“The likelihood of showers Tuesday is 80%.”
“The odds of winning the championship is 24 to 1.”
𝑃(𝐴) =
# 𝑜𝑓 𝑜𝑢𝑡𝑐𝑜𝑚𝑒𝑠 𝑜𝑓 𝐴
# 𝑜𝑓 𝑡𝑜𝑡𝑎𝑙 𝑜𝑢𝑡𝑐𝑜𝑚𝑒𝑠
“The frequency of death in an auto accident is 0.000176.”
We deal with probability most every day in one form or another. The law does as well: “probable cause,”
“probative,” “probability of an accidental match,” “weight of evidence,” and “beyond a reasonable doubt” are all probability terms used in our legal system.
Probability Questions
1.
Usually, evidence is used to link or associate a suspect to a crime. More often than not, the evidence is class evidence. For example, a blue fiber is found at a crime scene. Look at your classmates; how many could have transferred a blue fiber from the clothes they are wearing? How many suspects, then, are in your class?
2.
Suppose the entire student body (1150 students) had access to the crime site. How many suspects would there be based on the statistics from your class? (Clearly show all relevant calculations!) Is the blue fiber evidence of value? That is, does it do a good job of reducing the number of suspects down to a small number?
3.
Suppose that along with the blue fiber, an orange fiber was also found. Does the combination of a blue fiber and an orange fiber improve the evidentiary value? Explain why. (Clearly show all relevant calculations!)
4.
Does the number of characteristics of a material and/or the number of different, relevant objects found at a crime scene improve the probability of matching the evidence to a single suspect? Why or why not?
5.
Camden has the following demographics:

Female residents 2206

Male residents 1787

Unmarried residents 1712

Asian residents 20

American Indian residents 2

Caucasian residents 3937 a.
A suspect is described as a male, Asian resident of Camden. How many people fit that profile?
Would that characterization be of value? b.
A suspect is described as a female, married, and Caucasian. How many people fit that profile?
Would that characterization be of value?

Class vs. Individual
Classify each piece of evidence as either class or individual. Provide an explanation/support for your answer.
1.
A torn T-shirt was found in the back seat of a suspect’s car. Two pieces of torn cloth were found at the scene of the crime. Can the two pieces be individualized to the T-shirt? Explain.
2.
A broken test tube was found at a crime scene. Would small glass fragments be considered class or individual evidence? Explain.
3.
A note was found at a crime scene at a bank. A notepad with a similar type of paper was found in the desk of the suspect. Can the note be traced to the pad? Explain.
4.
A shoe print was found at the scene of a hit and run automobile accident. How can a suspect’s shoe be individualized to match a print? Explain.
5.
Some powder was found in a plastic bag in a suspect’s pocket. Some similar powder was found on the victim. Can the first powder be individualized to the second powder? Explain. If the two powders were determined to be identical, does that prove they came from the same source?
6.
Cut pieces of paper.
7.
Torn pieces of paper.
8.
A piece of duct tape was taken from a victim’s body at the crime scene. A roll of duct tape was found in the suspect’s car. Can the duct tape be matched back to the roll? Explain.
9.
Striation marks on a bullet recovered at a crime scene are analyzed. Can the striation marks be individualized to the gun that fired the bullet?
10.
Hair samples at the crime scene compared to a suspect’s hair.