Weighing Testimony of the Accused
When an accused testifies and their credibility is at issue, the trial judge must apply what is known as the "WD test" to determine what weight to put on their evidence.
The purpose of the WD test is to "ensure that the jury know how to apply the burden of proof to the issue of credibility. The jury must be cautioned that a trial is not a contest of credibility between witnesses, and that they do not have to accept the defence evidence in full in order to acquit." 
The purpose of the test is not "based on a choice between the accused's and the Crown's evidence, but on whether, based on the whole of the evidence, [the trier-of-fact] is left with a reasonable doubt".
At no time should the trier-of-fact ever shift the burden "from the Crown to prove every element of the offence beyond a reasonable doubt".
Where the accused an a complainant give contradictory evidence, the judge must apply the test from the case of R v D.W.
The WD steps apply not only to the accused's testimony but also to other exculpatory evidence that emerges during trial.
In the context of a voir dire, the principles of D.W. do not apply. Guilt or innocence is not at issue and the standard of proof is one of reasonable doubt, thus an accused will be considered in the same manner as any other witness. Thus if the accused's version conflicts with a police officer, for example, then the court must determine who is telling the truth. If the court cannot decide who is telling the truth then the applicant must fail.
Where the accused and another witness testifies for the defence, the W(D) test is applied differently.
R v D.W. 1991 CanLII 93 (SCC),  1 SCR 742
R v Fowler, 1993 CanLII 1907 (BCCA)
R v C.L.Y., 2008 SCC 2 (CanLII),  1 SCR 5
R v McKenzie (P.N.) 1996 CanLII 4976 (SK CA), (1996), 141 Sask. R. 221 (Sask. C.A.) at para 4
R v Rose (A.) (1992), 20 BCAC 7 (BCCA), 1992 CanLII 987 (BC CA)
R v Currie, 2008 ABCA 374 (CanLII)
R v B.G.S., 2010 SKCA 24 (CanLII)
- R v Warren, 2011 CanLII 80607 (NL PC) at 24
- R v F.E.E., 2011 ONCA 783 (CanLII) at para 104
R v B.D., 2011 ONCA 51 (CanLII) at paras 113-114
R v Cyr, 2012 ONCA 919 (CanLII) at para 50
- See R v Kocovic, 2004 ABPC 190 (CanLII)
- see R v Van, 2009 SCC 22 (CanLII),  1 SCR 716 paras 20-23
The "W.D." Test
Where the defence calls the accused to give evidence that contradicts the crown evidence, the trier of fact must determine:
- If you believe the evidence of the Accused obviously you must acquit.
- If you do not believe the testimony of the Accused but you are left in reasonable doubt by it, you must acquit.
- Even if you are not left in reasonable 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.
The order of the steps are not significant but the steps must all be applied separately.
It is wrong when considering conflicting evidence of credibility to "weigh" one story over the other. The trier of fact cannot "prefer" one story over the other or consider who is "most" credible. The “either/or” approach, preferring one over the other should be avoided. To prefer one testimony over another has the effect of reversing the onus upon the accused.
There is nothing preventing a judge from believing both the complainant and the accused even where they gave divergent or contradictory evidence
The real issue is not who is telling the truth, but instead, whether, on the entirety of the evidence, the crown has proven the case beyond a reasonable doubt.
The purpose of the WD test is to ensure that the trier of fact "understand that the verdict should not be based on a choice between the accused’s and Crown’s evidence, but on whether, based on the whole of the evidence, they are left with a reasonable doubt as to the accused’s guilt". It further intends to make clear that the burden never shifts from the Crown to prove every element of the offence.
The steps of WD are not considered "watertight" compartments. The analysis at each step should take into account the evidence as a whole.
