What is the difference between the burden of proof and the evidential burden?

Sopinka J. explained the difference between the burden of proof and the evidential burden in Snell v. Farrell, 1990 CanLII 70 (SCC), [1990] 2 S.C.R. 311, a medical malpractice case. Medical malpractice cases are an apt comparison to discrimination cases because as Sopinka observed at p. 322, “The physician is usually in a better position to know the cause of an injury than the patient”. At pp. 328-329 he said that in medical malpractice cases because “the facts lie particularly within the knowledge of the defendant…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”. He recognized that “[t]his has been expressed in terms of shifting the burden of proof” and went on to explain why that is not correct. At pp. 329-330 he said: …It is not strictly accurate to speak of the burden shifting to the defendant when what is meant is that evidence adduced by the plaintiff may result in an inference being drawn adverse to the defendant. 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. This is sometimes referred to as imposing on the defendant a provisional or tactical burden. In my opinion, this is not a true burden of proof, and use of an additional label to describe what is an ordinary step in the fact-finding process is unwarranted. [Citations omitted].