What is the test for determining whether a manufacturer of a product is responsible for a product's adverse effects?

The presumption eliminates the necessity of proof on a brand by brand basis. The presumption appears neither illogical nor unfair. Section 13.1(4) provides that the manufacturer may offer evidence in rebuttal. It may be assumed a manufacturer would be most familiar with the effects of his own product and have access to the necessary evidence to demonstrate a brand differential. [Snell v. Farrell, supra, Sopinka J., at p.300]: In many malpractice cases, the facts lie particularly within the knowledge of the defendant. In these circumstances, very little affirmative evidence on the part of the plaintiff will justify the drawing of an inference of causation in the absence of evidence to the contrary. …