The Nature of Evidence Chapter 3 ©2010 Elsevier, Inc

The Nature of EvidenceChapter 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


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


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


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


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


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



– 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



– 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


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 quality-centered protocols regarding handling and packaging of evidence


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


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


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


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


KNOWN AND QUESTIONED ITEMS

– The original sources of questioned evidence are unknown

– The origins of known evidence are known


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



– 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




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


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


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


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


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


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©2010 Elsevier, Inc.


– 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?



• 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


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


Documents

The Nature of Evidence Chapter 3 ©2010 Elsevier, Inc