Evidence - Expert Report. St. Marthe v. O’Connor
In St. Marthe v. O’Connor (Ont CA, 2021) the Court of Appeal considered expert reports, and particularly those of 'non-party experts' as opposed to 'litigation experts':
The Law of Expert Testimony
Expert Opinions Generally
 In civil proceedings, experts retained by a party to testify in a proceeding must prepare an expert report to be shared with all parties prior to trial. This is provided for in r. 53.03 of the Rules of Civil Procedure, R.R.O. 1990, Reg. 194. These experts are referred to as “litigation experts”: Westerhof v. Gee Estate, 2015 ONCA 206, 124 O.R. (3d) 721, at para. 6, leave to appeal refused, 36445 (October 29, 2015) and 36451 (October 29, 2015).
 The expert report includes an outline of the litigation expert’s opinion and anticipated testimony. This sets limits on the expert testimony that may be given and ensures that all parties have proper notice of the roadmap for the opinion evidence to be provided: Bruff-Murphy v. Gunawardena, 2017 ONCA 502, 138 O.R. (3d) 584, at para. 62, leave to appeal refused,  S.C.C.A. No. 343. The report also includes an acknowledgment of the expert’s duty: Imeson v. Maryvale (Maryvale Adolescent and Family Services), 2018 ONCA 888, 143 O.R. (3d) 241, at para. 60, leave to appeal refused,  S.C.C.A. No. 35.
 Litigation experts are distinguished from non-party experts. In Westerhof, at paras. 60-62, this court defined a participant or non-party expert as “a witness with special skill, knowledge, training, or experience who has not been engaged by or on behalf of a party to the litigation”. Non-party experts are subject to different rules than litigation experts.
Non-Party Expert Evidence
 Non-party medical experts such as Dr. Mussett who have special skill, knowledge, training, or experience, may give opinion evidence for the truth of its contents provided the opinion is given as part of the exercise of their skill and knowledge while observing or participating in the events at issue, the opinion is not a “complex vocational opinion requiring highly specialized expertise”, and it was disclosed to the opposing party: Westerhof, at paras. 60-62, 165.
 Non-party expert opinions are usually contained in clinical notes and records, or reports prepared for the purpose of consultation and treatment: Imeson, at paras. 61-63.
 A non-party expert need not provide a r. 53.03 expert report, where the opinion is based on the witness’s observation of or participation in the events at issue and the opinion was formed as part of the exercise of skill, knowledge, training, and experience while observing or participating in the events: Westerhof, at para. 60.
 One reason the requirement to provide a r. 53.03 expert report does not generally apply to non-party experts, is that disclosure problems do not exist in relation to the opinions of non-party experts requiring that they comply with r. 53.03. Non-party experts typically prepare documents summarizing their opinions contemporaneously with their involvement in the events. These summaries, or the expert’s notes and records, can be obtained through the discovery process: Westerhof at para. 85.
 Where the opinion evidence extends beyond the witness’s observation of or participation in the events and the opinion was not formed while observing the events, the expert must comply with the requirements in r. 53.03 with respect to the portion of the opinion that exceeds these limits: Westerhof, at para. 63. Rule 53.03 provides that experts must include the instructions given, a summary of the opinion, and the reasons for the opinion, among other requirements.
 Problems arise where disclosure of an opinion of a non-party expert is given only on the eve of or during the trial: Westerhof, at para. 90.
 It is not an error for a trial judge to prevent a non-party expert from giving opinion evidence that goes beyond the expert’s observations and comments in the original report if the opinion was not disclosed well in advance of trial: Hoang v. Vicentini, 2016 ONCA 723, 352 O.A.C. 358, at para. 30.
The Judge’s Role in the Admission of Evidence
 In White Burgess, the Supreme Court established a two-part framework for admitting expert evidence. First, the court must assess whether the opinion evidence meets the threshold requirements of admissibility: relevance, necessity, absence of an exclusionary rule, and has been provided by a properly qualified expert: White Burgess, at para. 23; see also R. v. Mohan, 1994 CanLII 80 (SCC),  2 S.C.R. 9, at pp. 20-25. If it does, the court engages in a discretionary analysis to determine if its probative value outweighs its prejudicial effect: White Burgess, at para. 24.
 As part of its gatekeeper function, the court may exclude evidence from a non-party expert if it does not meet the test for admissibility: Westerhof, at para. 64; Imeson, at para. 83.