SDCL § 19-19-606: Juror's competency as a witness.

(a) At the trial. A juror may not testify as a witness before the other jurors at the trial. If a juror is called to testify, the court must give a party an opportunity to object outside the jury's presence.

(b) During an inquiry into the validity of a verdict or indictment.

(1) Prohibited testimony or other evidence. During an inquiry into the validity of a verdict or indictment, a juror may not testify about any statement made or incident that occurred during the jury's deliberations; the effect of anything on that juror's or another juror's vote; or any juror's mental processes concerning the verdict or indictment. The court may not receive a juror's affidavit or evidence of a juror's statement on these matters.

(2) Exceptions. A juror may testify about whether:

(A) Extraneous prejudicial information was improperly brought to the jury's attention;

(B) An outside influence was improperly brought to bear on any juror; or

(C) A mistake was made in entering the verdict on the verdict form.