Appeal - Standard of Review (SOR) - Written HearingsThis is an interesting issue, of which there may be contradictory Court of Appeal cases.
Where a hearing is held in writing, which is allowed both in administrative tribunals [SPPA 5.1] and courts [under their inherent jurisdiction], the issue arises as to whether the SOR should be deferential at all ... since the original decision-maker and the appeal court are both faced with precisely the same record and the absence of oral evidence (and therefore no credibility issues, which are the natural basis of deference). While the appeal court will naturally use a 'correctness' SOR on law, the argument exists (sound to my mind) that the fact issues and issues of mixed fact and law should also be reviewed on a correctness standard, not the Vavilov-dictated 'palpable and overriding error' standard.
The THMR and Boutette cases (below), both at the Court of Appeal level, would seen to be in conflict on this point.
. Flood v. Boutette
In Flood v. Boutette (Ont CA, 2021) the Court of Appeal makes the point that appellate deference to fact findings does not weaken just because the lower level proceedings were in writing:
 This submission runs contrary to the settled principle that appellate deference to a trial judge’s factual findings does not change simply because the trial record is in writing. As this court explained in FL Receivables Trust 2002-A (Administrator of) v. Cobrand Foods Ltd., 2007 ONCA 425, 85 O.R. (3d) 561, at para. 46:. THMR Development Inc. v. 1440254 Ontario Ltd.
The principle of appellate deference to a trial judge's fact-finding and inference-drawing applies even when the entire trial record is in writing. That is so because the principle of deference is grounded in more than a trial judge's ability to see and hear the witnesses. Deference recognizes that even on a written record, the trial judge “lives through” the trial while a court of appeal reviews the record only through the lens of appellate review. Deference also preserves the integrity of the trial process, maintains the confidence of litigants in that process, reduces the number and length of appeals and therefore, the cost of litigation, and appropriately presumes that trial judges are just as competent as appellate judges to resolve disputes justly. In IFP Technologies (Canada) Inc. v. EnCana Midstream and Marketing, 2017 ABCA 157, 53 Alta. L.R. (6th) 96, at paras. 66-77, leave to appeal refused,  S.C.C.A. No. 303, the Alberta Court of Appeal considered the standard of review applicable in similar circumstances. The trial judge passed away before rendering a decision and, on consent, a new judge rendered a decision on a written record. On appeal, the court found that the usual standard of appellate review governs. As Fraser C.J. explained, appellate review is not intended to be a retrial, duplicating the trial judge’s efforts. The principal function of the appellate courts is to ensure consistency in the law, not correctness of factual findings in particular cases. Even when they do not hear evidence in person, trial judges have expertise in weighing evidence and navigating complex records. Appellate courts, on the other hand, must be mindful that a consistent standard of review helps maintain the effectiveness of appellate review for all litigants in the justice system.
In THMR Development Inc. v. 1440254 Ontario Ltd. (Ont CA, 2018) the Court of Appeal makes this straightforward point when the hearing below is in writing:
 This court is in as good a position as the application judge to deal with the issue of remedy because this case was argued on a paper basis without the involvement of live witnesses.