What is the test for reasonableness in a judicial review?

This, of course, is the nub of the difficulty. My colleagues write: In judicial review, reasonableness is concerned mostly with the existence of justification, transparency and intelligibility within the decision‑making process. But it is also concerned with whether the decision falls within a range of possible, acceptable outcomes which are defensible in respect of the facts and law. [para. 47] I agree with this summary but what is required, with respect, is a more easily applied framework into which the judicial review court and litigants can plug in the relevant context. No one doubts that in order to overturn an administrative outcome on grounds of substance (i.e. leaving aside errors of fairness or law which lie within the supervising “function” of the courts), the reviewing court must be satisfied that the outcome was outside the scope of reasonable responses open to the decision maker under its grant of authority, usually a statute. “[T]here is always a perspective”, observed Rand J., “within which a statute is intended [by the legislature] to operate”: Roncarelli v. Duplessis, 1959 CanLII 50 (SCC), [1959] S.C.R. 121, at p. 140. How is that “perspective” to be ascertained? The reviewing judge will obviously want to consider the precise nature and function of the decision maker including its expertise, the terms and objectives of the governing statute (or common law) conferring the power of decision, including the existence of a privative clause and the nature of the issue being decided. Careful consideration of these matters will reveal the extent of the discretion conferred, for example, the extent to which the decision formulates or implements broad public policy. In such cases, the range of permissible considerations will obviously be much broader than where the decision to be made is more narrowly circumscribed, e.g., whether a particular claimant is entitled to a disability benefit under governmental social programs. In some cases, the court will have to recognize that the decision maker was required to strike a proper balance (or achieve proportionality) between the adverse impact of a decision on the rights and interests of the applicant or others directly affected weighed against the public purpose which is sought to be advanced. In each case, careful consideration will have to be given to the reasons given for the decision. To this list, of course, may be added as many “contextual” considerations as the court considers relevant and material.