A Scottish personal injury case with some useful insight into the need for and use of expert evidence.
Lord Reed and Lord Hodge said,
The evidence of skilled witnesses
38. In our view four matters fall to be addressed in the use of expert evidence. They are
(i) the admissibility of such evidence,
(ii) the responsibility of a party’s legal team to make sure that the expert keeps to his or her role of giving the court useful information,
(iii) the court’s policing of the performance of the expert’s duties, and
(iv) economy in litigation.
39. Skilled witnesses, unlike other witnesses, can give evidence of their opinions to assist the court. This gives rise to threshold questions of the admissibility of expert evidence. An example of opinion evidence is whether Miss Kennedy would have been less likely to fall if she had been wearing anti-slip attachments on her footwear.
40. Experts can and often do give evidence of fact as well as opinion evidence. A skilled witness, like any non-expert witness, can give evidence of what he or she has observed if it is relevant to a fact in issue. An example of such evidence in this case is (the expert’s) evidence of the slope of the pavement on which (the Claimant) lost her footing. There are no special rules governing the admissibility of such factual evidence from a skilled witness.
41. Unlike other witnesses, a skilled witness may also give evidence based on his or her knowledge and experience of a subject matter, drawing on the work of others, such as the findings of published research or the pooled knowledge of a team of people with whom he or she works. Such evidence also gives rise to threshold questions of admissibility, and the special rules that govern the admissibility of expert opinion evidence also cover such expert evidence of fact. There are many examples of skilled witnesses giving evidence of fact of that nature… when an engineer describes how a machine is configured and works or how a motorway is built, he is giving skilled evidence of factual matters, in which he or she draws on knowledge that is not derived solely from personal observation or its equivalent. An expert in the social and political conditions in a foreign country who gives evidence to an immigration judge also gives skilled evidence of fact.
43. Counsel agreed that the South Australian case of R v Bonython (1984) 38 SASR 45 gave relevant guidance on admissibility of expert opinion evidence. [The citation from Bonython is the subject of an earlier entry]
44. In Bonython the court was addressing opinion evidence. As we have said, a skilled person can give expert factual evidence either by itself or in combination with opinion evidence. There are in our view four considerations which govern the admissibility of skilled evidence:
(i) whether the proposed skilled evidence will assist the court in its task;
(ii) whether the witness has the necessary knowledge and experience;
(iii) whether the witness is impartial in his or her presentation and assessment of the evidence; and
(iv) whether there is a reliable body of knowledge or experience to underpin the expert’s evidence.
All four considerations apply to opinion evidence, although, as we state below, when the first consideration is applied to opinion evidence the threshold is the necessity of such evidence. The four considerations also apply to skilled evidence of fact, where the skilled witness draws on the knowledge and experience of others rather than or in addition to personal observation or its equivalent. We examine each consideration in turn.
45. Assisting the court:
It is for the court to decide whether expert evidence is needed, when the admissibility of that evidence is challenged. In R v Turner  QB 834 (CA), Lawton LJ stated at p 841:
“If on the proven facts a judge or jury can form their own conclusions without help, then the opinion of an expert is unnecessary.”
46. Most of the Scottish case law on, and academic discussion of, expert evidence has focused on opinion evidence to the exclusion of. In our view, the test for the admissibility of )skilled evidence of fact) cannot be strict necessity as, otherwise, the court could be deprived of the benefit of a skilled witness who collates and presents to the court in an efficient manner the knowledge of others in his or her field of expertise. There may be circumstances in which a court could determine a fact in issue without an expert collation of relevant facts if the parties called many factual witnesses at great expense and thus a strict necessity test would not be met.
In Daubert v Merrell Dow Pharmaceuticals Inc (1993) 509 US 579, the United States Supreme Court referred to rule 702 of the Federal Rules of Evidence, which in our view is consistent with the approach of Scots law in relation to skilled evidence of fact. The rule, which Justice Blackmun quoted at p 588, states:
“If scientific, technical or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.”
47. The advantage of the formula in this rule is that it avoids an over-rigid interpretation of necessity, where a skilled witness is put forward to present relevant factual evidence in an efficient manner rather than to give an opinion explaining the factual evidence of others. If skilled evidence of fact would be likely to assist the efficient determination of the case, the judge should admit it.
48. An expert must explain the basis of his or her evidence when it is not personal observation or sensation; mere assertion or “bare ipse dixit” carries little weight, as the Lord President (Cooper) famously stated in Davie v Magistrates of Edinburgh 1953 SC 34.
If anything, the suggestion that an unsubstantiated ipse dixit carries little weight is understated; in our view such evidence is worthless. Wessels JA stated the matter well in the Supreme Court of South Africa (Appellate Division) in Coopers (South Africa) (Pty) Ltd v Deutsche Gesellschaft für Schädlingsbekämpfung mbH 1976 (3) SA 352, 371:
“[A]n expert’s opinion represents his reasoned conclusion based on certain facts or data, which are either common cause, or established by his own evidence or that of some other competent witness. Except possibly where it is not controverted, an expert’s bald statement of his opinion is not of any real assistance. Proper evaluation of the opinion can only be undertaken if the process of reasoning which led to the conclusion, including the premises from which the reasoning proceeds, are disclosed by the expert.”
As Lord Prosser pithily stated in Dingley v Chief Constable, Strathclyde Police 1998 SC 548, 604: “As with judicial or other opinions, what carries weight is the reasoning, not the conclusion.”
