At common law a distinction developed between expert witnesses engaged to provide opinion evidence in relation to a proceeding (litigation expert) and an expert not engaged by a party to form his/her opinions, and who does not form his/her opinions for the purpose of the litigation (participant expert). An example of the latter is a treating physician who gives opinion evidence directly related to his/her treatment of a patient, such as a working diagnosis and prognosis.
The distinction between these types of expert witnesses has been codified in amendments to the Rules of Civil Procedure as of 2010.
The Rules of Civil Procedure provides a comprehensive framework addressing a specific class of expert witnesses (see Rules 4.1.01, 53.03 and Form 53): those “engaged by or on behalf of a party to provide [opinion] evidence in relation to a proceeding.” Witnesses, albeit ones with expertise, testifying to opinions formed during their involvement in a matter, do not come within this description. They are not engaged by a party to form their opinions, and they do not form their opinions for the purpose of the litigation.
As a result, this type of expert (a participant expert) may give opinion evidence for the truth of its contents without complying with the procedural requirements set out in rule 53.03.
Westerhof v. Gee Estate, 2015 ONCA 206 at para 14.
The Court of Appeal for Ontario has been clear that a party does not “engage” an expert “to provide [opinion] evidence in relation to a proceeding” simply by calling the expert to testify about an opinion the expert has already formed.
Westerhof v. Gee Estate, 2015 ONCA 206 at para 82.
A participant witness—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:
o the opinion to be given is based on the witness’s observation of or participation in the events at issue, and
o 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.
However, if the participant experts also proffer opinion evidence extending beyond the scope of an opinion formed in the course of participation or observation for purposes other than the litigation, they must comply with rule 53.03 with respect to that part of their evidence.
Westerhof v. Gee Estate, 2015 ONCA 206 at para 63