28 USC Appendix Rule 606 - Competency of Juror as Witness

(a) 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 juror is sitting. 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.

(b) 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 jurys deliberations or to the effect of anything upon that or any other jurors mind or emotions as influencing the juror to assent to or dissent from the verdict or indictment or concerning the jurors mental processes in connection therewith. But a juror may testify about

(1) whether extraneous prejudicial information was improperly brought to the jurys attention,

(2) whether any outside influence was improperly brought to bear upon any juror, or

(3) whether there was a mistake in entering the verdict onto the verdict form. A jurors affidavit or evidence of any statement by the juror may not be received on a matter about which the juror would be precluded from testifying.