The Admissibility of Expert Opinion Evidence
| Author | Alan D. Gold |
| Pages | 39-78 |
Chapter
3: The
Admissibility
of
Expert
Opinion
Evidence
No
Opinions
Except...
In the
ordinary course
of
things
in our
courtrooms, witnesses
may not
give
opinion evidence. They
can
only
testify
about
facts
within their
knowledge through observation
and
personal sensory experience.
This
is
a
basic tenet
of our law of
evidence.1
Witnesses
may
testify
as to
what they
saw or
heard, assuming that
what they
saw or
heard
is
relevant
to an
issue
in the
case
and
otherwise
admissible.
But
witnesses
may not
give evidence
as to
what they conclud-
ed,
inferred,
or
came
to
believe
as a
result
of
what they
saw or
heard.
It is
for
the
trier
of
fact
to
draw inferences
and
other opinions
or
conclusions,
not for any
witness.
This
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
main exception
to the
rule
against opinion evidence.
The
courts describe this admission
as
"excep-
tional"
and
based upon necessity:
1 R. v.
A.K.
and
N.K.
(1999),
27
C.R. (5th)
226 at
para.
71
(Ont. C.A.).
[39]
THREE:
THE
ADMISSIBILITY
OF
EXPERT OPINION EVIDENCE
[Expert
evidence
and its
associated dangers]
... are
tolerated
in
those
exceptional cases where
the
jury
would
be
unable
to
reach their
own
con-
clusions
in the
absence
of
assistance
from
experts with special
knowledge.2
The
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 apparent age,
the
identification
of
persons
or
things,
and
ordi-
nary,
everyday mental
or
emotional states such
as
intoxication, anger,
and
the
like.3
The
theory
is
that
in
such cases
"it is
virtually impossible
to
sep-
arate
the
witness'
inference
from
the
facts
on
which
the
inference
is
based
[and]
...
the
admission
of
opinion
evidence
... is
merely
a
compendious
way
of
ascertaining
the
result
of the
witness's
observations."4
Evidence
from
an
expert witness need
not
necessarily
be
opinion evi-
dence. Sometimes
an
expert
may be
giving what
is
simply factual evi-
dence devoid
of any
opinion.
An
example
of
this
was R. v.
Collins5
where
the
witness testified regarding
a
simple experiment
he
conducted
to
determine
the
path
of a
bullet
fired
from
a
certain
location.6
A
useful
fourfold
division
of the
possible species
of
expert evidence
has
been suggested
as
follows:7
The
traditional role
of the
expert witness
is to
express
an
opinion predicat-
ed
upon
facts
in
evidence.
In
some cases, that opinion
is
predicated upon
facts
personally observed
by
the
expert. Having testified
to
those facts,
the
expert then states
his
opinion based upon those facts. Normally, such experts
as
fingerprint
2
R. v.
para.
51.
3 For
statements
of
this principle
and
examples,
see R. v.
Graat,
ing lay
opinion
by
police
officers
as to the
accused's state
of
intoxication,
and R. v.
Hill
(1986),
32
C.C.C.
(3d)
314
(Ont.
C.A.),
regarding
lay
opinion
as to the
similarity
of
shoe
treads.
4 R. v.
Graat,
ibid,
at
823.
5
(2001),
160
C.C.C.
(3d)
85
(Ont. C.A.).
6
See
also
R. v.
A.K.
and
N.K.,
above note
1 at
para.
72. The
distinction
is
discussed
at
length
in
Robert
B.
White,
The Art of
Using
Expert Evidence (Aurora,
ON:
Canada
Law
Book,
1997)
at
18-26.
In
United
States
v.
Norris,
(5th Cir. 2000), evidence related
to
"experimental" burning
of
$500,000
cash
to
refute
accused's explanation
for
missing
money.
7
Melvin
B.
Lewis, "The Expert Witness
in
Criminal Cases: General Considerations"
(undated,
unpublished paper)
at
3-5, reproduced with permission
in
Mark
J.
Mahoney,
"Materials
on
Examination
of the
Expert Witness" (Ontario Criminal Lawyers Associa-
tion
for the
Program "Experts
and
Junk Science,"
Toronto,
5
April 1997).
[40]
EXPERT
EVIDENCE
IN
CRIMINAL
LAW
examiners, ballistics specialists,
and the
physician
who has
actually treated
the
person
who is the
subject
of his
testimony,
offer
expert opinions based
upon their personal observations.
In
other
cases,
the
expert
has no
personal
knowledge
of the
facts.
The
facts
are
placed
in
evidence
by
those
who
observe them.
The
expert then
takes
the
stand
and is
confronted with
a
hypothetical question
in
which
he
is
asked
to
assume
the
truth
of the
facts
submitted
by
others
and to
offer
an
expert opinion, based upon
the
hypothesis that those
facts
are
true.
For
example,
the
opinion
of
most pathologists
testifying
for
criminal
defen-
dants will
be
based
entirely
upon
facts
observed
by
others.
The
corpse
is
usually
no
longer available
for
examination
at the
time
the
defense pathol-
ogist
is
consulted. Accordingly,
he is not
asked
to
make
his own
examina-
tion; 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
different
conclusion
from
that expressed
by the
prosecution's expert,
he may
then
testify
hypothetically
to
that conclusion.
Thus
a
defense pathologist, operating
on the
assumption that
the
prosecu-
tion expert
who
performed
the
autopsy properly observed
and
recorded
the
condition
of the
corpse,
may
express
a
conclusion concerning time
or
cause
of
death which
is
completely
different
from
the
conclusion reached
by the
prosecution pathologist.
Admixtures
are,
of
course, readily conceivable.
It is
quite possible that
an
expert
may
testify
to
certain
facts
personally observed
by him and
then
in
response
to a
hypothetical question which postulates additional
facts,
render
an
opinion which
is
based
upon
the sum of all of
those facts.
A
third area
of the
proper employment
of
expert witnesses does
not
require
that
the
witness
be
furnished
any
facts
whatever. Instead,
the
wit-
ness,
as an
expert,
testifies
to the
validity
of
certain principles commonly
accepted
by
practitioners within
his
discipline.
Thus,
a
criminal 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
death
is to be
determined accurately.
The
significance
of his
testimony will
probably
lie in the
fact
that those examinations were
not
made
by the
pros-
ecution's pathologist.
But the
"pure"
expert,
who
testifies
to
abstract prin-
ciples
and
ignores
the
facts
of the
case,
is not
concerned with what
the
examining experts actually
did.
He
draws
no
conclusions concerning
the
facts
of the
case.
He
merely states,
as an
abstract proposition, that unless
certain principles
are
observed
or
certain procedures followed,
valid
results
will
not be
obtained.
[41]
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