From McClellan v. I-Flow Corp., 710 F. Supp. 2d 1092 (D. Or. 2010):

Rule 702 permits expert testimony that is helpful to the trier of fact, reliable, and relevant. To that end, Daubert was intended to exclude "junk science" — unsupported testimony or evidence cloaked in the credentials of a testifying expert — that would confuse or mislead rather than "assist the trier of fact." Best v. Lowe's Home Ctrs., Inc., 563 F.3d 171, 176-77 (6th Cir. 2009) ("Daubert attempts to strike a balance between a liberal admissibility standard for relevant evidence on the one hand and the need to exclude misleading 'junk science' on the other."). Daubert did not, however, impose an "exacting standard of causality" beyond the preponderance of the evidence "simply because scientific issues are involved." In re Ephedra Prods. Liab. Litig., 393 F. Supp. 2d 181, 190 (S.D.N.Y. 2005). "It would be unreasonable to conclude that the subject of scientific testimony must be 'known' to a certainty; arguably, there are no certainties in science." Daubert, 509 U.S. at 590.... Moreover, "[l]ack of certainty is not, for a qualified expert, the same thing as guesswork." Primiano v. Cook, ___ F.3d ___, 2010 U.S. App. LEXIS 8859, 2010 WL 1660303, at *5 (9th Cir. Apr. 27, 2010).

Consequently, the proper focus under Daubert is whether an expert's testimony rests on evidence reliably derived from scientific methodology and is relevant to the facts of the case, not whether plaintiffs' experts can prove the point of their testimony. Ambrosini v. Labarraque, 101 F.3d 129, 135, 322 U.S. App. D.C. 19 (D.C. Cir. 1996) ("The dispositive question is whether the testimony will 'assist the trier of fact to understand the evidence or to determine a fact in issue,' not whether the testimony satisfies the plaintiff's burden on the ultimate issue at trial.") (citation omitted). Indeed, establishing reliability should not mean that plaintiffs "have to prove their case twice — they do not have to demonstrate to the judge by a preponderance of the evidence that the assessments of their experts are correct, they only have to demonstrate by a preponderance of evidence that their opinions are reliable." In re Paoli R.R. Yard PCB Litig., 35 F.3d 717, 744 (3d Cir. 1994); Allison v. McGhan Med. Corp., 184 F.3d 1300, 1312 (11th Cir. 1999) (accord); In re Ephedra, 393 F. Supp. 2d at 193 ("Daubert's dictum about scientific validity" is not "authority for increasing the burden of proof imposed by substantive law"). Reliability under Daubert does not depend on "the correctness of the expert's conclusions but [on] the soundness" of the methodology. Daubert II, 43 F.3d at 1318.

***"Under Daubert, the district judge is 'a gatekeeper, not a fact finder.'" Primiano, 2010 U.S. App. LEXIS 8859, 2010 WL 1660303, at *4 (quoting United States v. Sandoval-Mendoza, 472 F.3d 645, 654 (9th Cir. 2006)).