# Chapter 3: Physical Evidence

CHAPTER 3: PHYSICAL EVIDENCE 1. 2. 3. 4. 5. Learning Objectives: List the common types of physical evidence found at

crime scenes Explain the difference between the identification and comparison of physical evidence. Define individual and class characteristics. Give examples of physical evidence possessing these characteristics. Understand the value of class evidence to a criminal investigation. Explain the role the physical evidence plays in reconstructing the events surrounding the

I. List the types of physical evidence commonly found at crime scenes A. Blood, semen, saliva- V. Define individual and class characteristics. Give examples of physical evidence possessing these characteristics. A. Individual characteristics: properties of evidence that we can be highly certain came from a single source. Example: DNA, fingerprint

** can tell who the evidence came from** B. Class characteristics: properties of evidence that can only be associated with a group and never a single source. Example: blood type, hair VI. What value does class evidence have in a criminal investigation if we cant be certain exactly which person or object the item came from?

Class evidence (hair, fibers, etc) can Incriminating. For example: a fiber found a the scene matches a fiber from a carpet in the suspects home Having more than one type of class evidence makes it less likely that the evidence came from different sources.

Narrows down the possibilities help tie various parts of the investigation together Probability and Class Evidence A young person was seen leaving a high school parking lot after

having been near a car with a broken window; the cars CD player was missing. The suspect was identified as having light brown hair and wearing a white shirt, blue jeans, and darkcolored athletic shoes. In a school of 1600 students, how common are these characteristics? uestion 1: How many students would be expected to be wearing a white shirt n any given day? Lets say in a class of 33 students, 7 are wearing a white shirt ow may students in the school are likely to be wearing a white shirt? Question 2: How many student would be wearing blue jeans? In your class, yo count 12.

Question 3: Ask how many students are wearing a white shirt AND blue jeans: Now, multiply by the number of students in your school: We have narrowed the field a bit by just looking at two general pieces of class evidence. Question 4: Now determine how many students would be likely to have light brown hair. In your class you count five students with light brown hair:

How many students in school? How many students would be likely to be wearing a white shirt and blue jeans and to have light brown hair? So to determine how many students of the whole student body meet all those descriptors: Question 5: Now lets calculate how four pieces of class evidence could affect VII. What role does physical evidence play in

reconstructing the events surround a crime? Support or contradict witness or suspect testimony Helps establish a likely sequence of events (timeline)

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