What is the test for abuse of a motion for a new trial based upon an allegation of lack of jury impartiality?

We review for abuse of discretion the denial of a motion for a new trial based upon an allegation of lack of jury impartiality. United States v. Aguon, 851 F.2d 1158, 1170 (9th Cir.1988) (en banc). In addition, "[a]n evidentiary hearing must be granted unless the alleged misconduct could not have affected the verdict or the district court can determine from the record before it that the allegations are without credibility." United States v. Navarro-Garcia, 926 F.2d 818, 822 (9th Cir.1991) (emphasis added).