R v W(D), 1991 CanLII 93 (SCC),  1 SCR 742 per Cory J
R v PNM (1995), 1996 CanLII 4976 (SKCA), per Tallis JA - frames it as a four step inquiry
R v Minuskin, 2003 CanLII 11604 (ON CA), (2004), 181 CCC (3d) 542 (Ont. C.A.), at p. 550 R v B.D., 2011 ONCA 51 (CanLII), at paras 102-14
R v Turmel,  BCJ No. 2265 (C.A.), at paras 9-17
- R v JHS, 2007 NSCA 12 (CanLII) - on appeal to SCC
- R v B.G.S., 2010 SKCA 24 (CanLII) at para 9
R v Challice (1979) 45 CCC 2d 546 (Ont CA)
R v Morin 1988 CanLII 8 (SCC),  2 SCR 345
R v Chan (1989) 52 CCC 3d 148 (Alta CA)
R v Jaura,  O.J. No. 4157, 2006 ONCJ 385 (CanLII) paras 12, 13
- R v Abdirashid, 2012 ABPC 22 (CanLII),  A.J. No 131 at para 6
- R v Nadeau 1984 CanLII 28 (SCC),  2 SCR 570
- Canadian Criminal Evidence, Second Edition, by P.R. McWilliams, Q.C., at page 652
R v Nykiforuk (1946), 86 CCC 151 (Sask. C.A.)
R v CLY, 2008 SCC 2 (CanLII),  1 SCR 5 at para 8
JHS, supra at para 13
R v Berg, 2016 SKPC 55 (CanLII) - commentary by Kovatch PCJ
Looking at Evidence as a Whole
The first two steps in the WD test require the "weighing [of] the accused's evidence together with the conflicting Crown evidence."
It is essential that the court not look at any witnesses' evidence in a vacuum and instead look at it in relation to all the evidence presented as a whole. 
R v Humphrey, 2011 ONSC 3024 (CanLII),  O.J. No. 2412 (Sup. Ct.), at para 152
see also R v Newton, 2006 CanLII 7733 (ON CA) at para 5
R v Hull, 2006 CanLII 26572 (ON CA) at para 5
R v Snider  O.J. 879, at para 37
R v Hoohing, 2007 ONCA 577 (CanLII) at para 15
- R v Lake, 2005 NSCA 162 (CanLII), (2005), NSJ No. 506
Rejection of Accused's Evidence
The accused's evidence should be the evidence considered first.
It is necessary that in any case that turns on the accused's credibility, the judge's reasons "should disclose whether she believes or disbelieves the accused."
In explaining the reason to reject the accused's evidence it can be sufficient to justify it based on the reasoned acceptance beyond a reasonable doubt of a fact that conflicts with the evidence rejected. 
Impugning the accused's credibility is a permissible form of post-offence conduct.
The rejection of the accused's evidence does not amount to evidence in favour of the Crown.
A trial judges observation that the accused testimony was "self-serving" can lead to the appearance that the judge inherently suspects the accused testimony because it is and fantasias to misrepresent events in order to acquit himself.
Disbelief of the accused evidence cannot be used as "positive proof of guilt by moving directly from disbelief to a finding of guilt.
It is not permissible to reject the accused's evidence due to the fact that the accused was unable to explain why the accuser would have made allegations against him.
When in Conflict with the Complainant's Evidence
It is crucial that the judge not discount the accused's evidence for the reason that the complainant is believed. Otherwise, the defence is completely neutered before even testifying.  However, it is permissible to reject the accused's evidence on the basis that when "stacked beside" all the other evidence.
There is some support to suggest that "trial judge can reject the evidence of an accused and convict solely on the basis of his acceptance of the evidence of the complainant, provided that he also gives the evidence of the defendant a fair assessment and allows for the possibility of being left in doubt, notwithstanding his acceptance of the complainant's evidence."
Simply rejecting the accused's evidence on the basis that it conflicts with the complainant's evidence which has been accepted without explanation shifts the burden of proof and is not acceptable.
R v C.L.Y., 2008 SCC 2 (CanLII),  1 SCR 5
R v Lake, 2005 NSCA 162 (CanLII) at para 14 - however an implied conclusion is sufficient, see para 17
- R v D. (J.J.R.), 2006 CanLII 40088 (ON CA),  215 CCC (3d) 252 at para 53 (C.A.) Doherty J. ("An outright rejection of an accused’s evidence based on a considered and reasoned acceptance beyond a reasonable doubt of the truth of conflicting evidence is as much an explanation for the rejection of an accused’s evidence as is a rejection based on a problem identified with the way the accused testified or the substance of the accused’s evidence.")
