Judicial assessment of expert evidence

AuthorEvan Bell
PositionMaster, Queen's Bench and Matrimonial Divisions, Court of Judicature, Northern Ireland
2010] Judicial Assessment of Expert Evidence 55
While the test for admissibility of expert evidence differs
from jurisdiction to jurisdiction, judges in all jurisdictions face
the common responsibility of weighing expert evidence and
determining its probative value. This is no easy task. Expert
opinions are admissible to furnish courts with information which
is likely to be outside their experience and knowledge.
The evidence of experts has proliferated in modern litigation and
is often determinative of one or more central issues in a case.1
Without it judges could not hope to determine where truth lies.
Yet expert evidence presents its own hazards. The phenomenon
of the “hired gun” expert is well known. Particular experts
become associated with certain positions and accordingly are
called by the parties seeking to establish that position. Opposing
experts are then called by the opposing parties.2 The disparities in
the evidence of experts within the same discipline are notorious,
ineluctably favouring the party who has instructed them. As a
result, judges sometimes suggest that the courts should be
prepared, where appropriate, to approach the evidence of experts
with “a healthy scepticism”3 or “with reserve and scepticism”.4
Expert testimony, like all other evidence, must be given
only appropriate weight. It must be as influential in the overall
decision-making process as it deserves: no more, no less.
This article seeks to identify the criteria which judges use to
weigh the probative value of expert evidence. Some of these
* Master, Queen’s Bench and Matrimonial Divisions, Court of Judicature,
Northern Ireland.
1 State v. Pearson and Others (1961) 260 Minn. 477.
2 McLachlin, “Judging in a Democratic State”, Sixth Templeton Lecture on
Democracy, University of Manitoba, Winnipeg, 3 June 2004.
3 R v. Griffin [2001] 3 N.Z.L.R. 577.
4 Tomislav Lipovac Bhnf Maria Lipovac v. Hamilton Holdings Pty Ltd and
others [1996] A.C.T.S.C. 98.
Judicial Studies Institute Journal [2010: 2
criteria are applicable in all cases; others only in some, depending
on the nature of expert evidence which has been called5 and what
other evidence has been offered. Certain of the criteria are
interrelated and overlap. No single criterion is determinative.
Some of the criteria are particular to expert evidence alone; others
are used to assess the weight of all witness testimony. The article
illustrates how judges have applied these criteria in weighing
expert evidence in order to demonstrate how they operate in
The weight to be given to expert evidence will derive from
how that evidence is assessed in the context of all other evidence.
This is because, while expert evidence is important evidence, it is
nevertheless merely part of the evidence which a court has to take
into account.6 Four consequences flow from this.
Firstly, expert evidence does not “trump all other
evidence”.7 It is axiomatic that judges are entitled to disagree
5 Courts have distinguished between different types of evidence offered by
expert witnesses. In The Torenia [1983] 2 Lloyd’s Rep. 210 Hobhouse J.
identified different evidential categories. Firstly, evidence is adduced which
can be described as direct factual evidence which bears directly on the facts of
the case. Secondly, opinion evidence is given with regard to those facts as they
have been proved; and thirdly, evidence, which might be described as factual,
is used to support or contradict the opinion evidence. This is evidence which is
commonly given by experts, because in giving their evidence they rely upon
their experience, and they refer to that experience in their evidence. So an
expert may say what he has observed in other cases and what these have taught
him for the evaluation of the facts of a particular case. Similarly, experts give
evidence about experiments which they have carried out in the past or which
they have carried out for the purposes of their evidence in the particular case in
question. In Harrington-Smith v. Western Australia (No.7) [2003] F.C.A. 893
Lindgren J. suggested four categories: expert opinion evidence, expert non-
opinion evidence, non-expert opinion evidence, and (implicitly) non-expert
non-opinion evidence. It may be the difficult to apply the concept of “opinion”
to certain evidence which may amount to an analysis, synthesis and summary
of factual material.
6 Huntley (also known as Hopkins) (a protected party by his litigation friend,
McClure) v. Simmons [2010] E.W.C.A. Civ 54.
7 Woodhouse v. Britannic Assurance p.l.c., Employment Appeal Tribunal,
U.K.E.A.T. 0132/03/RN para. 25.
2010] Judicial Assessment of Expert Evidence 57
with an expert witness.8 Expert evidence should be tested against
known facts, as it is the primary factual evidence which is of the
greatest importance. It is therefore necessary to ensure that expert
evidence is not elevated into a fixed framework or formula,
against which actions are then to be rigidly judged with a
mathematical precision.9 In HKSAR v. Chan Sze Pui10 the
defendant switched the price label on a box of DVDs with that of
a cheaper box. She paid the cheaper price and then left the shop.
At her trial she testified she had heard a voice in her mind telling
her to do so. The defence called a psychiatrist whose opinion was
that she was suffering from obsessive-compulsive disorder and a
major depressive disorder. On appeal against conviction it was
held that a court is not compelled to accept the evidence of an
expert but is entitled to accept or reject that evidence like any
other, bearing in mind the whole of the evidence in the case.
The court had placed the expert testimony in the context of the
whole of the evidence and determined what weight could be
placed upon it. It was entitled to make findings as to whether the
symptoms the expert said would be present, and the distress that
the defendant said she experienced, were to be seen on the
surveillance video. The court found that the signs of obsessive-
compulsive behaviour given by the expert, such as handing-
washing, checking, and counting, were absent at the material
time. Secondly, a judge must not consider expert evidence in a
vacuum.11 It should not therefore be “artificially separated” from
the rest of the evidence. To do so is a structural failing.12
A court’s findings will often derive from an interaction of its
views on the factual and the expert evidence taken together.
The more persuasive elements of the factual evidence will assist
the court in forming its views on the expert testimony and vice
8 AB v. BG and Others [2009] E.W.C.A. Civ. 10. This proposition also has an
equally obvious corollary. There must be material upon which the judge in
question can safely found his disagreement, and he must fully explain the
reasons for rejecting the expert’s evidence.
9 Stewart v. Glaze [2009] E.W.H.C. 704.
10 HKSAR v. Chan Sze Pui, Gloria [2004] H.K.C.U. 298.
11 Wang Din Shin v. Nina Kung [2004] H.K.C.U. 730.
12 HE (DRC – Credibility and Psychiatric Reports) [2004] U.K.I.A.T. 00321.

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