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Legal vs. Scientific Questions
PHYSICAL EVIDENCE
Questions Science May Answer
What crime was committed?
How was the crime committed?
“Who dunnit?
dunnit?”
Where is the true “scene of the crime”
crime”?
When was the crime committed in time?
The law must establish that a crime has
been committed by defining the corpus
delecti (body of the crime)
All elements that legally define any
particular crime must be present to
proceed
The forensic scientist must translate the
legal question into a scientific question
Question Science Can’
Can’t Answer
Why was the crime committed?
The Framework
Determine the legally relevant question
Determine the assumptions
Analyze the evidence to provide facts
about the question
Make an inference based on the
assumptions & facts
Fundamental Principles
Transfer
Locard Exchange Principle
Identification
Individualization
placing objects in a class
narrowing the class to one
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Probative Value of Evidence
Fundamental Concepts of Forensic Science
Association
Reconstruction
linking a person with a crime scene
understanding the sequence in which
events occurred
Strength vs. Significance
Strength of an association depends only on a
pinhole view of the evidence
can determine an exclusive relationship between a
finger and an object on which a fingerprint is found
Principle of Divisible Matter
Significance of an association uses a widewide-angle
lens & includes the circumstances of the crime
a fingerprint on the outside of a stolen car is
meaningless
a fingerprint on the steering wheel of the stolen car is
highly probative
Creates physical traits not present in the
original undivided material
These new traits may be used to associate
the two separated items at a later time
When force is applied, matter divides
When matter is divided into smaller units,
it will
Process of Division
Crucial to articulate the significance of
analytical similarities
This is probably the most frequent
contention between experts in the
courtroom
retain characteristics of the larger piece
acquire characteristics created by the process
of division
Process of Division
New traits created at
the fracture boundary
Boundary roughness is
a natural consequence
Boundary surfaces
similar not completely
complementary
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Principle of Divisible Matter
Corollary 1
Some characteristics retained by smaller pieces are
unique to the original item or to the division process
Principles of Divisible Matter
Corollary 3
useful for individualizing all pieces to the original
Corollary 2
Some characteristics retained by the smaller pieces
are common to the original item as well as to other
items of similar manufacture (class characteristics)
Every time an object comes into contact
with another object, it either leaves a
portion of itself or takes a portion of the
other object with it
confounds the attempt to infer a common source
useful for associating pieces to each other
The Locard Exchange Principle
Some characteristics from the original item
will be lost or changed during or after the
moment of division and subsequent dispersal
Transfer
Physical movement of material from one
place to another
1 mm fiber transferred from a sweater (its
source) to a sweatshirt (its destination) during
contact
Every contact leaves its trace
Transfer
No physical movement of
material
a pattern formed when an
object makes an impression
on some receptive substrate
shoe stepping in mud (no
detectable amount of leather
left by shoe
The principles of divisible matter and
transfer are the two fundamental scientific
principles upon which the forensic analysis
of physical evidence are based
Division & transfer of matter occurs before
and during the crime
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Mechanism of Transfer Example
Mechanism of Transfer Example
9 a.m. -- hair falls from my head to floor
2 p.m. -- stranger walks across floor &
hair picked up on shoe
7 p.m. -- hair is removed from shoe in an
investigation
Future time -- comparison of hair on shoe
& reference hair from my head shows
them to be indistinguishable
Might infer that I and the person wearing
the shoe were in contact
A WRONG INFERENCE
Mechanism of Transfer Example
Scenario
PrePre-pubescent girl asserts that her uncle
that lives in the same house molested her.
After event, she took a shower. The towel
she used is collected as evidence. Pubic
hair is present on the towel. Comparison
of the evidence hair and reference hairs
from the uncle show them to be
indistinguishable.
Mechanism of Transfer Example
What is the significance of this
comparison?
How strong is the association?
Mechanism of Transfer Example
Mechanism of Transfer Example
Method of hair examination used
microscopic comparison
DNA comparison
Since the uncle lives in the house & uses
that bathroom
Inference of common source is strong
Significant likelihood of finding his pubic hair
on a towel in that bathroom
Association is strong
Significance of the association is minimal
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Physical Evidence
Types of Physical Evidence
Almost anything can be physical evidence.
The purpose of recognizing physical
evidence is so that it can be collected and
analyzed.
It is difficult to ascertain the weight a
given piece of evidence will have in a case
Ultimately the weight will be decided by a
jury.
Blood, semen, and saliva
Documents
Drugs
Explosives
Fibers
Fingerprints
Firearms and ammunition
Glass
Hair
Impressions
Organs and physiological fluids
The examination of physical evidence by a
forensic scientist is usually undertaken for
identification or comparison purposes.
Identification : the determination of the physical
or chemical identity of a substance with as near
absolute certainty as existing analytical
techniques will permit.
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 chemical composition of an illicit drug.
Identify gasoline in residues recovered
from the debris of a fire
Identify the nature of explosive
residues—
residues—for example, dynamite or TNT.
Identification of blood, semen, hair, or
wood
The object of an identification is to determine the
physical or chemical identity with as much certainty as
existing analytical techniques will permit.
The process of identification first requires the
adoption of testing procedures that give characteristic
results for specific standard materials.
Once these test results have been established, they
may be permanently recorded and used repeatedly to
prove the identity of suspect materials.
Second, identification requires that the number and
type of tests needed to identify a substance be
sufficient to exclude all other substances.
Comparison
Common Types of Identification
Paint
Petroleum products
Plastic bags
Plastic, rubber, and other
polymers
Powder residues
Soil and minerals
Tool marks
Vehicle lights
Wood and other vegetative
matter
Identification
Purpose of Examining Physical Evidence
A comparative analysis has the role of determining
whether or not a suspect specimen and a
standard/reference specimen have a common origin.
origin.
