What is the test for excluding an expert from a witness testimony?

2. The district court precluded testimony by an expert witness that the defendants intended to call regarding the credibility of cooperating witnesses. "We review a district court's evidentiary rulings for abuse of discretion, and will reverse only if we find that there was a violation of a substantial right." United States v. Ebbers, 458 F.3d 110, 122 (2d Cir. 2006).

"It is a well-recognized principle of our trial system that determining the weight and credibility of a witness's testimony belongs to the jury . . . ." Nimely v. City of New York, 414 F.3d 381, 397 (2d Cir. 2005) (internal quotation marks, alterations, and ellipsis omitted). "Thus, this court, echoed by our sister circuits, has consistently held that expert opinions that constitute evaluations of witness credibility, even when such evaluations are rooted in scientific or technical expertise, are inadmissible under [Federal Rule of Evidence] 702." Id. at 398. The defendants argue that their expert witness would have testified regarding different issues: the credibility of cooperating witnesses generally, and the benefits such witnesses receive in exchange for government cooperation, and would not have opined as to the credibility of cooperating witnesses in this case. But as the district court ruled, even general testimony of that sort would represent an "evaluation[] of witness credibility" that we have held is impermissible under Rules 702 and 403. Id. Indeed, the defendants' theory of relevance for the expert's intended testimony is that it would have provided the jury with a logical basis for concluding that the cooperating witnesses in this case testified falsely. "Such testimony does not assist the trier of fact, but rather undertakes to tell the jury what result to reach, and attempts to substitute the expert's judgment for the jury's." Id. (internal citation and quotation marks omitted). The district court therefore did not err in granting the government's motion in limine to exclude the expert testimony.

3. Finally, the defendants argue that the evidence at trial was insufficient to support the defendants' convictions. "A defendant who challenges the sufficiency of the evidence to support his conviction bears a heavy burden." United States v. Jackson, 335 F.3d 170, 180 (2d Cir. 2003) (internal

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