Competency. R v Nguyen
In the criminal case of R v Nguyen (Ont CA, 2015) the Court of Appeal commented on several basic principles of evidence law while considering whether the spousal incompetency rule (that spouses are not competent to testify against spouses) should be extended to common law spouses by virtue of the s.15 equality provisions of the Charter.
On competency the court stated:
 Spousal competence, compellability and privilege are distinct but related concepts. As they lie at the heart of these appeals, it is useful to consider their meanings now.
 Competence refers to a person’s legal capacity to give evidence in a court of law. A person who is incompetent cannot testify, even if he or she wishes to do so. Competence is a threshold requirement for the admissibility of testimony, the purpose of which is to “exclude at the outset worthless testimony, on the ground that the witness lacks the basic capacity to communicate evidence to the court”: R. v. D.A.I., 2012 SCC 5,  1 S.C.R. 149, at para. 16.
 Historically, at common law, numerous categories of potential witnesses were deemed incompetent to testify, including individuals with a criminal record, those who were unable to swear an oath, those with an interest in the proceeding (such as the accused), and the spouses of accused persons. However, legislation has materially changed this situation: see, generally, Sidney N. Lederman, Alan W. Bryant, & Michelle K. Fuerst, Sopinka, Lederman & Bryant: The Law of Evidence in Canada, 4th ed. (Markham, Ontario: LexisNexis, 2014) at pp. 869-91. A person is now generally presumed competent to testify: s. 16 of the Canada Evidence Act, R.S.C. 1985, c. C-5 (the “CEA“); D.A.I., at para. 16.