Opinion and Expert Evidence

AuthorDavid M. Paciocco/Lee Stuesser/Palma Paciocco
1.1) Important Distinctions Introduced
This chapter is about opinion and expert ev idence. Two disti nctions are
important to this body of law: (1) t he difference between fact and opin-
ion; and (2) the difference between speci alized and ordinary know-
ledge. The first distinction matters because mere observations of fact
are generally admissible, whereas opinion evidence is presumptively
inadmiss ible. The second di stinction matters because only expert s may
give opinions that require spec ialized knowledge; ordinar y witnesses
may not. In most cases, the t wo key distinctions can be made easi ly, but
there are many inst ances that require the exerci se of careful judgment.
1.2) The Difference Between Fact and Opinion and the
General Exclusionary Rule
“In the law of evidence, an opinion means an ‘ inference from observed
fa ct.’”1 An inference from obser ved fact is different than the observed
fact itself. A witnes s who says a wound was life-threatening, for
example, is drawi ng an inference from an observed fact and is therefore
offering an opinion. If that same witness merely describes the wound
1 R v Collins (2001), 160 CCC (3d) 85 (Ont CA) at para 17 [ Collins].
by saying either “the victim had a wound in his neck” or “the carotid
artery was severed,” that witness is simply reporting an observed fact.
This distinction between inferences and facts is import ant to the law of
evidence, to the extent that it can be drawn. In our system of trial, it is
the neutral, impartial trier of fact who is to determine what inferences
or conclusions to draw from facts. The role of the witness is ordinarily
to describe obser ved facts that the impartial trier of fact will then use
to draw their own in ferences or conclusions. “A basic tenet of our law
is [therefore] that the usual witness may not give opinion ev idence,
but testify only to facts w ithin his knowledge, observation and experi-
en ce .”2 In other words, t here is a general exclusionary rule that oper-
ates to make opinion evidence presumptively in admissible.
1.3) The Difference Between Specialized and Ordinary
Knowledge and Its Implications
There are two exceptions to the general exclusionar y rule for opinion
evidence, one for lay witnesses and one for expert witnesses. Both
exceptions are based on the need, in the circumstances, to resort to
opinion evidence in order to gain access to relevant information.
As will be ex plained,3 lay witnesses are permitted to offer opinions
or conclusions where there is no other meaningful way for them to
communicate ordinary knowledge they possess. A lay wit ness could
express an opinion, for example, that the assailant str uck quickly. Per-
mitting such a statement allows for more effective communication th an
does confining the witness to a mechanistic description of the move-
ments of the assail ant. Similarly, a lay witness could testify that “the
victim had a deep wound to his neck,” even though the depth of a
wound is a matter of judgment, or arguably “opinion.” There would
be no other meaningful and straightforward way for the witne ss to
describe the wound they obser ved, and this is the kind of judgment lay
people are fully capable of making.
The exception to the general exclusion of opinion evidence that
attracts the gre atest legal attention is for expert witnesses. We let expert
witnesses offer opinions where triers of fact do not have the special
training or ex perience required to make the relevant and worthwhile
inferences the exper t witnesses are offering. In effect, we sometimes
allow experts to testify about their opinions because, without expert
help, we could not gain access to useful information — information that
2 R v D(D), [2000] 2 SCR 275 at para 49 [D(D)].
3 See Section 2, “The ‘L ay Opinion Evidence’ Rule,” below in this c hapter.
Opinion and Ex pert Evidence 235
is “beyond the ken [or understanding] of the average juror” because it
takes “special k nowledge or experience” to acquire.4 To return to the
example above, medical doctors have specia l training in the prognosis
of injuries and are able to describe such things as the impact of having
a severed carotid arter y. We would therefore perm it a properly quali-
fied medical doctor to offer the opinion, where relevant, that such a
wound was life threatening. We determine the ad missibility of expert
opinion evidence through several rule s, described in thi s chapter, that
we will refer to collectively as the “exper t evidence admissibility rule s.5
Given that there are two exceptions to the opinion ev idence rule,
it is important to identify which exception to consider when opinion
evidence is being offered. If all t hat is required to form the opinion is
ordinary ex perience, then the lay opinion evidence rule should be used.
For example, the lay opinion evidence rule governed whether a police
officer could testify about the direction in which a person was runn ing
when they made footprints in the snow, since “any school child would
deduce this from the track s . . . observed.”6 By contrast, if special train-
ing or experience is needed to form t he opinion, then the expert evi-
dence admissibility r ules apply. For example, evidence predicting the
effects that the pharmaceutical Antabuse would have on alcohol abuse
had to meet the expert ev idence admissibility rules.7
There are, of course, close cases where it is difficult to determine
whether the lay opinion evidence rule or the expert evidence admissi-
bility rules apply. To avoid confusion, it is helpful to keep two principles
in mind. First, the key considerat ion is not who is offering the opinion,
but the nature of the opinion that is being offered — and more particu-
larly, whether the opinion being offered could be formed only by some-
one with special training or expert ise. In R v Collins, for example, the
expert evidence adm issibility rules did not apply to testimony about
the results of an experiment conducted by police officers to determine
whether bullets would ricochet off the water, striking a t arget on the
opposite shore.8 That experiment did not require ex pertise to conduct,
4 R v Lavallee, [1990] 1 SCR 852 at para 35 [Lavallee]. Se e Section 4.3(b), “Thr esh-
old Requirement 2: Nece ssity,” b elow in this chapter.
5 See below in thi s chapter, Section 4, “Expert Witnes ses”; Section 5, “A dditional
Expert Ev idence Admissibility Ru les”; S ection 6, “The Forms of Expert Evi-
dence”; and Section 7, “Special Rules Re lated to Presentation and Ev aluation.”
6 R v Lee, 2010 ABCA 1, aff’d [2010] 3 SCR 99 at p ara 6.
7 Children’s Aid Society of Simcoe County v J D, 2 010 ONSC 3542. But see Arct ic
Foundations of Can ada Inc v Mueller Canada Inc, 200 9 MBQB 309, where the trial
judge improperly used t he lay opinion rule to exam ine the admission of test i-
mony about the failure of a t hermosyphon system on the s tability of structure s.
8 Collins, above note 1.

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