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Unit 2 Evidence Targets-By the end of this chapter you will be able to:
summarize Locard’s exchange principle identify four examples of trace evidence distinguish between direct and
circumstantial evidence summarize the seven steps of a crime-
scene investigationsketch a crime scene to scale with
measurementsDistinguish between direct and indirect
graphical relationships.
Forensic Science: Fundamentals & Investigations, Chapter 2 4
Principle of ExchangePrinciple of Exchange
Dr. Edmond Locard, director of the world’s first forensic lab (1910, Lyon, France), established the idea of the exchange principle; namely that:
1. When a person comes in contact with an object or another person, a cross-transfer of physical material can occur.
2. Study of the material can determine the nature and duration of the transfer.
Forensic Science: Fundamentals & Investigations, Chapter 2 5
Types of Evidence Types of Evidence
Direct evidence Testimony by a
witness about what that witness saw,
heard, or did
Forensic Science: Fundamentals & Investigations, Chapter 2 6
Types of Evidence Types of Evidence
Circumstantial evidence
• also called Indirect
• trace evidence
Physical evidence
• e.g., fingerprints
Biological evidence
• e.g., blood or hairs
Forensic Science: Fundamentals & Investigations, Chapter 2 7
Types of Evidence Types of Evidence
Class Individual
Identifies group of person/things
Identifies specific person/thing
e.g. Blood Type e.g. fingerprint
Forensic Science: Fundamentals & Investigations, Chapter 2 8
Types of EvidenceTypes of Evidence
Explain why this would be or would not be trace evidence
Soils, Minerals, Wood, and Other Vegetative Matter.
• Location • Link suspect• Trace Evidence
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Firearms and Ammunition • Circumstances of discharge• Link to suspect• Link to weapon• Link to victim
bsapp.com
Forensic Science: Fundamentals & Investigations, Chapter 2 24
The Seven The Seven SS’s of Crime Scene ’s of Crime Scene Investigation Investigation
1. Secure the scene2. Separate the witnesses3. Scan the scene4. Seeing the scene5. Sketch the scene 6. Search for evidence7. Secure the collected evidence
Forensic Science: Fundamentals & Investigations, Chapter 2 27
Analyze the EvidenceAnalyze the Evidence
Lab results can: o Show reliability of witness accountso Establish the identity of suspects or victimso Show suspects to be innocent or link them
with a scene or victim
Forensic Science: Fundamentals & Investigations, Chapter 2 28
. . . . . . . . . . . . . . . . . . . . . . . . . . . . SummarySummary . . . . . . . .
o Locard’s exchange principle: • Contact between people and objects can
transfer material that can determine the nature and duration of the transfer
o Evidence can be direct or indirect • Physical or biological traces
o Investigations consist of recognizing, documenting, and collecting evidence.
Graph Relationships
o When you look for patterns in the data that you have graphed
o There are 2 different patterns you may find:• direct• inverse
Forensic Science: Fundamentals & Investigations, Chapter 2 29
Direct relationship
• Relationship Statement:
When the independent variable is at its highest, the dependent variable is at its highest
Forensic Science: Fundamentals & Investigations, Chapter 2 30
Inverse Relationship
• Relationship Statement:
When the independent variable is at its highest, the dependent variable is at its lowest
What if no relationship is shown?
• If the graph looks like
• Relationship Statement: When the independent variable increases, the dependent variable doesn’t change