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What is an expert witness

The role of an expert witness
© Image: Michigan Municipal League (2010). Retrieved from

The role of the expert witness

An expert witness may be anyone who holds specialist knowledge or experience in a field or discipline considered to be beyond the ken of a typical layman. In contrast to non-expert witnesses, who are allowed to testify only to facts, expert witnesses are allowed to testify to their opinions within their specified realm of expertise, but not beyond. Either party in an adversarial legal system may employ expert advisors to assist them with technical matters, data, preparation of strategy and examination, but this is not an expert witness. An expert witness’s overriding duty is to the court – whose duty is the provision of a fair trial – rather than to either the prosecution or the defence, no matter which side called the witness or is paying them for their time.

Michigan Municipal League 2010

The court must give permission for an expert witness to be called to testify in a matter before them. Motions to exclude expert witness testimony may be filed by either party in a case if there is a reasonable basis for the assertion that the expert witness is not qualified or cannot offer unbiased and objective opinions. Criminal trials are not required to contain any type of forensic evidence or expert witness testimony and more often than not, will proceed with exclusively fact witnesses.

Opinions given by an expert witness may be based on prior knowledge, facts in evidence, testing results or reasoning. Uncertainty (whether the opinion given is accurate) is separate from an assessment of evidential value (the significance of a given opinion in context), both of which should be communicated by the expert witness if known. There is often an unavoidable level of uncertainty in opinion evidence and this must be explained and quantified to the extent possible by the expert witness, either in the report or testimony.

Evidential value, or weight of evidence, refers to how important to the questions at issue the conclusion of the expert is; for example the meaning of a ‘match’ is different if the analysis is comparing shoe prints rather than DNA, or if a defendant’s fingerprint is found to match a fingerprint from a potential murder weapon and that weapon is a kitchen knife from the defendant’s home where they cook regularly rather than a gun they claim to have never seen. Often, the weight of the link provided by expert evidence cannot be quantified and so to ascribe a specific value to it can be misleading.

Expert witness statement and testimony

An expert witness is usually expected to produce a report or statement prior to the trial containing the results of their analysis. Such a statement must include details of the expert witness’ qualifications, experience and certification/accreditation if applicable, as well as their proffered scope of expertise. The basis for any opinions or conclusions drawn must be stated, such as published literature, recovered data or case information provided by either party or the police. If colleagues or other similar experts are expected to or have offered differing conclusions from the same evidence, this should be included in the statement and an explanation of the reasons for the expert witness’s own opinion articulated. The report will be disclosed to both prosecution and defence in a criminal trial.

An expert witness must be able to communicate complex ideas to a lay audience in a succinct manner. Presentation skills are important for an expert witness as they need to explain their scientific or technical discipline in brief to the fact finder and then articulate their conclusions and the bases for them without losing the attention of non-experts. The expert witness may opine on the evidence and their analysis in response to questions asked by the attorneys or the court, and should put such answers in appropriate context, but they may not offer unsolicited opinions or advocate for either party.

© Image: Michigan Municipal League (2010). Retrieved from
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