Expert witness

An expert can be anyone with knowledge or experience of a particular field or discipline beyond that to be expected of a layman. An expert witness is an expert who makes this knowledge and experience available to a court to help it understand the issues of a case and thereby reach a sound and just decision. The duty of an expert witness is to give or prepare evidence for the purpose of court proceedings.

Expert evidence is sought most obviously in disputes requiring detailed scientific or technical knowledge. Expert evidence must provide as much detail as it is necessary to convince the judge that the expert’s opinions are well founded. It will often include:

• factual evidence obtained by the witness which requires expertise in its interpretation and presentation;
• explanations of technical terms or topics;
• hearsay evidence of a specialist nature;
• opinions based on facts adduced in the case.

The primary ethical duty of an expert witness is to be truthful to the court, thorough in technical reasoning, honest as to opinion and complete in coverage of relevant matters. This applies to written reports as much as oral evidence, and regardless of whether the witness is on oath.

At the same time, when accepting instructions the expert assumes a responsibility to the client to exercise due care with regard to the investigations he carries out and to provide opinion evidence that is soundly based. This necessitates that the expert undertakes only those tasks he is competent to carry out and gives only those opinions he is competent to provide.

To fulfill these duties adequately it is, of course, vital that the expert should also have kept up to date with current thinking and developments in his field.

Expert evidence should be independent, objective and unbiased. In particular, an expert witness must not be biased towards the party responsible for paying his fee. The evidence should be the same whoever is paying for it.

Clearly, too, an expert witness should have:

• a sound knowledge of the subject matter in dispute, and, usually, practical experience of it;
• the powers of analytical reasoning required to fulfill his assignment;
• the ability to communicate findings and opinions clearly, concisely and in terms adapted to the tribunal before which evidence is being given;
• the flexibility of mind to modify opinions in the light of fresh evidence or counter-arguments.