• With the courts permission they may remain in court prior to giving evidence, subject to argument about disputed factual evidence they may give, in order to hear evidence of both fact and circumstance.


• They may express an opinion on matters within their field of expertise.


• They may refer to the works of others within their field of expertise without infringing the rules against hearsay.

An expert witness may give opinions to assist in resolving issues concerning matters of knowledge which can only be acquired by special training or experience. His/her role is to assist the court or jury on matters where their ordinary, everyday experience does not enable them to adequately consider the issues in the case.

The facts on which the expert’s opinions are based must be either admitted or proved by admissible evidence. Usually, such facts are established by other witnesses but they may include facts observed by the expert him/herself.

An expert’s opinion must be objective and unbiased. Although a defendant will be entitled to call their own expert evidence, it is the duty of an expert witness instructed by the prosecution (and indeed an expert witness called by any defendant) in a criminal case to act in the cause of justice. An expert’s duty to the court overrides any obligation to the party instructing him/her and s/he should maintain professional objectivity and impartiality at all times whilst complying with the rules of evidence.

The responsibilities in relation to expert evidence are shared between the expert and the prosecutor, but it is the prosecutor who has overall responsibility for the case; the expert is expected to assist with that task.

The duties and responsibilities of expert witnesses in criminal cases have been summarised and include the following:

• Expert evidence presented to the court should be (and be seen to be) the independent product of the expert and should not be influenced as to form or content by the demands of litigation;


• An expert witness should provide independent assistance to the court in relation to matters within his or her expertise. An expert witness should never assume the role of an advocate;


• An expert witness should state the facts or assumptions upon which his/her opinion is based. S/he should consider all material facts in drawing conclusions and should not omit to consider those that detract from his/her conclusions;


• An expert witness should make it clear when a particular question or issue falls outside his/her expertise. The opinion of an expert is only admissible if s/he is competent (i.e. qualified), through study, training or experience, in a field relevant to the case;


• If an expert considers that insufficient data is available to allow him/her to come to a final conclusion, this must be stated with an indication that the opinion is a provisional one;


• Experts who are civil servants must also carry out their duties in accordance with the Civil Service Code and its core values of integrity, honesty, objectivity and impartiality.


The Code reinforces the standards expected of all experts by the courts.