1. The General Exclusionary Rule and its Exceptions

• Author: David M. Paciocco - Lee Stuesser
• Profession: Justice of the Ontario Court of Justice - Professor of Law, Bond University
• Pages: 181-182

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"In the law of evidence, an opinion means an ‘inference from observed fact.’"¹In our system of trial, it is the neutral, impartial trier of fact who is to determine what inferences to draw from facts. "A basic tenet of our law is [therefore] that the usual witness may not give opinion evidence, but testify only to facts within his knowledge, observation and experience."²In spite of this general exclusionary rule, opinion evidence is often offered. It is therefore more instructive to focus on when opinion evidence will be allowed. As will be developed in this chapter, there are two categories of admissible opinion evidence, one for lay witnesses and one for expert witnesses. In simple terms, we let lay witnesses offer opinions when there is no other meaningful way for them to communicate ordinary knowledge that they possess. We let expert witnesses offer opinions where triers of fact do not have the special training or experience required to make the relevant and worthwhile observations that the witness is offering.

It is therefore a straightforward matter as to which set of rules apply in a given case. If all that is required to form the opinion is ordinary

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experience, the lay opinion rule should be used. For example, the lay opinion evidence rule governed whether a police officer could testify to the direction the person who made footprints in the snow was running, since "any school child would deduce this from the tracks . . . observed."³

By contrast if special training or experience is needed to form the opinion, the expert opinion rules apply. For example, evidence predicting the effects that the pharmaceutical "antabuse" would have on alcohol abuse by an alcoholic had to meet the expert evidence rules.⁴The expert evidence rules are, of course, intended to determine when individuals can offer testimony about things that lay persons do not have the special training or experience to observe. As a result, the expert evidence rules that govern opinion evidence are not confined to opinions per se. Any time a witness offers observations - even descriptive ones - that cannot competently be made without special training or experience, the expert evidence rules described in this chapter apply. For example, a lay witness would not be permitted to describe the anatomy of the eyeball.

Confusion can be avoided if it is remembered that the issue in determining which rules to apply is not whether the witness is an expert. It is...