Chapter 3 – Physical Evidence
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
List the common types of physical evidence
encountered at crime scenes
Explain the difference between the identification and
comparison of physical evidence
Define individual and class characteristics
Discuss the value of class evidence to a criminal
investigation
Explain the purpose physical evidence plays in
reconstructing the events surrounding the commission
of the crime scene.
Common Types of Physical
Evidence
1.
Blood, semen, and saliva - identity and
possible origin
a.
b.
2.
Documents – authenticity or source
a.
b.
3.
Liquid or dried
Human or Animal
Handwriting
Typewriting
Drugs –
a.
Any seized substance
Common Types of Physical
Evidence
4.
Explosives
a.
b.
5.
Fibers – transfers establish relationships
a.
b.
6.
Devices containing explosive charge
Objects suspected to contain residues of explosive
Natural
Synthetic
Fingerprints
a.
b.
Latent
Visible
Common Types of Physical
Evidence
7.
Firearms & Ammunition
a.
b.
8.
Glass
a.
b.
9.
Firearm
Discharged or intact ammunition
Glass particle or fragment
Windowpanes w/ holes made by bullet
Hair
a.
b.
Animal
human
Common Types of Physical
Evidence
Impressions
10.
a.
b.
c.
d.
e.
Tire markings
Shoe prints
Depressions in soft soils
Glove, fabric impressions
Bitemarks in skin, food
Organs and physiological fluid
11.
a.
b.
Test existence of drugs or poisons
Blood analyzed for alcohol or drugs
Paint
12.
a.
b.
Liquid or dried
Transferred from one surface to another
Common Types of Physical
Evidence
Petroleum products
13.
a.
b.
Gasoline – residues from arson
Grease and oil stains
Plastic Bags
14.
a.
Polyethelene disposable bags
Plastic, rubber, other polymers
15.
a.
Remnants of man-made materials
Powder Residues
16.
a.
Item suspected of containing firearm discharge residues
Common Types of Physical
Evidence
Serial numbers
17.
Stolen property for restoration of erased I.D. #
a.
Soil and Minerals
18.
a.
b.
Imbedded in shoes
Safe insulation in garments
Tool Marks
19.
a.
Screwdriver or crowbar impressed into another object
Vehicle Lights
20.
a.
Light on or off at time of impact
Common Types of Physical
Evidence
21.
Wood & other vegetative matter
a.
b.
c.
d.
e.
Wood
Sawdust
Shavings
Vegetative matter
On shoes, clothing, or tools
Identification
Ultimate Goal in identifying a specific physical or
chemical substance is exclude other substances

1.
Requires adoption of testing procedures that have
characteristic results for specific standard materials
a.
2.
Once established, procedure can be used repeatedly to prove
identity
Requires that # and type of tests needed to identify a
substance be:
a.
b.
Sufficient to exclude all other substances
I.E. eliminate all but one substance
Identification

Determination of physical or chemical identity
of a substance with as near absolute certainty as
possible
Illicit drug preperation – heroin, cocaine,
barbiturates
 Gasoline residue from debris of a fire
 Nature of explosive residues – dynamite or TNT
 Species of Origin – blood, semen, hair, wood

Identification
3.
Each type of evidence requires a different test;
each test has a different degree of specificity
a.
b.
c.
d.
e.
Substance A – one test
Substance B – 5 or 6 tests
No control over quality or quantity of specimens
Forensic scientist determines at what point analysis is
concluded
Has to be beyond reasonable doubt for court of law
Comparison

Suspect specimen and a standard/reference
specimen run through same tests, examinations
to determine common origin
Hairs at crime scene to hairs in suspect head
 Paint chip from victim’s clothing to car in hit-andrun
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Comparison
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Procedure:
1.
Combinations of select properties chosen from
suspect and standard/reference sample
a.
2.
Which properties and how many properties depends on
type of material (more details later….)
Once testing complete, forensic scientist concludes
on origins of specimen
a.
b.
One or more properties doesn’t agree – not same origin
All properties agree – is it always the same origin?
Comparison


Not necessarily always the same
Evidential value – probability in ascertaining
origins of two or more specimens

Probability
a.
b.

Frequency of occurrence of an event
Odds at which a certain event will occur
What does this mean?


