In a 1993 shipping case known as The Ikarian Reefer[i], Mr Justice Cresswell set out what is regarded as the classic statement of the duties and responsibilities of expert witnesses. These principles have been endorsed by the Courts in other cases and are largely reflected in the Civil Procedure Rules 1998.
These are the principles.
1. Expert evidence presented to the Court should be, and should be seen to be, the independent product of the expert uninfluenced as to form or content by the exigencies of litigation.
The principle that expert reports should be independent stands and the Civil Procedure Rules (CPR) emphasise that experts have a duty to help the Court on matters within their expertise. That duty overrides any obligation to the person from whom experts have received instructions or by whom they are paid (CPR 35.3).
This may be a little pedantic but I do not see how it can really be said that expert reports should be influenced in form and content by the exigencies of litigation. CPR 35.1 says that expert evidence shall be restricted to what is reasonably required to resolve the proceedings. So the scope of an expert's report will be limited in line with the Court's directions and needs to address issues within an expert's expertise. The CPR also sets out at Part 35 some formal requirements for reports.
This principle has probably been modified by Practice Direction 35.2.1, which says: "Expert evidence should be the independent product of the expert uninfluenced by the pressures of litigation." That puts it rather better.
2. An expert witness should provide independent assistance to the Court by way of objective, unbiased opinion in relation to matters within their expertise. An expert witness in the High Court should never assume the role of an advocate.
This principle is again reflected in the CPR 35.3. PD 35.2.2 says: "Experts should assist the courts by providing objective, unbiased opinions on matters within their expertise, and should not assume the role of an advocate."
One test of the independence of an expert's evidence is whether the expert would give the same opinion if instructed by the other party.
3. An expert witness should state the facts or assumptions upon which his/her opinion is based. He/she should not omit to consider material facts which could detract from his/her concluded opinion.
PD 35.2.3 says: "Experts should consider all material facts, including those which may detract from their opinions."
4. An expert witness should make it clear when a particular question or issue falls outside his expertise.
R v Pabon[ii] is a recent case where an expert failed to do this. In a criminal case about rigging LIBOR (inter-bank lending rates), the expert gave evidence on short-term interest rates, a matter outside his expertise. The judge described him as no more than an "enthusiastic amateur" and criticised his lack of independence as corrosive to justice.
5. If an expert's opinion is not properly researched because insufficient data are available, then this must be stated with an indication that the opinion is no more than provisional.
In cases where an expert witness who has prepared a report could not assert that the report contained the truth, the whole truth and nothing but the truth without some qualification, that qualification should be stated in the report.
6. If, after the exchange of reports, an expert witness changes his view on a material matter having read the other side's expert's report or for any other reason, such a change of view should be communicated (through legal representatives) to the other side without delay and when appropriate to the Court.
I have never encountered this principle applied in practice and it does not appear in the Civil Procedure Rules. No doubt an expert who changes his or her mind should revise their report before disclosing or putting it before the trial judge. But experts' reports, like other witness evidence, are privileged, and I am doubtful that experts (or solicitors) have a duty to communicate an expert's change of mind when it happens. If there is still such a duty, I suspect it is widely disregarded.
7. Where expert evidence refers to photographs, plans, calculations, analyses, measurements, survey reports or other similar documents, these must be provided to the opposite party at the same time as the exchange of reports.
**Read Paul Sankey's blog, 'How to be a better medical expert witness: 10 top tips'.***
[i] The Ikarian Reefer  2 Lloyd's Rep. 68
[ii] R v Pabon  EWCA Crim 420