THE ROLE OF AN EXPERT WITNESS AND THE EVALUATION OF EXPERT EVIDENCE
WHAT DOES THE EXPRESSION ‘EXPERT WITNESS’ MEAN?
The expression ‘expert witness’ is ordinarily used to refer to a witness whose opinions, and the reasons on which they are based, are admissible in evidence in litigious matters, as his or her knowledge and experience in some or other subject matter or in a particular field, usually technical or scientific in nature, enables such a witness to draw inferences and form views in circumstances where a court or tribunal is unable to do so reliably, unless it receives assistance or guidance from someone with expertise on the relevant subject matter or in that particular field.
Various facets of the above formulation of what an expert witness is, have been reiterated or emphasised in many different ways in judicial pronouncements made over the years. So, for example, it has been said that:
- the prime function of an expert witness is ‘… to guide the court to a correct decision on questions falling within his specialised field. His own decision should not, however, displace that of the tribunal which has to determine the issue to be tried …’; and
- the true and practical test for the admissibility or receivability of an expert’s opinion is whether or not the court:
‘… can receive “appreciable help” from that witness on a particular issue; in other words, “the test is a relative one, depending on the particular subject and the particular witness with reference to that subject.”’; and
- the occasion or occasions when such ‘appreciable help’ will be needed by a court or arbitration tribunal is when:
‘… by reason of their special knowledge and skill, they are better qualified to draw inferences than the trier of fact. There are some subjects upon which the court is usually quite incapable of forming an opinion unassisted, and others upon which it could come to some sort of independent conclusion, but the help of an expert would be useful (see, Hoffman, S.A. Law of Evidence, 2nd ed., p. 78). For an example of evidence held to be admissible because it “could be of great assistance to the court” in drawing the inference it was required to draw, see R v Vilbro and Another, 1957 (3) SA 223 (AD) at p. 228 G – H. If a party intends calling a witness to give expert evidence of the kind discussed above, he is required to furnish the other party with “a summary” of such witness’s “opinions and his reasons therefor”. The meanings of the words “summary” and “opinions” are reasonably clear.’
THE ROLE OF AN EXPERT WITNESS
In litigious proceedings an expert witness will be called by one of the parties involved in such proceedings. This happens because the party requires the opinion of a skilled witness to support a particular contention, or, perhaps, a certain line of reasoning from which justifiable inferences may properly be drawn by the witness in an objective and unbiased manner. However, a cautionary note must be sounded, namely that:
‘An expert is not a hired gun who dispenses his or her expertise for the purposes of a particular case. An expert does not assume the role of an advocate, nor gives evidence which goes beyond the logic which is dictated by the scientific knowledge which that expert claims to possess.’
In the case from which the above-quoted passage is taken, i.e. Schneider NO and Others v AA and Another (Schneider), the court – per Davis J – also referred to the well-known South African textbook on the law of evidence (Zeffertt, Paizes & Skeen The South African Law of Evidence at 330), where the learned authors thereof, citing the English judgment of National Justice Compania Naviera SA v Prudential Assurance Co Ltd (The ‘Ikarian Reefer’), outlined the duties of an expert witness as follows:
- An expert witness should provide independent assistance to the court by way of objective, unbiased opinion in relation to matters within his expertise …. An expert witness should never assume the role of an advocate.
- An expert witness should state the facts or assumptions upon which his opinion is based. He should not omit to consider material facts which could detract from his concluded opinion.
- An expert witness should make it clear when a particular question or issue falls outside his expertise.
- If an expert opinion is not properly researched because he considers that insufficient data is available, then this must be stated with an indication that the opinion is no more than a provisional one. In cases where an expert witness who has prepared a report could not assert that the report contained the truth, the whole truth and nothing but the truth without some qualification, that qualification should be stated in the report.’
The duties and responsibilities of expert witnesses outlined above in Schneider, were also endorsed by the Supreme Court of Appeal (SCA) in its judgment in PricewaterhouseCoopers Incorporated and others v National Potato Co-operative Ltd and another (PricewaterhouseCoopers). In PricewaterhouseCoopers the Court – per Wallis JA (with whom Fourie and Koen AJJA concurred) – further emphasised that the abovementioned principles underlying an expert witness’s duties and responsibilities already had been echoed by the erstwhile Appellate Division some thirty-four (34) years earlier in its judgment in Stock v Stock.
