What is the test for a "patently unreasonable" decision at the Canadian import tribunal?

The courts have grappled with the precise meaning of “patently unreasonable”. It has been defined as a defect apparent on the face of a tribunal’s reasons (or in this case, of the board’s policy). According to this approach, if significant searching is required to find the defect, then the decision is unreasonable but not patently unreasonable. However, Wilson J. in National Corn Growers Association v. Canadian Import Tribunal, 1990 CanLII 49 (SCC), [1990] 2 S.C.R. 1324 (at 1347-1348) held that a court must be prepared to scrutinize the agency’s chain of reasoning in which it engaged in the course of reaching its conclusion, even though its interpretation of the disputed statutory provision was not “patently unreasonable” on its face.