Challenging the opposing expert

A challenge to the opposing expert succeeds on the record, not on rhetoric. The grounds recur across forums; the test that organises them depends on whether your forum applies Daubert or Frye.

The four grounds

Qualification. The credential does not cover the opinion: wrong discipline, lapsed licence, or experience outside the setting and period at issue. The registers settle most of this in an afternoon. Reliability. Under Daubert, attack the method: untested, unpublished, unknown error rate, no controlling standards — or the analytical gap between the data the expert had and the opinion offered. Under Frye, attack acceptance: the method has not gained general acceptance in the field that would know. Fit. A reliable method applied to the wrong facts, or an opinion that does not help the trier decide an issue actually in dispute. The record. Prior inconsistent testimony and publications, compensation and one-sided work history, and anything the CV omits that the registers and transcripts supply.

Sequence

Depose before you move, and use the deposition to lock the expert to the method and the materials — the motion then quotes the expert, not your characterisation. File with the forum's burden in mind: since December 2023, Rule 702 requires the proponent to show admissibility is more likely than not, which makes a thin foundation a ground, not a quibble.

Sources: Daubert, 509 U.S. 579 (1993); FRE 702 (as amended Dec. 1, 2023); National Jurist, "Working with expert witnesses" (consulted 2026-11-02). Information, not legal advice.