Legal requirements for admissibility of evidence of expert opinion; assumed factual basis for opinion; relevance
16 The general rule is that 'evidence of an opinion is not admissible to prove the existence of a fact about the existence of which the opinion was expressed': Evidence Act, subs 76(1). This 'opinion rule' assumes that the evidence of the opinion is relevant. Section 78 of the Evidence Act excepts from the opinion rule evidence of certain lay opinions, while s 79 excepts from that rule evidence of certain opinions of experts. Section 79 is as follows:
'If a person has specialised knowledge based on the person's training, study and experience, the opinion rule does not apply to evidence of an opinion of that person that is wholly or substantially based on that knowledge.'
In the present case there are many objections to the effect that particular expressions of opinion lie outside the established 'specialised knowledge based on the [author's] training, study and experience'.
17 By providing for an exception to the inadmissibility created by the opinion rule, s 79 goes to admissibility. Again, the section poses an objective test; no discretion is involved; a party raising an objection to admissibility on the ground that the section is not satisfied is entitled to a ruling on the objection; it is for a party, not the judge, to raise the objection; the possibility arises of supposedly expert opinion which does not satisfy s 79 being admitted because an objection is not raised (or, if raised, is not pressed); and in such a case it will be open to the judge to give the evidence no weight.
18 Unfortunately, in the case of many of the experts' reports, little or no attempt seems to have been made to address in a systematic way the requirements for the admissibility of evidence of expert opinion. Counsel protested that, in order to ensure that the requirements of admissibility are met, lawyers would have to become involved in the writing of the reports of expert witnesses. In the same vein, counsel said, in supporting the admission of certain parts of a report, that they were written in the way in which those qualified in the particular discipline are accustomed to write.
19 Lawyers should be involved in the writing of reports by experts: not, of course, in relation to the substance of the reports (in particular, in arriving at the opinions to be expressed); but in relation to their form, in order to ensure that the legal tests of admissibility are addressed. In the same vein, it is not the law that admissibility is attracted by nothing more than the writing of a report in accordance with the conventions of an expert's particular field of scholarship. So long as the Court, in hearing and determining applications such as the present one, is bound by the rules of evidence, as the Parliament has stipulated in subs 82(1) of the NT Act, the requirements of s 79 (and of s 56 as to relevance) of the Evidence Act are determinative in relation to the admissibility of expert opinion evidence.
20 In order to establish the admissibility of evidence of expert opinion, it must be shown:
(a) that the opinion is relevant (including that the field of knowledge is one in which expert opinion can properly be called (see Cross on Evidence (Australian edn) at [29050]) (Evidence Act ss 55, 56);
(b) that the person put forward as an expert possesses specialised knowledge in that field (Evidence Act s 79);
(d) that the specialised knowledge is based on the person's training, study or experience (Evidence Act s 79); and
(e) that the particular opinion tendered is based on the specialised knowledge (Evidence Act s 79).
21 In many instances the present reports do not:
· clearly expose the reasoning leading to the opinion arrived at (cf Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705 ('Makita') at [59] - [86] per Heydon JA); or
· distinguish between the assumed facts on which an opinion is based and the opinion itself (cf Trade Practices Commission v Arnotts Ltd (No 5)(1990) 21 FCR 324 (Beaumont J) at 327 - 330; upheld on appeal in Arnotts Ltd v Trade Practices Commission (1990) 24 FCR 313 at 347 - 353; HG v The Queen (1999) 197 CLR 414 ('HG v The Queen') at [39] per Gleeson CJ; Makita at [85] per Heydon JA).
22 Discussions in the cases about these matters do not always clearly distinguish between that which a statement of expert opinion must contain in order to be admissible, and that which it should contain in order to be useful.
23 In relation to the first matter (exposure of reasoning), in Makita, Heydon JA said (at [59]):
'If Professor Morton's report were to be useful, it was necessary for it to comply with a prime duty of experts in giving opinion evidence: to furnish the trier of fact with criteria enabling evaluation of the validity of the expert's conclusions.'
