Evidence - Expert Opinion - Litigation v Participant Experts
Evidence - Expert Opinion - Admissibility
Imeson v. Maryvale (Maryvale Adolescent and Family Services)
In this case the Court of Appeal considers the use and admissibility of evidence from an expert:
 In considering the proper scope of Dr. Smith’s evidence as a participant expert, it is important to remember how participant experts differ from litigation experts.
 In the civil litigation context, a litigation expert is subject to r. 53.03. This rule requires, among other things, an expert report that sets out the expert’s opinions, as well as an acknowledgment of the expert’s duty. Typically, an expert report provides a “roadmap of the anticipated testimony and specific limits may be placed on certain areas of testimony”: Bruff-Murphy (Litigation guardian of) v. Gunawardena, 2017 ONCA 502 (CanLII), 138 O.R. (3d) 584, at para. 62, leave to appeal refused,  S.C.C.A. No. 343. The expert’s report will provide the framework for discussion about the proper scope of the expert’s testimony.
 In contrast, under Westerhof, participant experts may give opinion evidence without complying with r. 53.03. Typically, any opinions that are sought to be introduced are found in the clinician’s clinical notes and records, or in reports prepared for the purpose of consultation and treatment.
 In Westerhof, this court explained the proper role of a participant witness, at para. 60, as follows:
[A] witness with special skill, knowledge, training, or experience who has not been engaged by or on behalf of a party to the litigation may give opinion evidence for the truth of its contents without complying with rule 53.03 where: If participant experts proffer opinion evidence extending beyond these limits, they must comply with r. 53.03 “with respect to the portion of their opinions extending beyond those limits”: Westerhof, at para. 63. In acting as a gatekeeper, trial judges have the important task of ensuring that participant experts do not exceed their proper role or, if they do, that there is compliance with r. 53.03.
• the opinion to be given is based on the witness's observation of or participation in the events at issue; and
• the witness formed the opinion to be given as part of the ordinary exercise of his or her skill, knowledge, training and experience while observing or participating in such events.
 There are two steps in assessing the admissibility of expert evidence under the Mohan/White Burgess framework: White Burgess, 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.
 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.