What is the standard of care in medical negligence?

The leading case on standard of care in medical negligence is Wilson v. Swanson, 1956 CanLII 1 (SCC), [1956] S.C.R. 804. At pages 811-812, Rand J. sets out the duty owed by a physician toward his patient: What the surgeon by his ordinary engagement undertakes with the patient is that he possess the skill, knowledge or judgment of the generality or average of the special group or class of technicians to which he belongs and will faithfully exercise them. In a given situation some may differ from others in that exercise, depending on the significance they attribute to the different factors in light of their own experience. The dynamics of the human body of each individual are themselves individual and there are lines of doubt and uncertainty at which a clear course of action may be precluded. There is here only the question of judgment; what of that? The test can be no more than this: Was the decision the result of the exercise of the surgical intelligence professed? Or was what was done such that, disregarding it may be the exceptional case or individual, in all the circumstances, at least the preponderant opinion of the group would have been against it? If a substantial opinion confirms it, there is no breach or failure.