Types of Evidence
2
1
Objectives
You will understand:
the value of direct and indirect evidence in a court of law
that eyewitness accounts have limitations
what physical evidence can and cannot prove in court
that the forensic scientist’s main goal is to find a
unique source for the evidence
2
Evidence
There are two basic types of
evidence: testimonial and
physical
testimonial evidence is a
statement made under oath
also called “direct”
evidence or “prima facie”
evidence
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Evidence
physical evidence is any object or material relevant
in a crime
also called “real” evidence
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Testimonial or Direct Evidence
eyewitness accounts are
an example here
problems here? you tell
me...
5
action hero
You and your table buddy
list as many problems as
you can regarding
eyewitness accounts
What could make them
be inaccurate?
6
problems with eyewitness testimony
crime scene too dark
age of witness
encounter too brief
mental state of witness
stress factors
leading questioning techniques
errors in memory
relationship b/t witness and
accused
time between crime and
questioning
et cetera...
new info clouds memory
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8
Case Study
2.2: Ronald
Cotton
(no “case
study,” but will
be on quiz)
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Physical Evidence
This is what we are most interested in.
We look for physical and chemical
properties in the physical evidence
physical properties: color, density,
solubility, refractive index
things that help ID something
without changing the object
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Physical Evidence
chemical properties: formation of
bubbles, color change, pH change...
things that help ID something
and change that thing
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ALERT!
The next slides have many
types of evidence
the evidence you are
looking at may be in more
than one category! all is
well!!! :)
(most = low priority)
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Low
some evidence is
fleeting, or transient
odor—putrefaction, perfume, gasoline, urine, burning, explosives,
cigarette or cigar smoke
temperature—surroundings, car hood, coffee, water in a bathtub, cadaver
imprints and indentations— footprints, teeth marks in perishable foods,
tire marks on certain surfaces
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Low
pattern evidence is produced by direct contact
between a person and an object or between
two objects.
mostly in the form of imprints, indentations,
striations, markings, fractures, or deposits
Blood spatter
Glass fracture
Fire burn pattern
Furniture position
Projectile trajectory
Tire marks or skid marks
Clothing or article
distribution
Gunpowder residue
Material damage
Body position
Toolmarks
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Low
conditional evidence is something at the scene that had better be
recorded right away
Light—headlight, lighting conditions,
lights on or off
Smoke—color, direction of travel,
density, odor
Fire—color and direction of the
flames, speed of spread,
temperature and condition of fire
Location—of injuries or wounds, of
bloodstains, of the victim’s vehicle,
of weapons or cartridge cases, of
broken glass
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Low
conditional evidence is something at the scene that had better be
recorded right away
Vehicles—doors locked or
unlocked, windows opened or
closed, radio off or on, odometer
mileage
Body—position and types of
wounds; rigor, livor, and algor
mortis
Scene—condition of furniture,
doors and windows, any
disturbance or signs of a struggle
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Low
transfer evidence is produced by contact between person(s) and object(s),
or between person(s) and person(s).
associative evidence is something that may associate a victim or suspect
with a scene or with each other; e.g., personal belongings.
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classification of evidence by its nature
Biological—blood, semen, saliva,
sweat, tears, hair, bone, tissues,
urine, feces, animal material, insects,
bacteria, fungi, botanical material
Chemical—fibers, glass, soil,
gunpowder, metals, minerals,
narcotics, drugs, paper, ink,
cosmetics, paint, plastic, lubricants,
fertilizer
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classification of evidence by its nature
Physical—fingerprints, footprints,
shoeprints, handwriting, firearms, tire
marks, toolmarks, typewriting
Miscellaneous—laundry marks, voice
analysis, polygraph, photography,
stress evaluation, psycholinguistic
analysis, vehicle identification
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indirect evidence or circumstantial evidence, does not prove a fact,
but something may be inferred from it
difference?
Testimony that the witness saw the defendant shoot the victim
gives direct evidence. A forensic scientist who testifies that
ballistics proves the defendant’s firearm killed the victim gives
circumstantial evidence, from which the defendant’s guilt may be
inferred.
Similarly, a witness who testifies that she watched the defendant
stab the victim gives direct evidence. A witness who says that she
saw the defendant enter a house, that she heard screaming, and
that she saw the defendant leave with a bloody knife gives
circumstantial evidence.
