What is the test for finding negligence in a medical malpractice case?

A leading decision respecting the basis on which to find negligence is Ryan v. Victoria (City), 1999 CanLII 706 (SCC), [1999] 1 S.C.R. 201. It summarizes the law as follows: 28. Conduct is negligent if it creates an objectively unreasonable risk of harm. To avoid liability, a person must exercise the standard of care that would be expected of an ordinary, reasonable and prudent person in the same circumstances. The measure of what is reasonable depends on the facts of each case, including the likelihood of a known or foreseeable harm, the gravity of that harm, and the burden or cost which would be incurred to prevent injury. In addition, one may look to external indicators of reasonable conduct, such as custom, industry practice, and statutory or regulatory standards.