Daubert principles apply beyond "scientific" experts.

Settling a debate that has divided courts since it decided Daubert v. Merrell Dow Pharmaceuticals Inc., 509, U.S. 579 (1993), the U.S. Supreme Court ruled that what it called the "general holding" of Daubert, which established the trial judge's "gatekeeping" function, applies not only to testimony based on "scientific" knowledge, but also to testimony based on "technical" and "other specialized" knowledge, as those terms are used in Federal Rule of Evidence 702. Kumho Tire Co. v. Carmichael, 199 S.Ct. 1167 (1999).

But Justice Breyer, who wrote the opinion of the Court, went on caution that, as Daubert itself said, the test of reliability of experts is flexible, and he added that "Daubert's list of specific factors neither necessarily nor exclusively applies to all experts or every case. Rather, the law grants a district court the same broad latitude when it decides how to determine reliability as it enjoys in respect to the ultimate reliability determination." (Emphasis in original.)

In the case before the Court, Dennis Carlson Jr., described by the Court as an "expert in tire failure analysis," testified in deposition that a defect in its manufacture or design caused a tire to blow out, which resulted in a minivan accident in which one person was killed and several injured. Carlson has a master's degree in mechanical engineering, worked 10 years for a tire company, and has testified in other cases. He reached his conclusion by applying his own four-point test.

The district court applied the Daubert factors--the theory's testability, whether it had been "a subject of peer review of publication," the "known or potential rate of error," and the degree of acceptance "within the relevant scientific...