What is the test for credibility of a witness in a medical malpractice case?

On critical points, I am cognisant of the comments of Mr. Justice N. Smith in Gill v. Bassi, 2014 BCSC 1972, at para. 149: The defendants’ credibility, or lack of it, is an important factor, but it cannot be allowed to obscure the burden of proof. The plaintiffs must prove the alleged representations and their own reliance on them. They cannot prove those facts simply by showing the defendants lack credibility because there is no burden on the defendants to disprove the allegations against them.