Interest - Standard of Review. MDS Inc. v. Factory Mutual Insurance Company
In MDS Inc. v. Factory Mutual Insurance Company (Ont CA, 2021) the Court of Appeal considered the standard of review that applies to an award of prejudgment interest (which it held to be discretionary):
The standard of review for the award of prejudgment interest
 The trial judge awarded prejudgment interest at MDS’ actual cost of borrowing, including compound interest, under ss. 128 and 130 of the Courts of Justice Act, R.S.O. 1990, c. C.43 (“CJA”).
 Provided that the exclusions to an award of interest set out in the CJA do not apply, the trial judge’s decision to award prejudgment interest at a rate higher or lower than provided for in ss. 128 and 129 is discretionary: see Tribute (Springwater) Limited v. Atif, 2021 ONCA 463, at paras. 26-27. In exercising discretion to award interest at a rate higher or lower than provided for in ss. 128 and 129, the trial judge must take into account changes in market interest rates, the circumstances of the case, the amount claimed and recovered, and other relevant considerations: CJA, s. 130(2).
 As this court held in Krieser v. Garber, 2020 ONCA 699, 70 C.C.L.T. (4th) 40, at para. 46:
The court will only interfere with the exercise of discretion if it was based on an error of law (determined on a correctness standard), a palpable and overriding error of fact, the consideration of irrelevant factors or the omission of factors that ought to have been considered, or if the decision was unreasonable in the sense that it is not compatible with the judicial exercise of discretion. [Citations omitted.]In particular, the trial judge must consider the factors set out under s. 130(2) of the CJA.
 As such, the standard of review in respect of this issue is that of error of law or palpable and overriding error.