Expert Blog

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



This post follows on from our earlier blog The Basis Rule: A question of weight and relevance. There we explained the basis rule as a supposed yet tenuous requirement for the facts upon which an opinion is based to be proved by evidence in order for the opinion to be valid in expert evidence. Specifically we highlighted the uncertainty that exists as to whether the basis rule is a requirement of admissibility or more simply, an explanation of the process courts use to determine first the relevance of an opinion and second how much weight it should be given. The two cases below Makita (Australia) Pty Ltd v Sprowles (Makita)[1] and Dasreef Pty Ltd v Hawchar[2] have provided some guidance on how to apply the so called ‘basis rule’.

Makita was a decision on whether an employer failed to provide a safe means of access between the car park and offices. A central issue was the whether the stairs were safe and an expert gave his opinion that the stairs were slippery at the time of the accident. The admissibility of his opinion was questioned as the tests forming the basis of his opinion were performed nine years after the accident. Most usefully, Makita explored the interaction between weight (a forensic consideration) and relevance (an evidentiary concern).[3] From this case, the basal principle was formed that required the facts upon which the opinion is based to be proved by admissible evidence, in order “to render the opinion of any value”.[4] This means the expert is required to identify the factual basis of an opinion so that the court is able to assess how the opinion was reached. Makita however did not establish a basis rule but merely highlighted the interaction between considerations of weight and relevance since it implies the failure to disclose such facts will reduce the weight of the opinion so as to render it irrelevant. In Makita this was as far as the court could go in assessing the rule since the report was admitted without objection.

The uncertainty surrounding this principle was further clarified in Dasreef v Hawchar. Mr Hawchar brought a cause of action against Dasreef after contracting silicerosis alleging that it was contracted as a result of his exposure to silica dust during his tenure at Dasreef. At trial, there was expert evidence led from an expert pathologist as well as Dr Basden, a chemical engineer. Despite being an expert in respirable dust concentrations he had no experience with the respirable fraction of the stone with which Mr Hawchar worked. This evidence was ultimately held to be inadmissible on the basis that Dr Basden did not have training, study or experience in the sandstone particles that Mr Hawchar was exposed to and therefore there was a “lack of connection” enabling them to avoid consideration of the basis rule. It has been suggested that this imposed the ‘basis’ rule into the application of s 79 of the Evidence Act despite the deliberate non-inclusion by the ALRC. Indeed Heydon JA went so far in his dissent as to say that the ALRC has incorrectly assumed the basis rule was not a part of common law and that as s 79 has not abolished the rule it still existed as to govern admissibility. He outlined the basis rule as requiring:

  • disclosure of the assumption and facts upon which the opinion is founded; and
  • the proof of these assumptions and facts; and
  • a statement outlining how the opinion relates to these assumptions and facts.

The majority opinion in Dasreef established that admissibility of expert opinions will continue to be decided in accordance with Uniform Evidence Legislation. Though it does not conclusively solve the question of whether the basis rule exists as an exclusionary rule, it provides guidance for litigators in preparing expert evidence. First, they must identify an area where specialised knowledge is required and secondly, the must determine whether their expert is sufficiently qualified in that specific area to express their opinion. This will be central in the court’s consideration of the weight granted to expert opinions and concurrently, its admissibility.

This article was prepared in conjunction with Susan Flynn.

[1] [2001] NSWCA 305.

[2] [2011] 277 ALR 61.

[3] Australian Law Reform Commission, Uniform Evidence Law, Report No 102, [9.61].

[4] Miiko Kumar, ‘Admissibility of Expert Evidence: Proving the Basis for an Expert’s Opinion’ (2011) 33(3) Sydney Law Review 436.