 Page
 1.
21
What is physical evidence?
 2. How
is physical evidence different
from testimonial evidence?
Cornell Notes
Page 22
EQ: What is the difference
between class and individualized
evidence?
 The
purpose of recognizing physical
evidence is so that it can be collected
and analyzed.
 It
is difficult to determine the weight a
given piece of evidence will have in a
case as ultimately the weight will be
decided by a jury.
 The
examination of physical evidence by
a forensic scientist is usually undertaken
for identification or comparison
purposes.
 Identification
is the use of scientific
testing to determine the identity of a
substance.
 A comparison analysis subjects a suspect
specimen and a standard/reference
specimen to the same tests and
examinations for the ultimate purpose of
determining whether or not they have a
common origin.



The crime laboratory is frequently requested to
identify the chemical composition of an illicit
drug.
It may be asked to identify gasoline in residues
recovered from the debris of a fire, or it may have
to identify the nature of explosive residues—for
example, dynamite or TNT.
The identification of blood, semen, hair, or wood
are also very common and, as a matter of routine,
include a determination for species origin.
A
comparative analysis has the important
role of determining whether or not a
suspect specimen and a
standard/reference specimen have a
common origin.
 Both
the standard/reference and the
suspect specimen are subject to the same
tests.
 Individual
Characteristics
• Evidence that can be associated to a common
source with an extremely high degree of
probability is said to possess individual
characteristics.
 Class
Characteristics
• Evidence associated only with a group is said to
have class characteristics.

Class—common to a group of objects or
persons

Individual—can be identified with a
particular person or a single source
 You
cannot ever say with 100% certainty
that two specimens are from a common
source.
 However, you
can give probabilities that
defy human comprehension.
 Examples:
• the matching ridge characteristics of two
•
•
•
•
fingerprints
the comparison of random striation markings
on bullets or tool marks
the comparison of irregular and random
wear patterns in tire or footwear impressions
the comparison of handwriting
characteristics
the fitting together of the irregular edges of
broken objects in the manner of a jigsaw
puzzle
Which examples do you
think could be
individual evidence?
Kendall/Hunt Publishing Company
12
 Surprising
to the inexperienced forensic
scientist is the frequent inability of the
laboratory to relate physical evidence to a
common origin with a high degree of
certainty.
 Evidence
is said to possess class
characteristics when it can be associated
only with a group and never with a single
source.
 Can
still be very significant in a case.
 Most
items of physical evidence
retrieved at crime scenes cannot be
linked definitively to a single person or
object.
 The
value of class physical evidence lies
in its ability to provide corroboration of
events with data that are, as nearly as
possible, free of human error and bias.
• How many striations are necessary to
individualize a mark to a single tool and no
other?
• How many color layers individualize a paint chip
to a single car?
• How many ridge characteristics individualize a
fingerprint?
• How many handwriting characteristics tie a
person to a signature?
 These
are all questions that defy simple
answers and are the basis of arguments in
forensic science.
 As
the number of different objects linking
an individual to a crime scene increases, so
does the likelihood of that individual’s
involvement with the crime.
 Just
as important, a person may be
exonerated or excluded from suspicion if
physical evidence collected at a crime
scene is found to be different from
standard/reference samples collected from
that subject
 Stretch
 Write
your questions and summary for
your notes.