Transcript The Nature of Evidence Chapter 3

The Nature of Evidence
Chapter 3
©2010 Elsevier, Inc.
INTRODUCTION
• Evidence is critical to a trial
– Foundation for the arguments attorneys plan to
offer
– Impartial, objective information that leads a judge
or jury to their conclusions
– Much goes into getting evidence ready for court
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WHAT IS EVIDENCE
• In a legal case, whoever determines guilt or
innocence is called the trier-of-fact
– Uses information or evidence to determine if
statements made by witnesses are true
– Evidence is information
• Personal testimony, documents, material objects
produced to make a fact or proposition more or less
likely
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KINDS OF EVIDENCE
– Most evidence is real
• Generated as a part of the crime and recovered at the
scene (i.e. hairs, fingerprints, paint, blood, shoeprints)
– Some evidence is created to augment or explain
real evidence
• Diagrams, computer simulations, demonstrations are
examples of such evidence
• This evidence is termed demonstrative evidence
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LEVELS OF EVIDENCE
– Some items of evidence have more
importance than others
• Context of crime, type, amount, and quality of
evidence dictate what can be determined and
interpreted
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FORENSIC SCIENCE IS HISTORY
• Events under scrutiny have already occurred
• Forensic scientists assist the investigation
through the analysis of physical remains of the
criminal activity
– Examine proxy data, or remnants of events
– Sort through all items at a crime scene to find the
few items of evidence that will help reconstruct
the crime
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THE BASIS OF EVIDENCE: TRANSFER
AND PERSISTENCE
• When two things come into contact, information is
exchanged (Locard’s Exchange Principle)
– Locard’s theory says that exchange of information occurs
even if the results are not identifiable or too small to be
found
– All evidence is transfer evidence
– A number of conditions affect amounts of evidence
transferred
•
•
•
•
•
Pressure
Number of contacts
How easily the item transfers material
Form of evidence
How much of the item is involved in the contact
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THE BASIS OF EVIDENCE:
TRANSFER AND PERSISTENCE
– Evidence transfer from
a source to a location
with no intermediaries
is a direct transfer
– Evidence transfer
involving one or more
intermediate objects in
an indirect transfer
• May be complicated
and potentially
misleading
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THE BASIS OF EVIDENCE:
TRANSFER AND PERSISTENCE
– Persistence describes the
duration in which evidence
remains in a location before
it is further transferred,
degraded, or collected as
evidence
– Persistence depends on a
number of conditions
• Type of evidence
• Location
• Environment surrounding
evidence
• Time from transfer to
collection
• Activity of or around the
evidence location
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CONTAMINATION
– Undesired transfer of information between items
of evidence in considered contamination
• When possible every item of evidence should be
packaged separately
• Contamination can be limited through properly
designed facilities, protective clothing and qualitycentered protocols regarding handling and packaging of
evidence
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IDENTITY, CLASS, AND
INDIVIDUALIZATION
– All things are unique in space and time
• Identification is the examination of the
chemical and physical properties of an object
and using them to categorize the object as a
member of a group
• A class is a group of objects that share similar
characteristics
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IDENTITY, CLASS, AND INDIVIDUALIZATION
• The concept of a common
source depends on the
material in question
– Mode of production
– Specificity of the
examination used to classify
the object
• Individualization of an
object means that it can be
classified into a group with
only one member
– Object is unique
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Individualization of Evidence
– The concept of individualization rests on two
assumptions: all things are unique in space and
time and the properties by which a thing is
classified are constant over time
– Assumption of uniqueness of space is inherently
non-provable
• Casework is not research
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Individualization of Evidence
– Forensic science is relegated to making
interpretive statements based on statistical
methods
– Uncertainty is everywhere
• Statistics help to describe the uncertainties in the real
world of criminal activities with varying physical objects
– Forensic scientists recognize the complexity of
their evidence and consequently adjust their
methods
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KNOWN AND QUESTIONED ITEMS
– The original sources of questioned evidence are
unknown
– The origins of known evidence are known
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RELATIONSHIPS AND CONTEXT
– Relationships between people, places and things
involved in crimes are critical to deciding what to
examine and how to interpret the results
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RELATIONSHIPS AND CONTEXT
– If two things, which have previously never been in
contact with each other, have items on them
which are analytically indistinguishable at a
certain class level, the question of coincidental
association arises
• Important to establish context of the crime and those
involved
• Helps to determine what evidence is significant,
methods to be most effective for collection or analysis,
and what may be ignored
– Using context for direction prevents the
indiscriminate collection of items
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RELATIONSHIPS AND CONTEXT
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COMPARISON OF EVIDENCE
– Two fundamental processes in the analysis of
evidence: identification and comparison
• Identification is the process of discovering physical and
chemical characteristics of evidence with an eye toward
putting it into successively smaller classes
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COMPARISON OF EVIDENCE
• Comparison is done to try to establish the source of evidence
– Questioned evidence is compared with objects from a known source
– Examine for common physical and/or chemical characteristics that
exist between both samples
– If common characteristics exist, an association, dependant on a
number of factors, can be made
» Kind of evidence
» Intra- and inter-sample variation
» Amount and location of evidence
» Transfer and cross transfer
» Number of different kinds of evidence associated to one or more
sources
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COMPARISON OF EVIDENCE
• Individualization occurs when at least one unique
characteristic is found to exist in both the known and
questioned samples
– Not accomplished by identification alone
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CONTROLS
– Controls are materials whose source is known and
which are used for comparison with unknown
evidence
• Used to determine if chemical tests are performed
correctly
• Used to look for interference
• Controls may be positive and negative
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CONTROLS
• A positive control is expected to give a positive result with
the test reagents and serves to show that the test is
working properly
• A negative control is expected to give a negative result with
the test reagents
• A false positive conclusion results when a test is
determined to be positive and it is incorrect
– May occur if a negative control is not used
– Type I error
– A person could be falsely accused
• A false negative conclusion results when a test is
determined to be negative and it is incorrect
– May occur if a positive control is not used
– Type II error
– A person could be falsely exonerated
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ANALYSIS OF EVIDENCE: SOME
PRELIMINARY CONSIDERATIONS
• Scientific method is the process of proposing and
refining plausible explanations about any
unknown situation
– Ask and answer questions and then draw conclusions
from the answers
• Questions must be testable
• Experiments or tests must be repeatable
– Questions to be tested are called hypotheses
• Data is collected to shed light on the hypothesis
• Data is examined to determine what value it has in proving
or disproving the hypothesis; this is the probative value
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ANALYSIS OF EVIDENCE: SOME
PRELIMINARY CONSIDERATIONS
– Preliminary questions must be answered before
hypothesis is formulated
• Sufficient material to analyze?
• How was evidence sampled?
• In what order should the evidence be analyzed?
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ANALYSIS OF EVIDENCE: SOME
PRELIMINARY CONSIDERATIONS
• How is a large
amount of
evidence
sampled?
• Do multiple types
of evidence need
to be examined?
Disease diagnosis, trace evidence,
DNA, questioned documents, ink
analysis, hand-writing, latent
fingerprints, content analysis
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CHAPTER SUMMARY
• Context of evidence is central to how it is
analyzed and interpreted in reconstruction of
criminal events
• Scientific method applies to forensic science
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