The following excerpt is from U.S. v. Webb, 115 F.3d 711 (9th Cir. 1997):
While this is not to say that expert testimony must be excluded in cases where the subject-matter falls within the comprehension of the average juror, 14 Rule 702's helpfulness requirement indicates that the proffered testimony must offer something not otherwise present that would be helpful to the trier of fact. Cf. United States v. Castillo, 924 F.2d 1227, 1232-33 (2d Cir.1991).
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