Judicial assessment of expert evidence

• Author: Evan Bell
• Position: Master, Queen's Bench and Matrimonial Divisions, Court of Judicature, Northern Ireland
• Pages: 55-96

2010] Judicial Assessment of Expert Evidence 55

JUDICIAL ASSESSMENT OF

EXPERT EVIDENCE

EVAN BELL*

INTRODUCTION

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

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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

practice.

I. THE CONTEXT OF ALL OTHER EVIDENCE

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.