The Basis Rule: A question of weight and relevance (Part 1 of 2)

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Our recent blog Expert Witnesses: Puppets of advocates or gift of the Court? highlighted that Australian courts have increasingly adopted the view that admissibility of expert evidence is no longer a given.  The courts have made it clear that they are skeptical of the oxymoronic notion of a paid impartial expert. As a result, the application of the Uniform Evidence Legislation has spurred debate over the requirements to be met for expert evidence to be admissible. One aspect of this debate, the opinion rule, is considered one of the “most controversial and litigated issues in expert evidence.”[1]

In a two part post we will consider a particularly contentious issue; the existence of ‘the basis rule’. In part one, the basis rule and why its application is ambiguous will be outlined. In our second post, the judicial application of the basis rule will be considered through a discussion ofMakita (Australia) Pty Ltd v Sprowles (Makita)[2] and Dasreef Pty Ltd v Hawchar[3] which have provided some but not unequivocal guidance in its application. Unlike most kinds of witnesses, expert witnesses are able to offer opinion as to the implications and meanings of facts. In order to avoid the introduction of irrelevant opinions a number of requirements have developed in the common law.

  • The opinion must be on an area of specialised knowledge;
  • The witness must demonstrate they are an expert by reasons of specified training, study or experience; and
  • the expert must state and the party calling the expert prove, the facts upon which the opinion is based. (the basis rule)

Such requirements are mirrored in the Uniform Evidence Legislation under s 79 which provides an exception to the inadmissibility of opinion evidence;

if a person has specialised knowledge based on the person’s training, study or experience, the opinion rule does not apply to evidence of an opinion that is wholly or substantially based on that knowledge.” [4]

While the requirements of specialised knowledge by reasons of training can be found in the legislative provision, the requirement for the opinion to be based upon proven facts is not expressed in the legislation. Thus, as the law currently stands there is substantial uncertainty surrounding what is known as the basis rule in both statutory and common law. The Australian Law Reform Commission (“ALRC”) in particular has expressed doubt as to whether a basis rule exists concluding that the “better view is that there is no basis rule operating as a rule of admissibility”. [5]

Rather, the ALRC suggests the rule exists to explain the interaction of two processes used in cases where expert opinions are given on evidence that is not conclusively proved.[6] The first process involves a determination on the relevance of the opinion. The second is the assessment of how much weight should be afforded to the opinion. This overlap partially explains why the basis rule has not been conclusively held as a rule of admissibility since the consequences of the failure to disclose or prove the facts upon which the opinion is based has differed. In some cases it has resulted in inadmissibility, in others it has merely lessened its weight. This will be discussed in the second chapter to this post through a closer look at the cases Makita and Dasreef.

This article was prepared in conjunction with Susan Flynn.

[1] Miiko Kumar, ‘Admissibility of Expert Evidence: Proving the Basis for an Expert’s Opinion’ (2011) 33(3) Sydney Law Review, 427, 428.

[2] [2001] NSWCA 305.

[3] (2011) 277 ALR 622. [4] Above n 1, 428.

[5] Ibid, 428.

[6] Australian Law Reform Commission, Uniform Evidence Law, Report No 102, [9.57].

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