When a trial court is presented with a suggestion of possible jury misconduct, the first question is whether to investigate the matter at all. This decision is often said to be entrusted to the trial court's discretion. (People v. Bradford, supra, 15 Cal. 4th 1229, 1351; People v. Castorena (1996) 47 Cal. App. 4th 1051, 1065.) However, a credible suggestion of grounds for discharge in effect gives rise to a mandatory duty to investigate. "The court must investigate reports of juror misconduct to determine whether cause exists to replace an offending juror with a substitute. '. . . Once a juror's [inability to perform his duty] is called into question, a hearing to determine the facts is clearly contemplated. Failure to conduct a hearing sufficient to determine whether good cause to discharge the juror exists is an abuse of discretion subject to appellate review." ( People v. Keenan, supra, 46 Cal. 3d 478, 532, quoting People v. Burgener, supra, 41 Cal. 3d 505, 519-520; italics added, some bracketed material in Keenan.) This assumes, of course, that there are "material, disputed issues of fact" requiring resolution. ( People v. Hedgecock (1990) 51 Cal. 3d 395, 415, 272 Cal. Rptr. 803, 795 P.2d 1260.) If misconduct is conclusively established by concession, stipulation, or other means, there is no occasion to investigate further. Here defense counsel disputed the sufficiency of Juror 1's "allegations" to warrant discharging Juror 8, and Juror 8's conflicting account confirmed the presence of "disputed issues of fact." Moreover, despite the trial court's supposed discretion to decide when to investigate, the applicable standard of review effectively assumes the truth of any suggestion of potentially disqualifying misconduct: " 'The decision whether to investigate the possibility of juror bias, incompetence, or misconduct--like the ultimate decision to retain or discharge a juror--rests within the sound discretion of the trial court. The court does not abuse its discretion simply because it fails to investigate any and all new information obtained about a juror during trial. . . . a hearing is required only where the court possesses information which, if proven to be true, would constitute "good cause" to doubt a juror's ability to perform his duties and would justify his removal from the case.' ( People v. Ray (1996) 13 Cal. 4th 313, 343 [52 Cal. Rptr. 2d 296, 914 P.2d 846].)" ( People v. Castorena, supra, 47 Cal. App. 4th 1051, 1065; See People v. McNeal (1979) 90 Cal. App. 3d 830, 838, 153 Cal. Rptr. 706, italics added ["Once the court is alerted to the possibility that a juror cannot properly perform his duty to render an impartial and unbiased verdict, it is obligated to make reasonable inquiry into the factual explanation for that possibility."].)