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

Causation need not be determined by scientific precision.[27] In the Snell v. Farrell case Sopinka J. discussed the implications of this principle for the quality of the evidence a plaintiff must adduce in a medical malpractice case in order to prove causation. He was not prepared to reverse the burden of proof for an injury that might very well turn out to be due to factors unconnected to the defendant and not the fault of anyone.[28] However, Sopinka J. made it clear that in a malpractice case it was not essential to have a positive medical opinion to support a finding of causation.[29] What was required, in his view, was the treatment of causation as “essentially a practical question of fact which can best be answered by ordinary common sense rather than abstract metaphysical theory.”[30] In considering whether the evidence adduced by a plaintiff might result in drawing an inference adverse to the defendant medical practitioner, Sopinka J. stated: Whether an inference is or is not drawn is a matter of weighing evidence. The defendant runs the risk of an adverse inference in the absence of evidence to the contrary. … 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. … It is not therefore essential that the medical experts provide a firm opinion supporting the plaintiff's theory of causation. Medical experts ordinarily determine causation in terms of certainties whereas a lesser standard is demanded by the law.[31]