In a September 23, 2013 opinion, Liebsack v. United States, the US Court of Appeals for the Ninth Circuit held that a state statute limiting who can testify as an expert is a rule of "witness competency" that applies under FRE 601 when state law governs liability in a federal case.

Madlyn Liebsack's guardian brought suit in state court against several defendants, including the United States, contending that negligence by Liebsack's healthcare providers caused her serious permanent injury. The United States removed the action to federal court and all other parties eventually settled, leaving only a claim for negligence against federal healthcare providers under the Federal Tort Claims Act (FTCA). The district court held a bench trial and found that several parties, including the government, were negligent. The district court found that the United States was 15% at fault and ordered the government to pay non-economic damages and 15% of Liebsack's past and future medical expenses. Liebsack appealed.

On appeal, Liebsack contended that the district court's finding was erroneous because the government did not present an expert qualified under an Alaska statute that limits what kind of professional can testify about the appropriate standard of care in an action based on professional negligence (for example, the statute requires that the testifying professional be board certified in the specific subject at issue).

The Ninth Circuit agreed, holding that the Alaska statute is both a state rule of witness competency and a substantive law.

FRE 601 provides that "state law governs the witness's competency regarding a claim or defense for which state law supplies the rule or decision." The court held that where state law governs liability, FRE 601 explicitly requires the application of state witness competency rules in federal court without requiring the court to undergo an Erie analysis. The court noted that state competency rules do not displace FRE 702 and the analysis required under Daubert. FRE 702 and Daubert concern the admissibility of particular testimony, not a witness' competency to testify in the first place.

The Ninth Circuit rejected the government's argument that witness competency rules include only laws relating to matters like a witness's ability to understand the proceedings. Laws governing who may testify as an expert also qualify as witness competency rules.

The court held that the statute at issue is a witness competency rule under FRE 601 as well as part of Alaska's substantive law. The court explained that the Alaska statute is intertwined with the state's professional negligence laws because it limits what kind of professional can testify to the standard of care. That limitation, in turn, affects the standard of care against which the defendant's conduct will be tested, which is an inherently substantive inquiry. Accordingly, because state substantive law governs liability in FTCA actions, the Alaska rule at issue applies to this action.

The Ninth Circuit reversed the judgment of the district court and remanded for a new trial because the government's evidence before the district court did not conform with the Alaska statute requiring specialized expert testimony in medical malpractice actions.