CHAPTER 3: PHYSICAL EVIDENCE 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 commission of a crime.

I. List the types of physical evidence commonly found at crime scenes A. Blood, semen, saliva-

V. Define individual and class characteristics V. Define individual and class characteristics. Give examples of physical evidence possessing these characteristics. 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 can’t be certain exactly which person or object the item came from? Class evidence (hair, fibers, etc) can help tie various parts of the investigation together Incriminating. For example: a fiber found a the scene matches a fiber from a carpet in the suspect’s home Having more than one type of class evidence makes it less likely that the evidence came from different sources. Narrows down the possibilities

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 car’s CD player was missing. The suspect was identified as having light brown hair and wearing a white shirt, blue jeans, and dark-colored athletic shoes. In a school of 1600 students, how common are these characteristics? Question 1: How many students would be expected to be wearing a white shirt On any given day? Let’s say in a class of 33 students, 7 are wearing a white shirt. How 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, you 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 let’s calculate how four pieces of class evidence could affect the

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)