What is the test for determining causation in medical malpractice cases?

Snell v. Farrell deals specifically with causation in medical malpractice cases. In that case the court held that although the plaintiff has as in all cases the onus of proving causation, the causation test is not to be applied too rigidly. A plaintiff who is unable to strictly prove causation may be able to adduce such evidence as will allow the court to rule out the defendant’s position and to infer that but for the conduct of the defendant the injury complained of would not have occurred. This proposition, often applied, has come to be known as the “but for” test. Sopinka J. goes on (at page 330): 33 The legal or ultimate burden remains with the plaintiff, but in 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. If some evidence to the contrary is adduced by the defendant, the trial judge is entitled to take account of Lord Mansfield’s famous precept. This is, I believe, what Lord Bridge had in mind in Wilsher [[1988] 2 W.L.R. 557, rev’g [1987] 2 W.L.R. 425] when he referred to a “robust and pragmatic approach to the ... facts” (p. 569). [Lord Mansfield’s famous preceipt: “It is certainly a maxim that all evidence is to be weighed according to the proof which it was in the power of one side to have produced, and in the power of the other to have contradicted.” (Blatch v. Archer (1774), 98 E.R. 969 at 970.)