What is the test for drawing an inference of causation?

“[I]n the absence of evidence to the contrary adduced by the defendant, an inference of causation may be drawn although positive or scientific proof of causation has not been adduced.” The amount of affirmative evidence required to justify drawing an inference of causation, in the absence of positive or scientific proof, depends upon the knowledge possessed by the respective parties and the evidence which one party has the ability to produce, in addition to the ability the other party has to contradict that evidence. [Snell v. Farrell, 1990 CanLII 70 (SCC), [1990] 2 S.C.R. 311, at paragraphs 30 to 35]