His Honour followed Lord President Cooper in Davie v Lord Provost, Magistrates and Councillors of the City of Edinburgh 1953 SC 34 at 39-40 in the view that 'the bare ipse dixit' or 'oracular pronouncement' of an expert witness can carry little weight.
24 In relation to the second matter (distinguishing between assumed facts and opinion) the Evidence Act does not, in terms, require, as a condition of admissibility, that an expert witness state distinctly and fully the facts assumed as the basis of his or her opinion: cf ALRC Report No 26 ('Evidence') vol 1, par 750; Quick v Stoland Pty Ltd (1998) 87 FCR 371 ('Quick') at 373 - 374 per Branson J; Daniel v State of Western Australia (2000) 178 ALR 542 ('Daniel') (RD Nicholson J) at [5]; Sydneywide Distributors Pty Ltd v Red Bull Australia Pty Ltd (2002) ('Sydneywide')55 IPR 354 per Branson J at [10].
25 But expert opinion will not be relevant if there is an insufficient correspondence between all the facts assumed by the expert as the basis for his or her opinion, and those proved or admitted: cf Ramsay v Watson (1961) 108 CLR 642 at 648-649; Paric v John Holland Constructions Pty Ltd [1984] 2 NSWLR 505 at 509-510; Paric v John Holland (Constructions) Pty Ltd (1985) 62 ALR 85 at 87. Absent that sufficient correspondence, it cannot be known whether the opinion proffered applies to the facts proved or admitted. If I cannot discern in a report, distinguishing between them, the expert's opinion and all facts assumed by the expert on which that opinion is based, I will not be able to be satisfied:
· that the assumed facts are, to a sufficient degree, the actual facts as I find them to be, to make that opinion 'relevant' for the purposes of s 56 of the Evidence Act (cf Quick at 374 per Branson J; Sydneywide at [14] per Branson J); or
· (perhaps) that the opinion proffered is one substantially based on the expert's specialised knowledge, for the purposes of s 79 of the Evidence Act (cf Makita at [85] per Heydon JA)
While perhaps it cannot be said that in all cases evidence of expert opinion will be inadmissible if an expert does not separately list all the factual assumptions underlying his or her report, if this is not done, there is a substantial risk that the court will not be able to be satisfied on reading the report, as to what they were, with the result that the opinion will be inadmissible. If the expert does not state and distinguish between all the factual assumptions underlying the opinion and the opinion itself, can I be satisfied that the two criteria of admissibility mentioned are met?
26 Admittedly, there are great practical differences in the present respect between, for example, Makita, an appeal concerned with a report of expert opinion given by a 'physicist who specialised in the investigation of slipping accidents' (Makita at [25]) in relation to the slipperiness of a stair, and a case such as the present one, concerned with reports of opinions given by historians and anthropologists in relation to the more complex question whether there are communal, group or individual rights and interests of Aboriginal peoples in relation to land or waters, where the rights and interests are possessed under the traditional laws acknowledged, and the traditional customs observed, by the Aboriginal peoples, and they, by those laws and customs, have a connection with the land or waters (see the definition of 'native title' in the NT Act, s 223).
27 Unfortunately, however, in the case of many of the present reports, it is difficult to avoid the impression that no attempt at all has been made to address the criteria of admissibility of expert opinion evidence. The difficulty of my task is increased as a result. My impression is that in some cases, beyond the writing of an initial letter of instructions to the expert, lawyers have left the task of writing the reports entirely to the expert, even though he or she cannot reasonably be expected to understand the applicable evidentiary requirements. Such a course may have been followed because of a commendable desire to avoid any possibility of suggestion of improper influence on the author. But I suggest that the distinction between permissible guidance as to form and as to the requirements of s 56 and 79 of the Evidence Act, on the one hand, and impermissible influence as to the content of a report on the other hand, is not too difficult to observe. It does not serve the interests of anyone, including those of the expert witness, to deny him or her the benefit of guidance of the kind mentioned.