What is the test for determining the truth of a witness's testimony in a medical malpractice case?

Further, in considering the evidence given by the worker in the context of the balance of the evidence on the claim file, including the medical evidence, I have applied the test found in Faryna v. Chorny 1951 CanLII 252 (BC CA), [1952] 2 D.L.R. 354 at 357: The test must reasonably subject his story to an examination of its consistency with the probabilities that surround the currently existing conditions. In short, the real test of the truth of the story of a witness in such a case must be its harmony with the preponderance of the probabilities which a practical and informed person would readily recognize as reasonable in that place and in those conditions.