THE EVALUATION OF AN EXPERT WITNESS’S EVIDENCE
In PricewaterhouseCoopers the SCA extensively quoted a passage from the judgment of Justice Marie St-Pierre in the Canadian case of Widdrington (Estate of) v Wightman because it found the approach reflected therein ‘helpful’ in dealing with the evaluation of expert witnesses. The passage in question reads as follows:
‘Legal principles and tools to assess credibility and reliability
 ‘Before any weight can be given to an expert’s opinion, the facts upon which the opinion is based must be found to exist.’
 ‘As long as there is some admissible evidence on which the expert’s testimony is based it cannot be ignored; but it follows that the more an expert relies on facts not in evidence, the weight given to his opinion will diminish’.
 An opinion based on facts not in evidence has no value for the Court.
 With respect to its probative value, the testimony of an expert is considered in the same manner as the testimony of an ordinary witness. The Court is not bound by the expert witness’s opinion.
 An expert witness’s objectivity and the credibility of his opinions may be called into question, namely, where he or she:
- accepts to perform his or her mandate in a restricted manner;
- presents a product influenced as to form or content by the exigencies of litigation;
- shows a lack of independence or a bias;
- has an interest in the outcome of the litigation, either because of a relationship with the party that retained his or her services or otherwise;
- advocates the position of the party that retained his or her services; or
- selectively examines only the evidence that supports his or her conclusions or accepts to examine only the evidence provided by the party that retained his or her services.’
The above approach to the evaluation of an expert witness’s evidence is undoubtedly helpful. Depending on the particular factual circumstances of each case where expert witnesses are called to testify, one, more or all of these legal principles and tools may find useful practical application in assessing and evaluating their evidence.
‘… the resolution of the dispute “must depend on an analysis of the cogency of the underlying reasoning which led the experts to their conflicting opinions” (Buthelezi v Ndaba  ZASCA 72; 2013 (5) SA 437 (SCA) paragraph 14).’
Leaving aside the issue of any procedural requirements for the admissibility of an expert witness’s evidence, which is not the topic addressed in this article, the principles outlined above should assist parties’ legal representatives, adjudicators and arbitrators in following the correct approach to the role of expert witnesses in litigious proceedings, as well as the considerations that are likely to inform a court or tribunal on how the evidence of such witnesses ought to be evaluated.
Eric Dunn SC
10 March 2021
 George Colman QC, Cross-Examination: A Practical Handbook, Juta & Company, Ltd, First Edition: 1970, p. 108. The late author of this book was a prominent judge of the erstwhile Supreme Court of South Africa.
 S v Gouws 1967 (4) SA 527 (E) at 528 D-F.
 Gentiruco AG v Firestone SA (Pty) Ltd 1972 (1) SA 589 (A) at 616 H in fin.
 Coopers (SA) (Pty) Ltd v Deutsche Gesellschaft für Schädlingsbekämpfung mbH 1976 (3) SA 352 (A) at 370 G in fin. See too: Ruto Flour Mills Ltd v Adelson (1) 1958 (4) SA 235 (T) at 236 G – 238 A.
 Schneider NO and Others v AA and Another 2010 (5) SA 203 (WCC) at 212 A – B.
 Schneider at 211 E – I.
  2 Lloyd’s Rep 68 at 81.
  2 All SA 403 (SCA) at para , p. 440.
 1981 (3) SA 1280 (A) at 1296 F – G, where the Court (per Diemont JA, with whom Jansen JA and Cillié JA concurred) stated that: ‘An expert … who is asked to testify in a case … must be made to understand that he is there to assist the Court. If he is to be helpful he must be neutral. The evidence of such a witness is of little value where he, or she, is partisan and consistently asserts the cause of the party who calls him. I may add that when it comes to assessing the credibility of such a witness, this Court can test his reasoning and is accordingly to that extent in as good a position as the trial Court was.’ (Emphasis added). See too: Jacobs v Transnet Ltd t/a Metrorail 2015 (1) SA 139 (SCA) at para , p. 148 B – D.
 2011 QCCS 1788 (Can LII).
 PricewaterhouseCoopers at para , p. 441.
  3 All SA 358 (SCA). See too the analysis and summary of this judgment by Alastair Hay in this issue of the e-periodical’s section A case in point: Recent cases.
 Ibid., at para , p. 371.