What is the test for excusing a juror from a jury?

The constitutional right to a fair trial requires that the jury decide the case solely on the basis of evidence from witnesses. (People v. Nesler (1997) 16 Cal.4th 561, 578, 66 Cal.Rptr.2d 454, 941 P.2d 87.)

" Before an appellate court will find error in failing to excuse a seated juror, the juror's inability to perform a juror's functions must be shown by the record to be a "demonstrable reality." The court will not presume bias, and will uphold the trial court's exercise of discretion on whether a seated juror should be discharged for good cause under section 1089 if supported by substantial evidence. " (People v. Jablonski (2006) 37 Cal.4th 774, 807, 38 Cal.Rptr.3d 98, 126 P.3d 938, quoting People v. Holt (1997) 15 Cal.4th 619, 659, 63 Cal.Rptr.2d 782, 937 P.2d 213.) The decision whether to investigate the possibility of juror bias and the extent of

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any investigation rests within the sound discretion of the trial court. (People v. Cleveland (2001) 25 Cal.4th 466, 478, 106 Cal.Rptr.2d 313, 21 P.3d 1225.)