failed to meet the threshold requirements of necessity and the
need for a properly qualified expert. The case turned on the
respondent’s credibility, and the appellant argues that if the
“oath-helping” evidence of Dr. Smith had been excluded, the jury
would never have concluded that the respondent was sexually
assaulted by Tony “Doe”.
 The respondent asserts that, applying Mohan/White Burgess, there is no basis to interfere with the trial judge’s decision
to admit Dr. Smith’s opinion evidence, including his reports. This
decision was an exercise of discretion, entitled to deference, and
free of any error in principle. The respondent asserts that Dr.
Smith’s opinion evidence was relevant, necessary and properly
admitted at trial on the question of whether the sexual assaults
occurred, as well as on causation and damages. He also contends
that the trial judge appropriately addressed the oath-helping
potential of such evidence by providing the hearsay caution and
setting limits on Dr. Smith’s testimony.
 There are two steps in assessing the admissibility of expert
evidence under the Mohan/White Burgess framework: White Bur-
gess, at paras. 23-24.
 The first step in determining admissibility is for the court
to assess whether the proposed expert evidence meets the threshold requirements that the evidence is ( i) logically relevant;
( ii) necessary to assist the trier of fact; ( iii) not subject to any
other exclusionary rule; and ( iv) proffered by a properly qualified
expert who is willing and able to provide evidence that is impartial, independent and unbiased: see White Burgess, at paras. 23,
53; Mohan, pp. 20-25 S.C.R.2
 If the proponent of the evidence establishes the threshold
for admissibility, the second discretionary gatekeeping step is for
the trial judge to determine whether the potential benefits of
admitting the evidence outweigh its potential risks to the trial
process: White Burgess, at para. 24.
 The trial judge was correct that participant expert witnesses
are subject to the Mohan/White Burgess test for the admissibility of
expert evidence: Westerhof, at para. 64. In my view, on a proper
application of the Mohan/White Burgess framework, Dr. Smith’s
2 In the case of an opinion based on novel or contested science or science
used for a novel purpose, which is not the case here, there is an additional
requirement: the proponent must establish the reliability of the underlying
science for that purpose: White Burgess, at para. 23.