The Expert Witness

Evidence of Opinion

As a general rule, under UK law witnesses may not express opinions when giving evidence; they are limited to giving their personal perceptions of the facts without drawing inferences from them. The courts sometimes find that they require more assistance than is available to them under such restrictive rules of evidence. Where the issues being tried require an understanding of specialist matters of which the judges do not have knowledge, they may permit the introduction of evidence of opinion by someone who is expert in the specialisms concerned:

… if matters arise in our law which concern other sciences or facilities, we may commonly apply for the aid of that science or faculty which it concerns. Which is an honourable and commendable thing in our law. For thereby it appears that we don’t dispute other sciences but our own, but we approve of them and encourage them as things worthy of commendation..’ [1]

Defining the Role of an Expert Witness

The role of the expert witness developed to serve the needs of the courts and is defined accordingly. In the 18th century, the eminent engineer, Mr Smeaton, was called to give his opinion to the court as he; ‘understands the construction of harbours, the causes of their destruction and how remedied’.[2]

More recently the courts set out the duties of an expert witness as follows:

‘Their duty is to furnish the judge or jury with the necessary scientific criteria for testing the accuracy of their conclusions, so as to enable the judge or jury to form their own independent judgement of the application of these criteria to the facts provided in evidence’.[3]

The role of the expert in UK law is now addressed in the civil procedure rules.

In civil proceedings an expert may give evidence on any issue in a proceeding.[4] The experts first give their evidence-in-chief under questioning from their client’s advocate.[5] They may then be cross-examined by the opposing side’s advocate and thereafter re-examined by their own client’s advocate. In practice, in building disputes, experts’ reports are normally accepted as their evidence-in-chief. This places constraints on the form and content of such reports. In particular, an expert’s report must be a personal statement written in the first person singular. Where reference is made to information which experts have by report, unverified by their own observations, they should make this clear. To the extent that such references are to hearsay evidence, they may be inadmissible.