What evidence is available to determine whether a juror is unable to perform their duty?

Often, the evidence bearing on whether the juror is unable to perform his or her duty may be in conflict. (People v. Barnwell, supra, 41 Cal.4th at p. 1053.) Under questioning by the court, the identified juror may deny it, while other jurors may testify about how the identified juror has revealed an inability to perform. (Ibid.) The trial court is entitled to weigh the credibility of the jurors, "taking into account the nuances attendant upon live testimony. The trial court may also draw upon the observations it has made of the jurors during voir dire and the trial itself. Naturally, in such circumstances, we afford deference to the trial court's factual determinations, based, as they are, on firsthand observations unavailable to us on appeal." (Ibid.)