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Difference Between Lay Witness and Expert Witness

Woman doctor with stethoscope looking at medical papers at her office working hard

Lay witnesses and expert witnesses serve different functions in litigation, and the formalities surrounding the admission of each form of testimony differ as a result. Read on for a discussion of the primary differences between lay witness and expert witness testimony. If you need expert testimony, consultation, or advice in your litigation, call a qualified medical expert for assistance.

Experts can offer opinions. The primary difference between lay witnesses and experts is that experts can offer informed opinions about matters. They can review other evidence in the case and draw a conclusion about what that evidence suggests (e.g., “These brain scans indicate the presence of cancer” or “These tire marks suggest that the driver was traveling at a high rate of speed.”). They are not limited to discussing their personal involvement in the matter, although their personal experience can form part of the basis for their opinion.

Lay witnesses are limited to what they see, smell, hear, taste, or touch. Lay witnesses can only testify about what they personally perceived or personally know. They cannot look at other evidence in the case and offer opinions; they can only speak about matters they personally experienced. They must generally avoid relying on hearsay (e.g., “I heard someone say …”) and must generally avoid conjecturing or speculating about matters they did not personally witness.

Lay witness testimony is generally admissible if it is relevant. Lay witness testimony has a low barrier to admissibility. The question is simply: Did the witness perceive something relevant to the case? Within limits, the party seeking to call a lay witness does not need to establish the reliability or qualifications of the witness, although those factors may go to the witness’s credibility and the weight the jury assigns to their testimony.

Experts must be qualified and their opinions admissible. Experts, on the other hand, must be specially qualified to offer their opinion. They must possess sufficient training, experience, and education to be established as an expert in the relevant field. Moreover, their testimony must be based on proper scientific principles and reliable data. Pursuant to Federal Rule of Evidence 702, to admit the testimony of an expert:

  • The expert’s scientific or other specialized knowledge must be helpful to the jury in understanding the case;
  • The testimony must be based on sufficient facts or data;
  • The testimony must be based on reliable methodology; and
  • The expert must have properly applied good methodology to the facts of the case.

Witness and testimony disclosure. The rules of evidence in federal court and in each state generally set higher disclosure requirements for expert testimony. Experts must be identified in advance by each party, and the party must disclose a report previewing the expert’s proposed testimony, with sufficient detail about all the opinions the expert intends to present and the support for those opinions.

In contrast, while the identity of lay witnesses must generally be identified at some point prior to trial, parties typically have no burden to disclose in advance what that witness will say on the stand. Parties are responsible for deposing opposing witnesses to elicit their testimony in advance in order to prepare for trial.
If you need a qualified and effective expert witness in a personal injury, medical malpractice, or product liability case, contact the offices of Neurosurgery Medlegal Services, LLC, at 866-659-8051.

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