed in the law of hearsay:
expert evidence (scientific orotherwise) must be ‘potentiallysafely reliable’ in the contextof the
evidence as a whole. The implications of this test will vary according to the relationship between
the expert evidence and the other evidence in the case. The article identifies three main patterns
into which this relationship falls. Whether the jury relies upon the evidence will depend upon
what they regard as the best explanation of the evidence and how far they trust the expert.
Whether their reliance is safe (as a basis for conviction) depends on whether they could
rationallyrule out explanations consistentwith innocence, and whether the degree to which they
take the expert’s evidence on trust is consistent with prosecution’s burden of proving the
essential elements of its case, including the reliabilityof any scientific techniques on whichit relies.
DNA evidence, England and Wales, epistemology of testimony, expert evidence, scientific
The principle governi ng judicial scrutiny of th e reliability of exper t evidence in criminal t rials in
England and Wales is ‘that in determining the issue of admissibility, the court must be satisfied that
there is a sufficiently reliable scientific basis for the evidence to be admitted. If there is then the court
leaves the opposing views to be tested before the jury.’
According to Criminal Practice Direction (CPD)
Corresponding author:
Tony Ward, Northumbria University, City Campus East, Newcastle upon Tyne, UK.
E-mail: tony.ward@northumbria.ac.uk
1. RvDlugosz [2013] EWCA Crim 2, [2013] 1 Cr App R 32 at [11].
The International Journalof
Evidence & Proof
2020, Vol. 24(3) 233–254
ªThe Author(s) 2020
Article reuse guidelines:
DOI: 10.1177/1365712720927622
19A.4, this principle applies ‘especially’, but not solely, to scientific evidence and courts are ‘encour-
aged’ to consider similar factors to those which the Law Commission (2011) recommended should
become statutory preconditions for admissibility. Possibly because of lack of awareness in the profession
of what the CPD requires (Davies and Piasecki, 2016), there is a dearth of case law on its interpretation.
This article sets out a view of how it ought to be interpreted.
The combination of the ‘sufficient reliability’ test with a reaffirmation of the jury’s role as the final
arbiter of the weight of evidence reflects a tension between two fundamental principles of the criminal
jury trial: that defendants should be convicted only when their guilt is proved to the criminal standard on
a ‘logically justifiable basis’;
and that it is for the jury, not any purported expert, to determine what
weight can be given to any piece of evidence in deciding whether the standard of proof is met. The
principle of rational fact-finding or ‘epistemic due process’ (Beecher-Monas, 2009; Brewer, 1998) can
be invoked to support an ‘exclusionary ethos’ (Edmond and Roach, 2011: 396–405), according to which
only evidence that demonstrably meets rigorous standards of scientific validity (or analogous standards
in non-scientific fields of expertise) can be admitted. On the other hand, the role of the jury in determin-
ing the weight of evidence according to its own lights can be taken to support the much criticised
‘laissez-faire approach’ (Law Commission, 2011: 11), which demands little more of expert evidence
than that it be relevant and capable of assisting the jury. The CPD cannot plausibly be interpreted as
embodying either of these opposing views. Being anchored in existing case law, it cannot be supposed to
embrace an ‘exclusionary ethos’ which would radically change the previously existing approach. But by
incorporating as many of the Law Commission’s recommendations as could be implemented without
legislation, it clearly signals a rejection of the laissez-faire approach.
In looking for a tenable middle ground between the exclusionary and laissez-faire approaches, it is
important to appreciate that the jury has different tasks and a different epistemic standpoint to that of a
scientist. Whether a lay tribunal, considering the evidence as a whole, can be sure of guilt is a different
question from whether a particular piece of evidence, considered in isolation, conforms to some test of
scientific validity. The jury performs two epistemic functions that cannot be appropriately performed
either by expert witnesses themselves or by regulatory bodies that provide quality control in various
fields of expertise. One is the task of putting all the evidence, expert and non-expert, together and
determining whether it satisfies them to the relevant standard of proof (i.e. the criminal one, or in cases
where the defendant has the onus of proving a defence, the civil one). The other is to determine on behalf
of the community at large what degree of trust can be placed in the experts. As the OJ Simpson case
famously demonstrated, where either the scientists themselves, or those who the collect trace evidence
they analyse, are viewed as untrustworthy, otherwise extremely powerful scientific evidence may fail to
satisfy a jury (Lynch et al., 2008: 113–120).
Because these two aspects of the weight of evidence—the explanatory inferences to be drawn from
the whole body of evidence of which it forms part, and the degree to which the witness can be trusted—
are matters reserved for the jury, the judge cannot determine the weight of the evidence by examining the
expert evidence in isolation. What the judge can determine is, to borrow a phrase from hearsay law,
whether the evidence is potentially safely reliable. As the Court of Appeal has explained in the hearsay
The critical word is ‘potentially’. The job of the judge is not to look for independent complete verification. It
is to ensure that the hearsay can safely be held to be reliable. That means looking ...at its strengths and
weaknesses, at the tools available to the jury for testing it, and at its importance to the case as a whole.
2. RvHenderson R vHenderson [2010] EWCA Crim 1269, [2010] 2 Cr App R 24 at [2].
3. See also Leveson LJ’s remarks in RvH[2014] EWCA Crim 1555 at [44].
4. RvIbrahim [2012] EWCA Crim 837, [2012] 2 Cr App R 32 at [107].
5. RvRiat [2012] EWCA Crim 1509, [2013] 1 WLR 2592 [33].
234 The International Journal of Evidence & Proof 24(3)

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