What is the standard of proof required by a plaintiff to establish causation in a medical malpractice action?

The Ontario Court of Appeal, in Cottrelle v. Gerrard (supra at p. 746), provides a helpful summary of the standard of proof that must be met by a plaintiff in a medical malpractice action in establishing causation. Speaking for the court, Sharpe J.A. says the following: I agree with the appellant's submission that in an action for delayed medical diagnosis and treatment, a plaintiff must prove on a balance of probabilities that the delay caused or contributed to the unfavourable outcome. In other words, if, on a balance of probabilities, the plaintiff fails to prove that the unfavourable outcome would have been avoided with prompt diagnosis and treatment, then the plaintiff's claim must fail. It is not sufficient to prove that adequate diagnosis and treatment would have afforded a chance of avoiding the unfavourable outcome unless that chance surpasses the threshold of "more likely than not".