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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

KNOWN AND QUESTIONED ITEMS – The original sources of questioned evidence are unknown – The origins of known evidence are known ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

RELATIONSHIPS AND CONTEXT ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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.

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? ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.

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 ©2010 Elsevier, Inc.