expert witness

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An expert witness is a person with extensive experience or knowledge in a specific field or discipline beyond that expected from a layperson. The expert witness’s duty is to apply their expertise to give a professional opinion to the tribunal or court on particular matters in dispute. 

There are two types of witness in the US legal system: a lay witness and expert witness. Unlike a lay witness, who cannot give an opinion about the topics that require special knowledge, an expert witness can testify their opinion within their expertise. An expert opinion must be based on sufficient facts or data and reliable principles or methods.

The major function of an expert witness is to express their independent expert opinion based on the information provided. An expert can be employed in different capacities at arbitrations, tribunals, and litigation

Different jurisdictions have different requirements for an expert witness, but there are some general guidelines regarding the expert testimony definition.  In federal courts, expert witness testimony is governed by Article VII of the Federal Rules of Evidence. Federal judges determine the credibility of expert witnesses in a pre-trial Daubert hearing. 

In considering witnesses' qualifications, judges may consider information that is not admissible as evidence. Before trial, all experts must prepare a report summarizing their analysis and conclusions, and share the report with all other parties. See: disclosure and Rule 26(a) of the Federal Rules of Civil Procedure. This allows other parties to effectively cross-examine the expert.

Expert testimony is not limited to matters beyond the understanding of the ordinary juror. Instead, experts may testify on any subject within their area of expertise so long as their testimony will assist the jury. See Rule 702.

  • For example: a digital forensics expert would be allowed to offer opinions on computers or digital evidence. An expert witness in a digital forensics investigation could assist the court by analyzing deleted computer files, consulting on how to preserve or retrieve data, and determining how a computer was used. A person with appropriate skills and knowledge might be called on to explain digital subject matter so that non-experts can understand. 
  • Another example: in a shooting case, people who saw or heard the shooting may have relevant information to testify how many shots were fired, but they cannot testify about the path of the bullet because answering this type of question requires special knowledge, education, or training. Only an expert who has qualifications would be allowed to answer this question and give their opinion.

See also: State Civil Procedure Rules

[Last updated in August of 2022 by the Wex Definitions Team]