Wisconsin Statutes Evidence (Ch. 901 to 937) § 906.06. Competency of juror as witness

(1) At the trial. A member of the jury may not testify as a witness before that jury in the trial of the case in which the member is sitting as a juror. If the juror is called so to testify, the opposing party shall be afforded an opportunity to object out of the presence of the jury.

(2) Inquiry into validity of verdict or indictment. Upon an inquiry into the validity of a verdict or indictment, a juror may not testify as to any matter or statement occurring during the course of the jury's deliberations or to the effect of anything upon the juror's or any other juror's mind or emotions as influencing the juror to assent to or dissent from the verdict or indictment or concerning the juror's mental processes in connection therewith, except that a juror may testify on the question whether extraneous prejudicial information was improperly brought to the jury's attention or whether any outside influence was improperly brought to bear upon any juror. Nor may the juror's affidavit or evidence of any statement by the juror concerning a matter about which the juror would be precluded from testifying be received.