In the recent decision of Dasreef Pty Ltd v Hawchar  HCA 21 the High Court has passed up an opportunity to resolve, once and for all, a number of important issues in relation to the admissibility of expert evidence.
Hawchar worked for Dasreef as a labourer and stonemason over a period of around five and a half years between 1999 and 2005. In 2006 he was diagnosed with early stage silicosis. In proceedings commenced in the Dust Diseases Tribunal, Hawchar relied on opinion evidence from Dr Kenneth Basden, a chartered chemist, chartered professional engineer and retired academic. In his report, Dr Basden said that “actual dust concentrations generated in [Hawchar’s] breathing zone ... most certainly would not be from half to two ten-thousandths of a gram [0.05 to 0.2mg] per cubic metre of air, but more realistically would be of the order of a thousand or more times these values or even approaching one gram, or thereabouts, per cubic metre. The trial judge used it as an integer in calculations leading to the conclusion that the time weighted average of Hawchar’s exposure to dust while working for Dasreef, assuming he was exposed for 30 minutes on each of five days per week, was 0.25mg/m3, which exceeded the limit of 0.2mg/m3 in the relevant Australian standard. That was a pleaded particular of a breach of statutory duty. The Court of Appeal upheld the trial judge’s use of the evidence in that way.
On appeal to the High Court, Dasreef argued that Dr Basden’s opinion was inadmissible by reason of s79(1) of the Evidence Act 1995 (NSW) which provides: “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 of that person that is wholly or substantially based on that knowledge.” The “opinion rule” is the general exclusionary rule contained in s76(1) of the Act that provides: “Evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed.”
The High Court unanimously ruled that Dr Basden’s evidence was not admissible to establish the numerical or quantitative level of exposure.
The plurality judgment (ie the whole of the High Court except Heydon J) based its conclusion on the basis that Dr Basden had not demonstrated how his opinion was based upon his training, study or experience. There was no evidence that his training, study or experience permitted him to provide anything more than what he called a “ballpark” figure estimating the amount of respirable silica dust to which a worker using an angle grinder would be exposed if that worker was using it in the manner depicted in the photograph of Hawchar or a video recording that Dr Basden was shown. Indeed, in his written report, Dr Basden had pointed out that he had seen the use of an angle grinder in this way only once before. And he gave no evidence that he had then, or on any other occasion, measured directly, or sought to calculate inferentially, the amount of respirable dust to which such an operator was or would be exposed.
In doing so, the plurality emphasized that while the admissibility of evidence was to be considered in the light of the provision of the Evidence Act:
It remains useful to record that it is ordinarily the case, as Heydon JA said in Makita, that “the expert’s evidence must explain how the field of ‘specialised knowledge’ in which the witness is expert by reason of ‘training, study or experience’, and on which the opinion is ‘wholly or substantially based’, applies to the facts assumed or observed so as to produce the opinion propounded”. The way in which s79(1) is drafted necessarily makes the description of these requirements very long. But that is not to say that the requirements cannot be met in many, perhaps most, cases very quickly and easily. That a specialist medical practitioner expressing a diagnostic opinion in his or her relevant field of specialisation is applying “specialised knowledge” based on his or her “training, study or experience”, being an opinion “wholly or substantially based” on that “specialised knowledge”, will require little explicit articulation or amplification once the witness has described his or her qualifications and experience, and has identified the subject matter about which the opinion is proffered.
Where the plurality judgment becomes more interesting is in relation to a submission by Hawchar that Dasreef was seeking to reintroduce the common law “basis rule” that had been discarded (so it was argued) by the Evidence Act. The “basis rule” was a rule that provided that opinion evidence was to be excluded unless the factual bases upon which the opinion was proffered are established by other evidence. The plurality said:
Whether that rule formed part of the common law of evidence need not be examined. It may be accepted that the Law Reform Commission’s interim report on evidence denied the existence of such a common law rule and expressed the intention to refrain from including a basis rule in the legislation the Commission proposed and which was later enacted as the Evidence Act.
