The two categories of expert evidence and the importance of reasoning

It is useful to keep well in mind the 2 roles that an expert may have as a court expert and the 2 separate, but often overlapping, reasons why the expert may have been instructed.  The recent Supreme Court decision in Kennedy v Cordia (Services) LLP [2016] UKSC 6 was concerned with the evidence of an engineer in relation to a slipping case and although dealing with an appeal from Scotland, the Court’s observations are equally applicable to England and Wales.

Like any other witness the expert may be an expert of fact giving evidence of what he or she observed, so long as it is relevant to a fact in issue in the case.  So a medical expert may give evidence of an examination of the claimant, or the appearance of the claimant on arriving or leaving the consulting room. An engineer may provide measurements of a machine or location.

As a skilled witness, an expert ‘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.’  A medical expert may therefore, for example, give evidence as to how an organ functions and the potential impact on the organ of disease.  An engineer may describe how a machine is configured or works.  This is not opinion evidence, but for such evidence to be admissible from the witness, the same rules that govern admissibility of expert opinion evidence also apply.  Therefore to be permitted to give this evidence, ‘the skilled witness must set out his qualifications, by training and experience, to give expert evidence and also say from where he has obtained information if it is not based on his own observations and experience.’

This expert factual evidence may be given by itself or in combination with opinion evidence.  Opinion evidence, in contrast to expert factual evidence, will address, for example in medical expert evidence, prognosis, causation or standard of care, and in engineering evidence measures which might reasonably have been taken to avoid an accident.

Whether giving skilled evidence of fact or expert opinion the admissibility of the skilled evidence is governed by the same four considerations:

  1. Will the evidence assist the court in its task?
  2. Does the witness have the necessary knowledge and experience?
  3. Is the witness impartial in the presentation and assessment of the evidence?
  4. Is there a reliable body of knowledge or experience to underpin the expert’s evidence?

What is essential is that the expert explains the basis of his or her evidence when it is not personal observation or sensation.  ‘Mere assertion carries little weight.’  Quoting from a South African case the Supreme Court stated:

‘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.’

And quoting from a Scottish case, the Supreme Court stated:

‘As with judicial or other opinions, what carries weight is the reasoning, not the conclusion.’

The expert in writing a court report should bear these points well in mind.  When giving evidence of factual matters known only as a consequence of having the particular skills for which reason the expert has been instructed, and even more so when giving an opinion, in addition to providing the appropriate expert credentials to justify the status as the appropriate expert, the expert must provide clear statements as to the reasoning process which led to each conclusion reached.  It is the reasoning process as much as, and often more than, the conclusion which carries weight with the judge.

Kennedy v Cordia also provides very useful guidance as to those matters which it is appropriate for a health and safety expert to address in an expert report and the line between what it is appropriate for the expert to address in opinion evidence and for the judge to resolve.

Finally the Supreme Court reminds lawyers instructing experts that it is their duty to assess whether the skilled witness has the necessary expertise and to ensure that the skilled witness is aware of all of the duties imposed on an expert witness.

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