What is an Expert Witness?

The follow is an extract from Getting Started as an Expert Witness, published by the UK Register of Expert Witnesses and is reproduced here with the permission of the authors.

Let us start by defining the terms ‘expert’ and ‘expert witness’.


An expert can be anyone with knowledge of or experience in 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 (or other judicial or quasi-judicial bodies, e.g. tribunals, arbitrations, official enquiries, etc.) to help it understand the issues in a case and thereby reach a sound and just decision

Moreover, an expert witness is paid for the time it takes to:

•form an opinion and, where necessary,
•support that opinion during the course of litigation.
An expert witness is not paid for the opinion given, and still less for the assistance that opinion affords the client’s case.
What is expert evidence?
The fundamental characteristic of expert evidence is that it is opinion evidence. Generally speaking, lay witnesses may give only one form of evidence, namely evidence of fact. They may not say, for example, that a vehicle was being driven recklessly, only that it ended up in the ditch. In this example:
•it is the function of the court (whether magistrate, judge or jury) to decide the cause of the accident based on the evidence placed before it, and
•it is the task of the expert witness (an accident investigator, say) to assist the court in reaching its decision with technical analysis and opinion inferred from factual evidence of, for example, skid marks.
To be practically of assistance to a court, however, expert evidence must also provide as much detail as is necessary to allow the court to determine whether the expert’s opinions are well founded. It follows, then, that it will often include:
•factual evidence supplied in the expert’s instructions which requires expertise in its interpretation and presentation
•other factual evidence which, while it may not require expertise for its comprehension, is linked inextricably to evidence that does
•explanations of technical terms or topics
•hearsay evidence of a specialist nature, e.g. as to the consensus of medical opinion on the causation of particular symptoms or conditions, as well as
•opinions based on facts adduced in the case.
When is expert evidence needed?
Expert evidence is most obviously needed when the evaluation of the issues requires technical or scientific knowledge only an expert in the field is likely to possess. However, there is nothing to prevent reports for court use being commissioned on any factual matter, technical or otherwise, providing:
•it is deemed likely to be outside the knowledge and experience of those trying the case, and
•the court agrees to the evidence being called.
Roles of the expert
The expert might take on a number of roles.
•When advising a party, but there is no intention of putting the expert’s opinions before the court, the expert is known as an expert advisor.
•If the expert advisor is working ‘behind the scenes’ in a claim before the courts, the term shadow expert is often used instead.
•An expert instructed by just one party in a claim, and whose opinion is to be put before the court, is an expert witness proper, or party-appointed expert.
•If the expert witness is instructed by all the parties in a claim, then the single joint expert epithet applies.
•In a complex claim, involving many experts, one expert witness may be appointed the lead expert.
•Finally, the Civil Procedure Rules (CPR) also introduced the role of a truly court-appointed expert. Known as an assessor, this type of expert appointment is seldom seen in practice.





Extract from Getting Started as an Expert Witness, number three in the UK Register of Expert Witnesses Little Book Series (ISBN 1-905926-04-6, 2008)

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