How Tax 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 — tax 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 tax 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 tax experts lose ground

Scope: testifying outside your lane

The cross-examiner's question sounds simple:

You opine the tax position was unreasonable — but reasonable practitioners can disagree on close questions, can't they?

Why it works: Anchor to the IRC/regs and the applicable confidence standard; acknowledge reasonable disagreement.

A stronger answer: “I analyzed the position against the Internal Revenue Code, regulations, and the applicable standards (e.g., substantial authority), and I addressed it as a question of reasonable professional judgment under those standards.”

Methodology: the reliability attack

The cross-examiner's question sounds simple:

Your opinion relied on the return and workpapers — you didn't interview the taxpayer, did you?

Why it works: Methodology / record basis.

A stronger answer: “I analyzed the documented positions and records against the standards, which is the proper basis, and noted that limit.”

Assumptions: the one premise that sinks the opinion

The cross-examiner's question sounds simple:

You assume the taxpayer disclosed all the facts to the preparer — but you don't know that, do you?

Why it works: The disclosure assumption. Tie to records.

A stronger answer: “Where the record didn't establish disclosure, I said so and analyzed the standard accordingly.”

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 returns, records, and IRS correspondence, they can produce it in seconds rather than fumble.
• Stress-tested the report against FRE 702 — finding the reliability gaps before opposing counsel does.

Questions? Contact us at [email protected].