Expert Witness
A witness qualified by knowledge, skill, experience, training, or education to give opinion testimony on specialized matters.
The MBE rarely asks “is this person an expert?” — it tests the basis of the opinion and whether the judge let unreliable material reach the jury. The classic stem hands you an expert who relied on hearsay (another doctor’s report, a lab printout): the answer turns on Rule 703 — the expert may rely on inadmissible facts if experts in the field reasonably rely on that kind of data, but those underlying facts reach the jury only if their probative value in evaluating the opinion substantially outweighs prejudice. The “tell” for a wrong answer excludes the opinion merely because the data was hearsay.
The trap is conflating the lay and expert lanes: a lay opinion must be rationally based on personal perception and never on specialized knowledge — that belongs to an expert. Remember the criminal carve-out under 704(b): an expert states no opinion on whether this defendant had the requisite mental state, yet may describe the condition or what most people think and leave the inference to the jury (Diaz v. United States, 2024).
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