As an attorney who trains expert witnesses I am often asked to explain or define what is an expert witness. The short answer is that an expert witness is a special kind of witness in a court case who is allowed to testify as to their opinions. In some states expert witnesses are actually called “opinion witnesses.”
An expert witness should be contrasted with fact witnesses.
Fact witnesses are only able to testify to what they have sensed. That is, what they saw, heard, smelled, tasted, or felt. For example, an eyewitness to a crime or a witness who testified to what one of the parties to the case told them. Fact witnesses need to have firsthand knowledge of what happened, expert witnesses do not. Expert witnesses typically form their opinions from information such as medical records, photographs, site inspections, lab reports, witness statements, etc.
Common examples where expert witnesses can testify include:
- a medical doctor offering an opinion on whether or not another doctor committed medical malpractice,
- an engineer opining on the whether or not a particular product was defective,
- an accountant offering an opinion on how much a party was damaged after a deal went bad, or
- an accident reconstructionist opining on how fast a vehicle was travelling right before an accident.
Expert witness testimony is governed by the requirements of Federal Rule of Evidence 702.
This rule states that not everyone can give opinions in court. To be an expert and give opinions you would need to be qualified to do so by your “knowledge, skill, experience, training, or education.” Rule 702 further requires that the expert base their opinions on sufficient facts and use a reliable methodology.
So if anyone asks you “What is an expert witness?”
You can tell them the correct answer: A witness who is allowed to testify in court by way of an opinion.