How Human Factors Expert Witnesses Get Excluded Under Daubert — and How to Survive the Cross

Daubert is not a medicine problem. Under Kumho Tire and the 2023 amendment to Federal Rule of Evidence 702, the trial court's gatekeeping applies to every form of specialized testimony — human factors included. A 20-year study of 2,842 challenges to non-medical experts found that roughly half of those opinions were excluded or partially excluded, and the single most-cited reason was “unreliable methodology.”

The exclusion rarely happens in a written motion alone. It is built, piece by piece, in the deposition cross-examination — where opposing counsel walks a human factors expert into conceding scope, methodology, or an assumption that unravels the whole opinion. Here are the three traps, and how a prepared expert answers each one.

The three ways human factors experts lose ground

Scope: testifying outside your lane

The cross-examiner's question sounds simple:

You opine the warning was inadequate — but you never tested it on actual users, did you?

Why it works: The scope/testing attack. Anchor to the human-factors standards; concede the limits of an untested opinion.

A stronger answer: “I applied accepted human-factors principles and the warnings literature; where user testing would add support I said so, and I disclosed my basis.”

Methodology: the reliability attack

The cross-examiner's question sounds simple:

Your perception-reaction time came from a published average — not this specific driver, correct?

Why it works: Methodology / population averages. Use ranges and disclose that individual times vary.

A stronger answer: “I used accepted PRT ranges from the literature; this individual's exact time cannot be known, so I presented a range rather than a point value.”

Assumptions: the one premise that sinks the opinion

The cross-examiner's question sounds simple:

You assume the user actually looked at the warning — but you have no evidence they did, do you?

Why it works: The user-conduct assumption. Speak to adequacy under the standard, not one user's behavior.

A stronger answer: “My opinion addresses the adequacy of the warning under accepted standards; I did not opine on what this specific user looked at.”

How to prepare for the cross before you're sworn in

Every one of those traps is defeatable — but not by reading your report one more time. The experts who survive the cross have done three things:

• Rehearsed the cross-examination out loud, repeatedly, against a realistic examiner — so the scope concession, the methodology defense, and the assumption hedge are second nature.
• Mastered the record, so that when counsel asks them to recall the one line buried in thousands of pages of the design, warnings, and incident records, they can produce it in seconds rather than fumble.
• Stress-tested the report against FRE 702 — finding the reliability gaps before opposing counsel does.

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