[¶ 1] In Howes v. Kelly Services, Inc., 2002 ND 131, ¶¶ 11-16, 649 N.W.2d 218, we held Kelly was entitled to rely upon prior demands for a nine-person jury by two co-defendants, even though both co-defendants were dismissed from the lawsuit before trial. We concluded the trial court abused its discretion in denying Kelly’s motion for a new trial on that issue, and we reversed and remanded for a new trial. Id. at ¶¶ 16, 18. On petition for rehearing, Howes claims the trial to a six-person jury was harmless error under N.D.R.Civ.P. 61 and Allen v. Kleven, 306 N.W.2d 629, 634 (N.D.1981).
[¶ 2] In Allen, 306 N.W.2d at 632, Allen, a passenger in a car driven by Williams, sued Eleven for injuries sustained in an accident involving the car driven by Williams and a car driven by Kleven. Allen’s complaint did not request a jury trial. Eleven answered, alleging Allen’s injuries were caused by the negligence of Allen and Williams. Eleven’s answer requested a jury trial without spec*423ifying the number of jurors desired. Rule 38(c), N.D.R.Civ.P., then provided a jury shall consist of six unless a request for a jury of twelve is made within the time for demanding a jury trial. Eleven later brought a third-party action against Williams, alleging Williams was negligent and seeking contribution if Eleven was found liable for Allen’s injuries. Williams’ answer to Eleven’s third-party action denied liability and requested a twelve-person jury. At a pretrial conference, Williams agreed to a trial by a six-person jury.
[¶ 3] On appeal, Allen argued the trial court erred in allowing a six-person jury. Allen argued William’s agreement to a six-person jury effectively withdrew Williams’ demand for a twelve-person jury. Allen claimed she did not consent to the withdrawal under N.D.R.Civ.P. 38(e), and she affirmatively objected. Williams and Eleven argued the issue about jury size was germane only to the third-party action. They also argued Allen waived her right to object because she did not demand a jury trial. This Court considered the parties’ arguments under the harmless error rule in N.D.R.Civ.P. 61, and concluded Allen’s reasons for a twelve-person jury were conjectural and did not demonstrate the alleged error was prejudicial. Allen, 306 N.W.2d at 634. We said the appealing party had the burden of establishing the trial court erred and the error was highly prejudicial. Id. We decided there was “nothing before us from which to conclude that if an error was made by the trial court when it allowed the jury to be reduced from 12 to six members, it was anything more substantial than harmless error.” Id.
[¶ 4] The facts in Allen are different from this case in one aspect. In Allen, a third-party defendant demanded a twelve-person jury in a third-party action after Allen had sued Eleven in the main action and failed to request a jury trial. Here, although Eelly’s answer did not demand a jury trial, we held Eelly was entitled to rely on prior demands for a nine-person jury by two co-defendants. Eelly’s right to rely on co-defendants’ prior demands for a nine-person jury is more compelling than Allen’s right to rely on a third-party defendant’s subsequent demand for a twelve-person jury.
[¶ 5] In Allen, 306 N.W.2d at 634, this Court applied the harmless error rule of N.D.R.Civ.P. 61 to the issue involving jury size. Under N.D.R.Civ.P. 61, no error is ground for granting a new trial or setting aside a verdict unless refusal to do so is inconsistent with substantial justice, and at every stage of a proceeding, a court must disregard any error or defect which does not affect the substantial rights of the parties. In another context, we have defined an error affecting substantial rights as an error that was prejudicial, or affected the outcome of a proceeding. See State v. Olander, 1998 ND 50, ¶ 15, 575 N.W.2d 658.
[¶ 6] We have applied the harmless error rule to other issues affecting the jury. See Larson v. Williams Elec. Co-op., Inc., 534 N.W.2d 1, 4-5 (N.D.1995) (applying harmless error to exclusion from jury panel of all defendant co-op’s members and customers and concluding error was not harmless); Kronberger v. Zins, 463 N.W.2d 656, 658-60 (N.D.1990) (applying harmless error to trial court’s ex parte communications with jury and concluding error was not harmless); Sathren v. Behm Propane, Inc., 444 N.W.2d 696, 698 (N.D.1989) (applying harmless error to juror’s honestly mistaken response to voir dire question and concluding error was harmless); Andrews v. O’Hearn, 387 N.W.2d 716, 723-26 (N.D.1986) (applying harmless *424error to communications with jury and concluding error was harmless).
[¶ 7] Here, before trial, Kelly claimed it was entitled to a nine-person jury. Kelly argued prior demands for a nine-person jury by two co-defendants could not be withdrawn without the consent of all parties, even though the two co-defendants had been dismissed from the action before trial. The trial court held Kelly had waived its right to a jury, and the case was heard by a six-person jury, as requested by Howes. In Howes, 2002 ND 131, ¶¶ 11-16, 649 N.W.2d 218, we held Kelly was entitled to rely upon the prior demands for a nine-person jury by its two co-defendants. Although not of constitutional dimension, Kelly was denied its statutory right to a larger jury with the likelihood of obtaining a more representative cross section of the community. This is an error which affects the framework within which the trial proceeds, rather than an error which occurs during the trial process itself. Under our law, it would be virtually impossible to show the prejudicial effect of seating of a six-person jury and denying a party the statutory right to a nine-person jury, because jurors may not testify as to matters concerning their mental process during deliberations. N.D.R.Evid. 606(b). See Sathren, 444 N.W.2d at 698 n. 2. In this narrow situation, the inherent possibility of prejudice is present but virtually impossible to assess.
[¶ 8] In an analogous situation, we concluded the automatic exclusion of a defendant co-op’s members and customers from a jury pool without an individualized inquiry was not harmless error. Larson, 534 N.W.2d at 4-5. Although we acknowledged the scope and expense of a new trial, our decision recognized the importance with which we regard the jury system. See Larson, at 5-6 (VandeWalle, C.J., concurring). We concluded the exclusion of an identifiable group of people from the jury pool deprived the parties of a fair cross section of the population from which to select a jury and was not harmless error. Larson, at 4-5. We said the refusal to order a new trial in that situation would be inconsistent with substantial justice. Id. at 5.
[¶ 9] Here, we similarly conclude the trial court’s pre-trial denial of Kelly’s assertion of its statutory right to a nine-person jury would be inconsistent with substantial justice. We therefore conclude the denial affected Kelly’s substantial rights and was not harmless. To the extent Allen is contrary to our conclusion, we overrule it.
[¶ 10] We deny Howes’ petition for rehearing.
[¶ 11] MARY MUEHLEN MARING, CAROL RONNING KAPSNER and DALE V. SANDSTROM, JJ., concur.