The Admissibility of Expert Opinion Evidence

AuthorAlan D. Gold
Chapter 3
The Admissibility of Expert
Opinion Evidence
   course of things i n our courtrooms, witnesses may not give
opinion evidence. ey can only testify about facts within their knowledge
through observation and personal sensory experience. is is a basic tenet of
our law of evidence.
Witnesses may testify as to what they saw or heard, assuming that what t hey
saw or heard is relevant to an issue in the case and otherwise admissible. But
witnesses may not give evidence as to what they concluded, inferred, or came
to believe as a result of what they saw or heard. It is for the trier of fact to draw
inferences a nd other opinions or conclusion s, not for any witness .
is limitation applies when the evidence is adduced as substantive evidence
of the opinion, inference, and belief for the truth of its content. If the witness’s
state of mind is a relevant issue, then such evidence from the witness becomes
admissible to prove the state of mind, but not its substantive content.
Expert opinion evidence is admissible as the ma in exception to the rule
against opinion evidence. e courts describe this admission a s “exceptional ”
and based upon necessity:
R. v. A.K. an d N.K. (),  C.R. (th)  at para.  (Ont. C. A.).
    
[Expert evidence and its associated da ngers] . .. are tolerated in those excep-
tional cases where the jury would be u nable to reach their own conclusions in
the absence of assistance from experts w ith special knowledge.
e other major exception for opinion evidence is the lay opinion exception
in respect of certain judgmental issues such as a vehicle’s speed, a person’s appar-
ent age, the identication of persons or things, and ordinary, everyday mental
or emotional states such as intoxication, anger, and the like. e theory is that
in such cases “it is virtually impossible to separate the witness’ inference from
the facts on which the inferenc e is based [and] . . . the adm ission of opinion evi-
dence . . . is merely a compendious way of ascertaining the result of the witness’
Evidence from an expert witness need not necessarily be opinion evidence.
Sometimes an expert may be giving what is simply factual evidence devoid of
any opinion. An example of this was R. v. Collins where the witness testied
regarding a simple experiment he conducted to determine the path of a bullet
red from a certain location.
A useful fourfold division of the possible species of expert evidence has been
suggested as follows:
e traditiona l role of the expert witness is to express an opinion predicated
upon facts in evidence.
In some case s, that opinion is pre dicated upon fac ts personally o bserved by
the expert. Having testi ed to those facts, the expert then states his opinion
based upon those facts. Normal ly, such experts as ngerprint exam iners, bal-
listics specialists , and the physician who has actually treated the person who is
R. v. D.D., []  S.C.R.  at para. .
For statements of this principle and ex amples, see R. v. Graat, []  S.C.R. ,
regarding lay opin ion by police ocers as to the accus ed’s state of intoxication, and R. v.
Hill (),  C.C.C. (d)  (Ont. C.A.), regarding lay opi nion as to the similar ity of
shoe treads.
R. v. Graat, ibid. at .
(),  C.C.C. (d)  (Ont. C.A .).
See also R. v. A.K. and N. K., above note  at para. . e distinc tion is discussed at
length in Robert B . White, e Art of Using Ex pert Evidence (Aurora, ON: Cana da Law
Book, ) at –. In United States v. Norris,  F.d  (th Cir. ), evidence
related to “experimental ” burning of , cash to refute accu sed’s explanation for
missing money.
Melvin B. Lewis, “e Ex pert Witness in Crimina l Cases: General Considerations”
(undated, unpublished pap er) at –, reproduced with permission in Ma rk J. Mahoney,
“Materials on Exa mination of the Expert Witnes s” (Ontario Criminal Law yers’ Associa-
tion for the Program “Ex perts and Junk Science,” Toronto,  April ).
Chapter : The Admissi bility of Expert Opini on Evidence
the subject of his testimony, oer expert opinions based upon their personal
observations .
In other cases, the exper t has no personal knowledge of the facts. e facts
are placed in evidence by those who observe them. e expert then takes the
stand and is confronted with a hypothetica l question in which he is asked
to assume the truth of the facts submitted by others and to oer an expert
opinion, based upon the hypothesis that those facts a re true. For example, the
opinion of most pathologists testify ing for criminal defendants will be based
entire ly upon f acts ob served by other s. e cor pse is u sual ly no long er avai lable
for examination at the time the defense patholog ist is consulted. Accordingly,
he is not asked to make his own exami nation; instead, he is asked to make an
independent evaluation of the facts reported by the prosecution’s pathologist
(and possibly other sources). If those facts lead him to a dierent conclusion
from that expressed by the prosecution’s expert, he may then testify hypothet-
ically to that conclusion. us a defense pathologi st, operating on the assump-
tion that the prosecution expert who performed the autopsy properly observed
and recorded the condition of the corpse, may expre ss a conclusion concern-
ing time or cause of death which is completely dierent from the conclusion
reached by the prosecution pathologist.
Admixtures are, of course, readily conceivable. It is quite possible that an
expert may testify to cer tain facts personally observed by him a nd then in re-
sponse to a hypothetical question which postu lates additional facts, render an
opinion which is based upon the sum of all of those fa cts.
A third area of the proper employment of expert witnesses does not requi re
that the witness be furn ished any facts whatever. Instead, the witness, as a n ex-
pert, testies to the validity of certain principles commonly accepted by prac-
titioners within his discipline. us, a cr iminal defendant may call an expert
pathologist to testify that certain types of examinations are not only routine
in autopsy procedures, but indispensable if cause of deat h is to be determined
accurately. e signicance of his testimony will probably lie in the fa ct that
those examinations were not made by the prosecution’s pathologist. But the
“pure” expert, who testies to abstract principles and ig nores the facts of the
case, is not concerned with what the exam ining experts actua lly did. He draws
no conclusions concerning the facts of the case. He merely states, a s an abstract
proposition, that unless certain principles are obser ved or certain procedures
followed, va lid results wi ll not be obtaine d.
ere is yet a fourth role which an expert may play in some cases: that of
the expert observer. is w itness oers no opinions of any kind, except to the
extent that every factu al assertion may philosophically b e regarded as a matter
of opinion. e ha llmark of this ex pert is that his obser vation is one which a

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