In Pennsylvania Labor Relations Board v. Sand's Restaurant Corp., 429 Pa. 479, 240 A.2d 801 (1968), our Supreme Court confirmed the principle that the Labor Relations Board may draw on its expertise to resolve issues of fact. However, it also clarified that inferences must, in every case, be drawn "from the established facts" in order to satisfy the substantial evidence test. Id. at 485, 240 A.2d 804. Stated otherwise, circumstantial evidence can constitute substantial evidence.