Variation of characteristics of each specimen need to be
taken into account
Individual vs. Class Characteristics
Class Characteristics
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
The properties that all the members of a certain
group of objects or substances have in common
Frequent problem: inability of lab to relate
physical evidence to a common origin with a
high degree of certainty
Probability is a determining factor
Class Characteristics


Initial categories are broad and then narrowed down as
more information is obtained
Example
 Blue substance
 Blue paint
 Blue car paint
 Blue car paint from Ford Manufacturing Company
 Blue car paint from Ford produced 1998-2000
 Blue car paint from Ford produced 1998-2000, used
on Mustang and Explorer
Class Characteristics



The previous “paint” example was a refined
class evidence identification
The “paint” was identified by comparing its
class characteristics with those of known
standards or previously established criteria.
The “paint” is considered CLASS EVIDENCE
Other Examples…
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Single layered paint
Soil
Glass fragments too
small to fit back
together
Hairs
Fibers
Class Characteristics

Probability
Two paint chips with one layer of similar color
 Two paint chips with 7 similar paint layers, not all
part of car’s original color
 Which has high probability of same origin?

Class Characteristics

Blood – 2 samples, both human origin, both
type A
Type A is 26% occurrence in population – not
enough for comparison -BUT Use more blood factors to compare – probability
increases
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Blood proteins, growth factors, clotting factors, enzymes
Class Characteristics

Product Rule – calculates overall frequency of
occurrence in a population
Factor A % occurrence * Factor B % occurrence=
probability that both events will occur
simultaneously
 Applies when using factors that occur independently
of one another
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Class Characteristics

Product Rule Example:
Blood Factors
Frequency
A
26%
EsD1
85%
PGM2+22%
 Multipy (.26) (.85) (.02)= .44% or 1 in 200
 Probability that a blood sample will contain all three
types - .44% or 1 in 200 people
Value of Class Evidence
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Many lawyers try to discredit class evidence because it
cannot be limited to just one possible source
However Class Evidence DOES HAVE VALUE.
Look around room, most have different clothes on. If
I were to pick a fiber from one persons clothes in this
room, that fiber could possibly pick out the single
source or at least narrow the field
Some class evidence holds little forensic value such as
fiber from jeans or white cotton shirts…they are too
common
Individual Characteristics

Properties of evidence that can be attributed to
a common source with extremely high degree
of certainty

1.
2.
3.
Examples:
Matching ridge characteristics of 2 fingerprints
Comparison of random striation markings on
bullets or tool marks (figure 3-1)
Comparison of irregular & random wear patterns
in tire or footwear impressions
Individual Characteristics
4.
5.
6.
Handwriting
characteristics
Fitting together of
irregular edges of
broken pieces like a
jigsaw puzzle
Matching sequentially
made plastic bags by
striation marks running
across the bags
(figure3-2)
Individual Characteristics


Not possible to state with mathematical
exactness probability that specimens are of
common origin
Conclusion must be made by practical
experience of the examiner

probability of 2 individuals having same fingerprints
is 1 x 1060
Individual Characteristics
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Individual characteristics can be used to distinguish
members of the same class
Forensic scientists try to individualize a piece of
physical evidence by some type of comparison process
Only a few types of physical evidence (primarily
physical pattern evidence) can be truly individualized
The Ultimate Goal
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Ultimate goal would be to move all class
evidence to individualistic which is not very
likely
Hair is trying to make the move...something like
paint probably never will.
For human evidence to be individualistic, the
odds of two people matching the same piece of
evidence must be 1 in about 7.5 billion, which is
the population of the earth.
Class vs. Individual

When does evidence cross the line that
distinguishes class from individual evidence?
This is debated and disagreed among many forensic
scientists.
 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?

Class vs. Individual
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How many handwriting characteristics tie a person to a
signature?
Up to forensic scientist to find as many characteristics
as possible to compare one substance to another.
Significant to consider:
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Quality
Composition of evidence
Case history
Examiner’s experience
Functions of the Forensic Scientist
A.
Analysis of Physical Evidence
1. Apply scientific techniques to analysis
of evidence
2. Be aware of demands and constraints of
legal system
Functions of the Forensic Scientist
B. Provision of expert testimony
1. Expert witness – an individual whom the
court determines possesses knowledge
relevant to the trial that is not expected of the
average person
2. Competency shown by degrees held,
professional organizations belong to,
professional articles published, years of
experience, participation in special courses
Expert Witness
3. Opposing attorney may cross-examine the
witness & point out weaknesses in background
or knowledge
4. Laypersons testifying may not state their
opinion, but an expert witness can
Functions of the Forensic Scientist
C. Furnishing Training in the Proper
Recognition, Collection, & Preservation of
Physical Evidence
1. often have Evidence Technicians on 24
hour call to aid criminal investigations in
retrieving evidence
2. where patrol officers or detectives gather
the evidence, forensic scientist should train all
officers engaged in fieldwork