How is credibility assessed in the context of criminal or quasi-criminal trials?

[73] The decision of Cory J. in Regina v. W.(D.), supra. explains how credibility is to be assessed in the context of criminal or quasi-criminal trials, commencing at paragraph 27 therein, as follows: In a case where credibility is important, the trial judge must instruct the jury that the rule of reasonable doubt applies to that issue. The trial judge should instruct the jury that they need not firmly believe or disbelieve any witness or set of witnesses. Specifically, the trial judge is required to instruct the jury that they must acquit the accused in two situations. First, if they believe the accused. Second, if they do not believe the accused’s evidence but still have a reasonable doubt as to his guilt after considering the accused’s evidence in the context of the evidence as a whole. … Ideally, appropriate instructions on the issue of credibility should be given, not only during the main charge, but on any recharge. A trial judge might well instruct the jury on the question of credibility along these lines: ‘First, if you believe the evidence of the accused, obviously you must acquit. Second, if you do not believe the testimony of the accused but you are left in reasonable doubt by it, you must acquit. Third, even if you are not left in doubt by the evidence of the accused, you must ask yourself whether, on the basis of the evidence which you do accept, you are convinced beyond a reasonable doubt by that evidence of the guilt of the accused.’ If that formula were followed, the oft repeated error which appears in the recharge in this case would be avoided. The requirement that the Crown prove the guilt of the accused beyond a reasonable doubt is fundamental in our system of criminal law. Every effort should be made to avoid mistakes in charging the jury on this basic principle.