What is the legal test for a motion for a new trial for jury misconduct in a criminal case?

When allegations of juror misconduct made in a criminal trial raise a presumption of prejudice, the court is not limited, as it would be in a civil case, to consideration of evidence presented by affidavit or declaration to resolve whether a new trial motion should be granted. The court may conduct an evidentiary hearing, at which jurors may testify, to determine the truth of the allegations if the court concludes this is necessary to resolve material, disputed issues of fact. (People v. Hedgecock (1990) 51 Cal.3d 395, 414-419, 272 Cal.Rptr. 803, 795 P.2d 1260 (Hedgecock).) "Upon an inquiry as to the validity of a verdict, any otherwise admissible evidence may be received as to statements made, or conduct, conditions, or events occurring, either within or without the jury room, of such a character as is likely to have influenced the verdict improperly. No evidence is admissible to show the effect of such statement, conduct, condition, or event upon a juror either in influencing him to assent to or dissent from the verdict or concerning the mental processes by which it was determined." (Evid.Code, 1150, subd. (a).)