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why physical evidence is so important
can prove that a crime took place (e.g. incendiary device at a fire scene)
can support or refute witness testimony (e.g. is that bloodstain really
from the witness?)
can link a suspect to the crime scene (e.g. broken glass on suspect’s
clothing)
can ID someone associated with crime (e.g. DNA)
can help investigators recreate crime scene (e.g. blood splatter)
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Your main role as forensic scientist?
See if there is a link between
evidence, victim, crime scene, and
suspect.
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quick quiz
1. T/F Eyewitness testimony is considerably more reliable than physical
evidence.
2. What kind of evidence (physical or chemical) can be used without
changing the evidence itself?
3. Odor, temperature, imprints, indentations are all what special type of
evidence (here now, gone in a few minutes)
4. There are four classes of evidence classified by their nature. What are the
four types?
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another role is to reconstruct a
crime scene
you may have to use the physical
evidence to show:
what happened
how the victim was killed
how many people were involved
when it happened
combined with witness accounts, the crime may be reconstructed
watch how many times this year that the detectives, prosecution, and
defense will reconstruct what they think happened based on the
evidence!
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often there is a questioned or unknown
sample (Q) to be compared to with a
known sample (K)
e.g. Did the paint on the suspect’s
bumper (Q) match the victim’s
bike (K)?
the control samples play a huge role
in making a conclusion
e.g. If Q and K and dozens of
other paints (controls) are heated
to a certain temperature and Q
and K change color but all the
controls do not, Q might be the
same as K
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HIGH
individual vs class evidence
two types of physical evidence: individual or class
individual evidence is nailed to a single source
DNA, fingerprints, broken glass pieces that match perfectly
class evidence has characteristics of a group of items
a red shirt, a footprint of a particular size
individual can ID an individual,
class can narrow to a group but not individual
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These fibers are class evidence;
there is no way to determine if
they came from this garment.
They may be “consistent with”
the fabric, but could be from
another piece.
The large piece of glass fits
exactly to the bottle; it is
individual evidence.
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Activity 2.2
Can the following evidence be individualized?
Number 1-12 and write for each class or individual
(Warning: There may be some “...unless, …” exceptions)
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cloth is class; BUT if tear marks match
exactly it can be individualized
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class; unless it was one of a
kind linked to a specific owner
by e.g. serial number
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individual if the pieces can
be matched, otherwise class
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class
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individual if the blood (DNA) or fingerprints can be linked to culprit
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class, unless there are
imperfections that can be
linked to a specific tire
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bullets can be individualized by
striations, the casings can be
individualized by firing pin marks
(more on that later)
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class; no
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class
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class; unless you can get
the DNA off of it
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class, unless you can match it to tear patterns from a suspected matchbook
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if you do enough analysis of
the soil you MIGHT be able to
individualize
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HIGH
p 43
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HIGH
white shirt ➡ 0.21 = 21%
white shirt and blue jeans ➡ 0.21 x 0.36 = 7.6%
white shirt & blue jeans & light brown hair ➡ 0.21 x 0.36 x 0.15 = 1.1%
white shirt & blue jeans & light brown hair & dark-colored athletic shoes ➡
0.21 x 0.36 x 0.15 x 0.12 = 0.13%
Therefore one can assume that of all 1600 students at the school, 0.13%
of those 1600 are likely to be wearing all that...
That is only about… 2 students.
Lesson learned? The more circumstantial evidence
you can collect the more you can narrow down
the list of suspects.
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Forensic Investigations, then,...
...include some or all of these seven major
activities:
1. Recognition—the ability to distinguish
important evidence from unrelated material
Pattern recognition
Physical property observation
Information analysis
Field testing
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2. Preservation through the collection and proper packaging of evidence
3. Identification using scientific
testing
Physical properties
Chemical properties
Morphological (structural)
properties
Biological properties
Immunological properties
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4. Comparison of class characteristics measured against those of known
standards or controls; if all measurements are equal, then the two
samples may be considered to have come from the same source or origin
5. Individualization in demonstrating that the sample is unique, even among
members of the same class
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6. Interpretation—giving meaning to all the
information
7. Reconstruction of the events in the case
Inductive and deductive logic
Statistical data
Pattern analysis
Results of laboratory
analysis
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quick quiz
1. What is the difference between K and Q in speaking of samples?
2. What kind of evidence can be used to nail an individual?
3. What kind of evidence at best can narrow it down to a group but not an
individual?
4. T/F Since class evidence is the weaker of the two, it is rarely used to
prove anything.
5. Name all 7 of the Big Responsibilities of a forensic scientist.
48
all except: 9 & 10
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Case Study
FF18 The Disappearance of Helle Crafts
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