49. In Davie the Lord President at p 40 observed that expert witnesses cannot usurp the functions of the jury or judge sitting as a jury. Recently, in Pora v The Queen  UKPC 9;  1 Cr App R 3, para 24, the Judicial Committee of the Privy Council in an appeal from New Zealand, stated:
“It is the duty of an expert witness to provide material on which a court can form its own conclusions on relevant issues. On occasions that may involve the witness expressing an opinion about whether, for instance, an individual suffered from a particular condition or vulnerability. The expert witness should be careful to recognise, however, the need to avoid supplanting the court’s role as the ultimate decision-maker on matters that are central to the outcome of the case.”
Thus, while on occasion in order to avoid elusive language the skilled witness may have to express his or her views in a way that addresses the ultimate issue before the court, expert assistance does not extend to supplanting the court as the decision-maker. The fact-finding judge cannot delegate the decision-making role to the expert.
50. The witness’s knowledge and expertise:
The skilled witness must demonstrate to the court that he or she has relevant knowledge and experience to give either factual evidence, which is not based exclusively on personal observation or sensation, or opinion evidence. Where the skilled witness establishes such knowledge and experience, he or she can draw on the general body of knowledge and understanding of the relevant expertise…
51. Impartiality and other duties:
If a party proffers an expert report which on its face does not comply with the recognised duties of a skilled witness to be independent and impartial, the court may exclude the evidence as inadmissible: Toth v Jarman  EWCA Civ 1028… paras 100-102.
In Field v Leeds City Council  1 EGLR 54, the Court of Appeal upheld the decision of a district judge, who, having ordered the Council to provide an independent surveyor’s report, excluded at an interim hearing the evidence of a surveyor whom the Council proposed to lead in evidence on the ground that his impartiality had not been demonstrated.
… But the requirement of independence and impartiality is in our view one of admissibility rather than merely the weight of the evidence.
52. The Scottish courts have adopted the guidance of Cresswell J on an expert’s duties in The Ikarian Reefer  2 Lloyd’s Rep 68 in both civil and criminal matters…[This quotation is dealt with in the earlier entry on the Ikarian reefer]
54. Reliable body of knowledge or experience:
What amounts to a reliable body of knowledge or experience depends on the subject matter of the proposed skilled evidence. In Davie v Magistrates of Edinburgh the question for the court was whether blasting operations in the construction of a sewer had damaged the pursuer’s building and the relevant expertise included civil engineering and mining engineering. In Myers, Brangman and Cox, as we have said, the subject matter was the activities of criminal gangs; a policeman’s evidence, which was the product of training courses and long term personal experience as an officer serving with a body of officers who had built up a body of learning, was admitted as factual evidence of the practices of such gangs.
55. In many cases where the subject matter of the proposed expert evidence is within a recognised scientific discipline, it will be easy for the court to be satisfied about the reliability of the relevant body of knowledge. There is more difficulty where the science or body of knowledge is not widely recognised… Lord Eassie’s opinion in Mearns v Smedvig Ltd 1999 SC 243 [was]…:
“A party seeking to lead a witness with purported knowledge or experience outwith generally recognised fields would need to set up by investigation and evidence not only the qualifications and expertise of the individual skilled witness, but the methodology and validity of that field of knowledge or science.”
56. We agree with that proposition, which is supported in Scotland and in other jurisdictions by the court’s refusal to accept the evidence of an expert whose methodology is not based on any established body of knowledge.
Thus in Young v Her Majesty’s Advocate 2014 SLT 21, the High Court refused to admit evidence of “case linkage analysis” because it was the subject of only relatively recent academic research and a methodology which was not yet sufficiently developed that it could be treated as reliable.
See also, for example, R v Gilfoyle  2 Cr App R 5, in which the English Court of Appeal (Criminal Division) refused to admit expert evidence on “psychological autopsy” for several reasons, including that the expert had not embarked on the exercise in question before and also that there were no criteria by reference to which the court could test the quality of his opinions and no substantial body of academic writing approving his methodology. The court also observed that the psychologist’s views were based on one-sided information and doubted that the assessment of levels of happiness or unhappiness was a task for an expert rather than jurors.
(ii) Making sure that the expert performs his or her role
57. It falls in the first instance to counsel and solicitors who propose to adduce the evidence of a skilled witness to assess whether the proposed witness has the necessary expertise and whether his or her evidence is otherwise admissible. It is also their role to make sure that the proposed witness is aware of the duties imposed on an expert witness. The legal team also should disclose to the expert all of the relevant factual material which they intend should contribute to the expert’s evidence in addition to his or her own pre-existing knowledge. That should include not only material which supports their client’s case but also material, of which they are aware, that points in the other direction, viz the court’s concerns about one-sided information in R v Gilfoyle. The skilled witness should take into account and disclose in the written report the relevant factual evidence so provided.
(iii) Policing the performance of an expert’s duties
58. … Where the court has significant powers of case management…a judge can address concerns about the evidence in the report by a skilled witness at a case management hearing and discuss with counsel how they are to be resolved…
59. [There follows discussion of proof in Scottish cases which may be of interest in a case management context in England and Wales]… the judge should, when assessing whether and to what extent to give weight to the evidence, test the evidence to ascertain that it complies with the four considerations which we have set out in para 38 above and is otherwise sound…
(iv) Economy in litigation
[Here the judges set out the practical application of the existing practice in England and Wales.]
The citations from the case have been edited for ease of reading. See other cases on expert evidence under this tag.