The plurality concluded by saying:
A failure to demonstrate that an opinion expressed by a witness is based on the witness’s specialised knowledge based on training, study or experience is a matter that goes to the admissibility of the evidence, not its weight. To observe, as the Court of Appeal did, that what Dr Basden said about the volume of respirable dust to which Mr Hawchar was exposed over time was “an estimate” that was “contestable and inexact” no doubt did direct attention to its worth and its weight. But more importantly, it directed attention to what exactly Dr Basden was saying in his evidence and to whether any numerical or quantitative assessment he proffered was admissible. And if, as the Court of Appeal observed, his opinion on that matter lacked reasoning, the absence of reasoning pointed (in this case, inexorably) to the lack of any sufficient connection between a numerical or quantitative assessment or estimate and relevant specialised knowledge.
In his concurring judgment, Heydon J let both barrels fly in defending the statement of the law relating to opinion evidence he had proffered when in the NSW Court of Appeal in Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705, which has been stridently criticized as at best a “counsel of perfection” and at worst an overstatement of the opinion rule provided for by the Evidence Act. In doing so, Heydon J emphasized that both at common law and continuing under the Evidence Act, in order to be admissible opinion evidence had to satisfy three requirements:
- First, the expert has to disclose the “facts” and “assumptions” on which the expert’s opinion was founded (the “assumption identification” rule).
- Secondly, the “facts” and “assumptions” stated must be proved before the evidence is admissible (the “basis” rule in the language of the ALRC, but which Heydon J refers to as “proof of assumption” rule).
- Thirdly, there must be a statement of reasoning showing how the “facts” and “assumptions” are related to the opinion so as to reveal that that opinion was based on the expert’s expertise (the “statement of reasoning” rule).
In relation to the assertion that the “proof of assumption” rule did not exist at common law, and the ALRC had deliberately decided not to include it in s79, Heydon J said:
The [ALRC’s] reasoning has misled both itself and some of its readers. A decision to refrain from including what was thought to be a rule which does not exist at common law does not demonstrate abolition of a rule which does in fact exist at common law. The [ALRC] wrongly thought that there is no proof of assumption rule at common law. On that hypothesis, as the [ALRC] correctly saw, the question was whether it should recommend that the legislature should enact one, and it decided not to make that recommendation. In fact there is a proof of assumption rule at common law, and the question for the [ALRC] thus should have been whether to recommend that it be abolished by legislation. To abolish it by legislation would have called for specific language. The [ALRC’s] misapprehension of the common law, and hence of its task, has resulted in a failure to have enacted specific language ensuring that s79 tenders need not comply with a proof of assumption rule.
There can be no doubt that Heydon J’s reference to “readers” being misled is directed towards Branson J’s complaint in Sydneywide Distributors Pty Ltd v Red Bull Australia Pty Ltd  FCAFC 157 that Heydon J’s statement of the law in Makita was a “counsel of perfection”. Heydon J’s rejection of that criticism was taken further in dealing with Branson J’s proposition that if “tenders depended on compliance with a proof of assumption rule, ‘the smooth running of trials involving expert evidence could be expected to be interrupted by the need to explore in detail, in the context of admissibility, matters more properly considered at the end of the trial in the context of the weight to be attributed to the evidence.’ " In response, Heydon J said:
Those complaints overlook a key distinction. It is a distinction between the need for the party tendering an expert opinion to establish that the other evidence (tendered or yet to be tendered) taken at its highest is capable of supporting the expert’s factual assumptions, on the one hand, and, on the other hand, the need for the tendering party to persuade the trier of fact to accept that the evidence tendered to support the factual assumptions actually does support them. And it is necessary to set against any practical difficulties caused by the proof of assumption rule a competing consideration. That is whether the reception of evidence which turns out to be useless because the assumptions on which it rests are unproven does not create countervailing difficulties – costs burdens, unduly long trials, the risk of misleading the trier of fact, and unnecessary appeals.
Ultimately, Heydon J ruled Dr Basden’s evidence inadmissible on the same basis as the plurality, ie that Dr Basden had not demonstrated how his opinion was based upon his training, study or experience.
As a footnote, it should also be noted that the plurality overruled previous decisions of the NSW Court of Appeal that had held that the DDT was entitled, as a specialist tribunal, to take into account its experience in determining what caused Hawchar’s silicosis.
The plurality concluded, nonetheless, that despite the inadmissibility of Dr Basden’s quantitative opinion, and the trial judge’s incorrect use of his experience as a specialist tribunal, nonetheless the evidence was sufficient to uphold the finding of liability in Hawchar’s favour.