The Basis of the ‘Basis Rule’: The Role of the Basis Rule in the Admissibility of Expert Opinion

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 and assumptions upon which the opinion is based. 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 weight. 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.