What is the test for granting a motion for a new trial based on comments made by a juror during deliberations during the penalty phase of a death penalty trial?

In exercising its discretion as to whether to grant a new trial based on juror comments during deliberations, courts must be cognizant of the reality that, "[n]ot all comments by all jurors at all times will be logical, or even rational, or, strictly speaking, correct. But such comments cannot impeach a unanimous verdict; a jury verdict is not so fragile." (People v. Riel (2000) 22 Cal.4th 1153, 1219 [no abuse of discretion denying motion for new trial where juror expressed an incorrect personal opinion during deliberations on penalty phase of death penalty trial that court will commute a death verdict].)