The Basis of the ‘Basis Rule’: The Role of the Basis Rule in the Admissibility of Expert Opinion
| Author | David Caruso,Trang Phan |
| DOI | 10.22145/flr.43.2.6 |
| Published date | 01 June 2015 |
| Date | 01 June 2015 |
| Published By | Sage Publications, Inc. |
THE BASIS OF THE 'BASIS RULE':
THE ROLE OF THE BASIS RULE IN THE ADMISSIBILITY OF
EXPERT OPINION
Trang Phan*and David Caruso **
ABSTRACT
The 'basis rule' is, in general terms, a rule which restricts expert witnesses to giving
opinion evidence in respect of which there is or will be proof, by other admissible
evidence, of the facts a nd assumptions upon which the opinion is b ased. There has been
no clear consensus as to whether the basis rule exists either at common law or under the
Uniform Evidence Legislation, or whether the rule goes to admissibility or weigh t. This
article examines the jurisprudence, with a particular focus on the recent High Court
decision of Dasreef Pty Ltd v Hawchar. The authors argue that the controversy
surrounding the basis rule has been the result of a misunderstanding and
misconstruction of the rule. They argue that the conflict may be resolved by
understanding the basis rule as simply a rearticulation, in the specific context of expert
evidence, of the requirement that evidence must be relevant to be admissible. The weight
of that expert evidence remains to be determined in accordance with ordinary principles.
IINTRODUCTION
Experts have knowledge that courts do not.Courts accordingly seek the assistance of
experts to understand reasoning and matters that may determine facts in issue.
However, it is the role of the court, and not witnesses, to reason and infer whether
material facts have been established.Because of this, together with the cost and time
involved in calling experts, courts have been reticent to accept only expert evidence
which adheres to increasingly strict rules of admissibilit y.One such rule which has been
employedto restrict the tender of expert evidence is the so called 'basis rule': a rule which
has been a source of confusion a nd disagreement among courts and commentators. 1
*Associate, Lipman Karas. The views expressed in this article are those of the authors and do
not necessarily represent the views of Lipman Karas.
**Foundation Director, Litigation Law Unit, The University of Adelaide; Lecturer, Adelaide
Law School, The University of Adelaide; Special Counsel, Fisher Jeffries | Gadens; President-
Elect, Law Society of South Australia; Director, Law Council of Australia.
1See generally Miiko Kumar, 'Admissibility of Expert Evidence: Proving the Basis for an
Expert's Opinion' (2011) 33 Sydney Law Review427; J D Heydon, Cross on Evidence(LexisNexis
Butterworths, 9thed, 2013) 967–73 [29070]; Ian Freckelton and Hugh Selby, Expert Evidence:
Law, Practice, Procedure and Advocacy(Lawbook Co, 4thed, 2009) ch 6 'The Basis Rule'; Robert
French, 'Expert testimony, opinion, argument and the rules of evidence' (2008) 36 Australian
314Federal Law ReviewVolume 43
_____________________________________________________________________________________
The exact ambit of the 'basis rule' is neither well-settled nor consistently explained
in its jurisprudence.The basis rule is, in general terms, a rule which restricts experts to
opinions in respect of which there is or will be proof of the fac tual basis for the opinion.
Thisis specifically identified by Heydon J in Dasreef Pty Ltd v Hawchar,2as the 'proof of
assumption' rule. In Dasreef, Heydon J said that the 'basis rule' includes the 'proof of
assumption rule' as well as two other distinct but interrelated rules: the 'assumption
identification rule', whereb y the expert must state the facts and assumptions upon which
his or her opinion is based; and the'statement of reasoning rule', whereby the expert
must state the process of reasoning by which the expert forms his or her opinion.3
Whatever the labels given, it is what Heydon J calls the 'proof of assumption rule' that
is identified in common law as the 'basis rule'. The focus of this article is on the ambit
and work of 'the proof of assumption rule', which we will refer to a s the 'basis rule'.
The premiseof the basis rule is entirely sensible.The rule is an acknowledgement
that opinions proffered by anexpert will only be of use to the court if there is admissible
evidence which can support the facts which form the basis for the expert's opinions.
There is still debate as towhether the basis rule existsat common law or under the
Uniform Evidence Legislation ('UEL')4and, critically,whether the rule goes to
admissibility or weightof the expert opinion. In Dasreef, Heydon J upheld the existence
of the basis rule as a rule of admissibility at common law and under the UEL. The
plurality in Dasreef expressly declined to address the question.
We respectfully agree with Heydon J that proof of the factual basis of an opinion
may be a matter affectingadmissibility,5but disagree to the extent thatwe contend there
is not a strict 'basis rule' of admissibility.Rather than being an exclusionary rulein its
own right, the basis rule is no more tha n a rearticulation, in the specific context of expert
evidence, of the requirement that evidence must be relevant to be admissible.As
indicated at the outset, experts assist the court to learn reasoning that can be applied to
determine facts.The usefulnessof expert opinion hinges on whether the facts and
assumptions, which the expert assertsleads to the opinion, are sufficiently similar to
facts that are capable of proofby other admissible evidence. Whether that is so is a
requirement of relevance, whichordinarily must be satisfied at the time of tender. Expert
opinion that cannot assist the court cannot be relevant and it isthiswant of relevance
Business Law Review263, 274–5; Christopher C Hodgekiss, 'Expert evidence in competition
litigation' (2006) 14 Trade Practices Law Journal6, 11–13.
2(2011) 243 CLR 588 ('Dasreef').
3See ibid 612 [61].
4Uniform evidence legislation has been enacted across a number of Australia jurisdictions
following an inquiry by the Australian Law Reform Commission into the law of evidence
which culminated in the publication of two reports: Evidence (Interim), Report No 26 (1985)
('ALRC 26'); and Evidence, Report No 38 (1987). The adoption of the Commission's
recommendations began with the enactment of the Evidence Act 1995(Cth). New South
Wales, Tasmania, Victoria and the Australian Capital Territory have since followed suit.
5Some distinction has been drawn between facts which are 'observed' by the expert and facts
which are 'assumed' or 'accepted' by the expert: see, eg, Makita (Australia) Pty Ltd v S prowles
(2001) 52 NSWLR 705, 743–4 [85]. As far as the basis rule is concerned, there is no practical
difference between the two categories. Both kinds of facts find probity in other evidence. The
difference is only in the source of that evidence: assumptions rely on proof by evidence not
provided by the expert, while facts observed rely on proof by evidence to be provided by the
expert him or herself. In this article, the term 'factual basis' of an opinion is employed to
cover both facts observed and assumptions.
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