What is the test for determining causation in a medical malpractice case?

The last issue is that of causation, and again the principles are not in dispute. The leading case is that of Snell v. Farrell (1990), 1990 CanLII 70 (SCC), 72 D.L.R. (4th) 289 (S.C.C.), in which Sopinka J. sets out the following principles for the court: - causation is an expression of the relationship that must be found to exist between the tortious act of the wrongdoer and the injury to the victim in order to justify compensation of the latter out of the pocket of the former; (p. 298) - causation need not be determined with scientific precision – it is essentially a practical question of fact that can best be answered by ordinary common sense rather than abstract metaphysical theory; (p. 300) - the allocation of the burden of proof is not immutable – in many malpractice cases, the facts lie particularly within the knowledge of the defendant, and 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; (p. 300) - while this has been expressed as the shifting burden of proof to the defendant, that is not correct - this is not a shifting of the burden of proof; rather, the evidence of the plaintiff may result in an inference being drawn adverse to the defendant based on the weight of the evidence; (p. 301) - 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 has not been adduced; (p. 301) - if some evidence to the contrary is adduced by the defendant, the trial judge is entitled to take a “robust and pragmatic” approach to the facts to overcome what had become an overly rigid application of the principle of causation; (p. 300(b) and 301(g)) - it is not essential that the medical experts provide a firm opinion supporting the plaintiffs’ theory of causation – medical experts ordinarily determine causation in terms of certainties, whereas a lesser standard is demanded by the law. (p. 301)