What is the test for judicial deference in determining whether a statutory interpretation is reasonable or unreasonable under reasonableness review?

The bottom line here, then, is that the Commission holds the interpretive upper hand: under reasonableness review, we defer to any reasonable interpretation adopted by an administrative decision maker, even if other reasonable interpretations may exist. Because the legislature charged the administrative decision maker rather than the courts with “administer[ing] and apply[ing]” its home statute (Pezim [Pezim v. British Columbia (Superintendent of Brokers), 1994 CanLII 103 (SCC), [1994] 2 S.C.R. 557], at p. 596), it is the decision maker, first and foremost, that has the discretion to resolve a statutory uncertainty by adopting any interpretation that the statutory language can reasonably bear. Judicial deference in such instances is itself a principle of modern statutory interpretation.