Both the standard/reference and the suspect specimen
are subject to the same tests.
The forensic comparison is actually a twotwo-step
procedure.
First, combinations of select properties are chosen from the
suspect and the standard/reference specimen for comparison.
Second, once the examination has been completed, the forensic
scientist must be prepared to render a conclusion with respect to
to
the origins.
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Role of Probability
To comprehend the evidential value of a
comparison, you must appreciate the role that
probability has in determining the origins of two
or more specimens.
Probability is the frequency of occurrence of an
event.
In flipping a coin, probability is easy to establish.
With many analytical processes exact probability
is impossible to define.
Classifying Characteristics
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.
characteristics.
Individual Characteristics
In all cases, it is not possible to state with
mathematical exactness the probability
that the specimens are of common origin.
It can only be concluded that this
probability is so high as to defy
mathematical calculations or human
comprehension.
Individual Characteristics
Examples:
Class Evidence
Class Characteristics
Surprising 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.
The high diversity of class evidence in our
environment makes their comparison very
significant in the context of a criminal
investigation.
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
Matching sequentially made plastic bags by striation
marks running across the bags
Weakness of forensic science is the inability
of the examiner to assign exact or even
approximate probability values to the
comparison of most class physical evidence.
What is the probability that a nylon fiber
originated from a particular sweater, or that a
paint chip came from a suspect car in a hit
and run?
There are very few statistical data available
from which to derive this information
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Class Evidence
Forensic scientists must create and update
statistical databases for evaluating the
significance of class physical evidence.
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 is its
ability to provide corroboration of events
with data that are free of human error and
bias.
Forensic Databases
• The Integrated Automated Fingerprint Identification System
(IAFIS), a national fingerprint and criminal history system
maintained by the FBI.
• The Combined DNA Index System (CODIS) enables federal, state,
and local crime laboratories to electronically exchange and compare
DNA profiles.
• The National Integrated Ballistics Information Network (NIBIN)
allows firearm analysts to acquire, digitize, and compare markings
made by a firearm on bullets and cartridge casings.
• The International Forensic Automotive Paint Data Query (PDQ)
database contains chemical and color information pertaining to
original automotive paints.
• SICAR (shoeprint image capture and retrieval) is a shoeprint
database.
Class Evidence
The chances are low of encountering two
indistinguishable items of physical evidence
at a crime scene that actually originated
from different sources.
When one is dealing with more than one
type of class evidence, their collective
presence may lead to an extremely high
certainty that they originated from the same
source.
The contribution of physical evidence is
ultimately determined in the courtroom.
Using Physical Evidence
Class Characteristic Example
Cocaine in a container
can be identified as
cocaine by its
chemical properties
These properties are
shared by all samples
of the same molecular
structure
As the number of different objects linking an
individual to a crime scene increases, so does
the likelihood of that individual’
individual’s involvement
with the crime.
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.
Class Characteristic Example
Cocaine from Columbia looks just like
cocaine from any other source
Sometimes (particularly in drug cases)
identification of the evidence item is all
that is required
The source of the cocaine is not important
its possession is illegal
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Another Example
White cotton fibers
are so common that
they will be present in
almost all trace
evidence collection
Fiber can be identified
as cotton via
polarizing light
microscope
Cotton Example
Origin from a specific source can’
can’t be
determined without looking at some nonnoncotton characteristic such as trace
elements
Discovery of fibers that fall into the class
of white cotton not very useful
Last example
A bullet can be
identified as .22
caliber by
measurement
May be isolated to a
specific manufacturer
by its markings
Bullet Example
Manufacture usually produces millions of
rounds of such ammunition
can’
can’t distinguish one round from another by
looking only at caliber
Firearms examiner would look at
individualizing characteristic such as bullet
striae to increase the probative value
Crossing Over
Crossing over the line from class to individual
does not end the discussions.
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?
Limitations
The strength of the evidence is an
inverse function of its limitations
These are all questions that defy simple answers
and are the basis of legal arguments.
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Questions Setting Limitations
What traits are being examined?
What distinguishes this item from others
in the same class?
How many different kinds of traits exist &
which of these have been examined?
How much variability might exist within a
single individual?
How much variability exists in the
population?
The Role of Forensic Science
Limitations
The more common the observed traits,
the less significant the similarity becomes
Insensitivity of detection method
Deteriorated state of sample
inability to observe a difference that exists
obliteration of a difference that actually
existed
Limitations
Limitations
The amount of information that can be
derived about the sources of the carpet is
limited
by the type of examination performed
the nature of the evidence
Provide scientific information to the legal
community
Done via reports and testimony
A statement of the limitations on
conclusions drawn from an experimental
result is an essential element
Analyst examines the chemical & physical traits
of an evidence fiber
These traits are similar to fibers from a
reference carpet in the suspect’
suspect’s house
Analyst knows that thousands of yards of that
carpet were installed in homes
The evidence fiber is consistent with the
reference carpet and the other thousands of
carpets
Natural vs. Evidential Limits
There are practical limits to the properties and
characteristics the forensic scientist can select for
comparison.
Modern analytical techniques have become so
sophisticated and sensitive that natural
variations in objects become almost infinite.
Carrying natural variations to the extreme, no
two things in this world are alike in every detail.
Evidential variations are not the same as
natural variations.
Distinguishing variations of evidential use from
natural variations is not always an easy task.
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Responsibility of the Analyst
May conclude that an evidence object & a
reference object show similar traits
Must understand and communicate the
limitations inherent in the conclusion
limitations are a combined function of
the nature of the traits
the sensitivity and resolution of the detection
methods
the state of the evidence
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