R v Jaw, 2009 SCC 42 (CanLII),  3 SCR 26 at para 39
R v White, 1998 CanLII 789 (SCC),  2 SCR 72 at para 26
- R v Nedelcu, 2012 SCC 59 (CanLII) (“rejection of an accused’s testimony does not create evidence for the Crown”)
- R v Drescher, 2010 ABQB 94 (CanLII)
- R v M.Q., 2010 ONSC 61 (CanLII), per Hill J.
R v J.C.H., 2011 NLCA 8 (CanLII) at para 18
- R v Lake, 2005 NSCA 162 (CanLII),  NSJ No 506 at para 21
- R v TS, 2012 ONCA 289 (CanLII), at para. 79 ("…as a matter of law, reasoned acceptance of a complainant’s evidence is a basis upon which a trial judge can reject the evidence of an accused and find guilt proven beyond a reasonable doubt. A reasoned and considered acceptance of the complainant evidence is as much as explanation for rejecting the contrary evidence of an accused as are problems inherent in an accused’s own testimony.")
R v Surana, 2013 ABPC 164 (CanLII) at para 78
- R v Y.M., 2004 CanLII 39045 (ON CA) at para 30
The court simply rejecting the accused story is not enough.  The purpose of the third part of the test is to convey that "a complete rejection of the [accused's] evidence does not mean that his guilt is established." 
It is an error to "use disbelief of the accused’s evidence as positive proof of guilt by moving directly from disbelief to a finding of guilt"
It is an error of law to use the disbelief of the accused's evidence as proof of guilt.
- R v B.C.G., 2010 MBCA 88 (CanLII), M.J. No. 290 (“reasonable doubt is not forgotten” simply because a trial judge rejects “the accused’s version of events.”)
R v Liberatore, 2010 NSCA 82 (CanLII),  NSJ No. 556, at 15 stated WD prevents “a trier of fact from treating the standard of proof as a simple credibility contest”
- R v Gray, 2012 ABCA 51 (CanLII) at para 40
R v Dore 2004 CanLII 32078 (ON CA), (2004), 189 CCC (3d) 526 (Ont. C.A.), at p. 527
R. v H(S),  O.J. No. 118 (C.A.), at paras 4-6
R v Dore, 2004 CanLII 32078 (ON CA), (2004), 189 CCC (3d) 526 (Ont. C.A.), at p. 527
R v H.(S.), at paras 4-6
Several Court's of Appeal recommend an additional element to the D.W. test after the first step directing the judge that "If after careful consideration of all of the evidence, you are unable to decide whom to believe, you must acquit".
A judge cannot take into account roadside statements in the assessment of the accused's credibility.
R v CWH 1991 CanLII 3956 (BC CA), (1991), 68 CCC (3d) 146 (BCCA)
R v PNM (1995), 1996 CanLII 4976 (SKCA)
- R v Huff,  O.J. No. 3487; leave to appeal to SCC denied  S.C.C.A. No. 562
It has sometimes been suggested that the proper approach should be to consider the accused's evidence first before looking at the complainant's evidence in order to avoid creating a burden upon the accused. But this approach has had some critics.
A judge may reject the accused evidence on the sole basis that it contradicts the accepted evidence.
An accused's testimony which merely denies the offence and provides no further detail cannot be the basis to dismiss the testimony as unworthy of belief.
- R v Moose, 2004 MBCA 176 (CanLII) at para 20
R v D.A.M., 2010 NBQB 80 at para 53 and 56
R v Schauman, 2006 ONCJ 304 (CanLII) at para 6
R v C.Y.L., 2008 SCC 2 (CanLII)
R v Currie, 2008 ABCA 374 (CanLII),  A.J. No. 1212
R v D. (J.J.R.) 2006 CanLII 40088 (ON CA), (2006), 215 CCC (3d) 252 (Ont. C.A.), at para 53
R v M. (R.E.), 2008 SCC 51 (CanLII), (2008) 235 CCC (3d) 290 (S.C.C.), at para 66
R v Thomas, 2012 ONSC 6653 (CanLII), at para 26
- R v Surana, 2013 ABPC 164 (CanLII) at para 71