When Do You Need an Expert Witness in Your Case?

When a disputed question can be answered only with specialized knowledge that jurors do not bring from everyday life. The advisory committee's note to the federal rule on expert testimony states the test in a sentence it quotes from the evidence literature: "There is no more certain test for determining when experts may be used than the common sense inquiry whether the untrained layman would be qualified to determine intelligently and to the best possible degree the particular issue without enlightenment from those having a specialized understanding of the subject involved in the dispute." The federal rule on lay opinion testimony draws the same line from the other side: a witness who is not testifying as an expert may give an opinion only if it is rationally based on what the witness perceived and is not based on scientific, technical or other specialized knowledge. Its committee note gives an example: a lay witness with experience could testify that a substance appeared to be blood, but a witness would have to qualify as an expert to testify that bruising around the eyes is indicative of skull trauma.

In injury, malpractice and employment cases the usual signals are these:

• earning capacity or employability is contested, not only the wages already lost;
• future care is claimed, as after a catastrophic injury with long-term care needs, and what it includes or costs is disputed;
• medical causation or the prognosis is at issue;
• the standard of care is disputed;
• the other side has disclosed an expert, whose report calls for review and, where its opinion is contested, a rebuttal expert in the same discipline; the plaintiff versus defense expert comparison sets the two roles side by side.

A past wage loss that the pay records document may need only arithmetic. The expert witness testimony guide explains how an expert's role differs from a fact witness's.

In federal court, a witness qualified as an expert by knowledge, skill, experience, training or education may give an opinion only if the party offering it shows the judge that it is more likely than not that:

• the expert's specialized knowledge will help the trier of fact (the judge or jury deciding the facts) understand the evidence or determine a fact in issue;
• the testimony is based on sufficient facts or data;
• the testimony is the product of reliable principles and methods; and
• the opinion reflects a reliable application of those principles and methods to the facts of the case.

That is the federal rule on expert testimony as amended in 2023. Its advisory committee's note says the amendment was made to clarify and emphasize that the party offering the opinion carries that burden, and to emphasize that each opinion must stay within what can be concluded from a reliable application of the expert's basis and methodology. The judge decides these questions before the jury hears the opinion: the Supreme Court gave trial judges that gatekeeping role (Daubert v. Merrell Dow Pharmaceuticals, Inc., 1993) and made clear that it applies to all expert testimony, not only to science (Kumho Tire Co. v. Carmichael, 1999). State courts apply their own admissibility standards, which the federal and state admissibility guide compares, and attorneys confirm the governing rule for their case.

The side that has to prove the point may be left without proof of it. Because a lay witness may not give an opinion based on specialized knowledge, an element of a claim or defense that turns on such an opinion has no evidence behind it until a qualified expert supplies one, and an expert whose opinion is excluded leaves the same gap. The Supreme Court has read the federal summary judgment rule to mandate judgment, after adequate time for discovery and upon motion, against a party who fails to make a showing sufficient to establish an element essential to its case on which it will bear the burden of proof at trial (Celotex Corp. v. Catrett, 1986). The reverse holds as well. Where every disputed question is one jurors can answer from their own experience, the case may need no expert at all, and the 1972 advisory committee's note to the federal rule on expert testimony explains that an opinion on such a question is excluded because it is unhelpful and therefore superfluous and a waste of time.

The disputed question decides the discipline, because experts in civil litigation each address distinct questions of liability and damages.

• Was the standard of care met? A physician expert reviews the care against the standard for the specialty.
• What did the event cause, and what is the prognosis? A treating or retained physician, such as a physical medicine and rehabilitation specialist, states the diagnosis, the causation opinion and the restrictions the damages experts rely on.
• What can the person physically do? A licensed physical or occupational therapist usually measures it in a functional capacity evaluation (FCE).
• What work and pay remain within the restrictions? A vocational expert states the earning capacity before and after the event.
• What care will be needed, and what will it cost? A life care planner builds the care schedule and prices each item.
• What is the loss worth today? A forensic economist projects the losses and states their present value.

The economic damages expert guide sets out which of the last three a case needs and how to judge the one proposed. Kincaid Wolstein Vocational and Rehabilitation Services (KWVRS) prepares vocational evaluations, life care plans and physician standard of care reviews for plaintiff and defense counsel, applying the same method whichever side retains it. KWVRS provides forensic economic analysis with its affiliated firm, Kincaid Wolstein Economics.

The role decides what the other side can see. A consulting expert is retained in anticipation of litigation or to prepare for trial and is not expected to be called as a witness, and under the federal civil rules the other side ordinarily may not discover that expert's facts or opinions; the exceptions are narrow, such as a showing of exceptional circumstances under which facts or opinions on the same subject cannot practicably be obtained by other means. A testifying expert is identified in a disclosure. Unless the parties stipulate or the court orders otherwise, the disclosure of one retained or specially employed to give expert testimony in the case comes with a written report that the expert prepares and signs, stating every opinion with its basis and reasons and the facts or data considered, and such an expert may be deposed once the report is served. A consultant can test a theory, read the other side's reports and suggest what discovery to take before anything is disclosed, and that work ordinarily stays outside discovery while the expert is not expected to testify. Once counsel designates the expert to testify, the report discloses the opinions to be offered and the facts or data the expert considered in forming them, which the 2010 committee note to the federal civil rules reads broadly to reach any material considered by the expert, from whatever source, that contains factual ingredients, not only what the expert relied on. That material can include what the expert considered while consulting, and it can be examined at deposition with the opinions, so the role is settled before the disclosure deadline. The expert witness disclosure guide lists what a disclosure contains, and the expert witness testimony page describes deposition and trial testimony.

Before the record is fixed, because an expert's first use is often in saying what the record needs. The decision comes up at four points in a case:

• At intake. A consultant can say whether the records support the claim or the defense as pleaded.
• During discovery. The expert can name the records, the testing and the deposition questions the opinion will need.
• Ahead of the disclosure deadline. The deadline falls when the court orders; absent a stipulation or an order, the federal civil rules set it at least 90 days before the date set for trial or for the case to be ready for trial, and the testifying expert needs the time before it to finish the report.
• When the other side discloses an expert. A federal rebuttal disclosure is due within 30 days after the disclosure it answers, unless a stipulation or the court sets another date.

Late retention risks gaps in the record and shortened report timelines. The considering an expert pages list, by case type, what the record should show before an expert adds value, and the vocational expert hiring guide covers the questions to ask before retaining one.