Transcript Document

Chapter 3
PHYSICAL EVIDENCE
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Physical Evidence
• It would be impossible to list all the objects
that could conceivably be of importance to a
crime.
• 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 as
ultimately the weight will be decided by a
jury.
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Types of Physical Evidence
• Blood, semen, and
saliva
• Documents
• Drugs
• Explosives
• Fibers
• Fingerprints
• Firearms and
ammunition
• Glass
• Hair
• Impressions
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Paint
Petroleum products
Plastic bags
Plastic, rubber, and
other polymers
Powder residues
Soil and minerals
Tool marks
Wood and other
vegetative matter
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Purpose of Examining Physical Evidence
• The examination of physical evidence by
a forensic scientist is usually undertaken
for identification or comparison
purposes.
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Identification
• The object of an identification is to determine
the physical or chemical identity with as near
absolute 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.
– Second, identification requires that the
number and type of tests needed to identify a
substance be sufficient to exclude all other
substances.
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Common Types of Identification
• 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.
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Comparison
• A comparative analysis has the important
role of determining whether or not a
suspect specimen and a standard or
reference specimen have a common
origin.
• Both the standard/reference and the
suspect specimen are subject to the same
tests.
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Comparison
• The forensic comparison is actually a
two-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 the origins.
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Role of Probability
• To comprehend the evidential value of a
comparison, one must appreciate the role
that probability has in ascertaining the
origins of two or more specimens.
• Simply defined, probability is the
frequency of occurrence of an event.
• The lower probability evidentiary item
has greater value in a court of law.
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Classifying Characteristics
• Individual Characteristics
– Evidence 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.
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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.
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Individual Characteristics
• 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
– matching sequentially made plastic bags by striation
marks running across the bags
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Class Characteristics
• Evidence is said to possess class characteristics
when it can be associated only with a group and
never with a single source.
• Here again, probability is a determining factor.
• Nevertheless, the high diversity of class
evidence in our environment makes their
comparison very significant in the context of a
criminal investigation.
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Class 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 lies in its
ability to provide corroboration of events with
data that are, as nearly as possible, free of
human error and bias.
• Finally, the contribution of physical evidence
is ultimately determined in the courtroom.
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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?
• These are all questions that defy simple
answers and are the basis of arguments.
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Using Physical Evidence
• 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.
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Reconstruction
• The method used to support a likely sequence
of events by the observation and evaluation of
physical evidence, as well as statements made
by those involved with the incident, is referred
to as reconstruction.
• Crime-scene reconstruction relies on the
combined efforts of medical examiners,
criminalists, and law enforcement personnel to
recover physical evidence and to sort out the
events surrounding the occurrence of a crime.
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The Role of Physical Evidence
• The physical evidence left behind at a crime
scene plays a crucial role in reconstructing the
events that took place surrounding the crime.
• Although the evidence alone does not describe
everything that happened, it can support or
contradict accounts given by witnesses and/or
suspects.
• Information obtained from physical evidence
can also generate leads and confirm the
reconstruction of a crime to a jury.
• The collection and documentation of physical
evidence is the foundation of a reconstruction.
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