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Chapter 3 - Physical Evidence
Transcript of Chapter 3 - Physical Evidence
What is evidence?
Something that tends to disprove or establish a fact.
Two types of Evidence
Eye witness accounts provide important evidence
Eye witnesses Heavily influence juries
Fear and stress involved in witnessing a crime can either sharpen the senses or confuse them
But are they accurate?
New information affects eye witness accounts
Can change memory of witness, even those trying to be fair or honest
Time between the crime and questioning of witness can affect what they remember
Encounter too brief
Presence of weapon diverted witnesses attention
Points to consider in Reliability of Eye Witness Accounts
Type of crime and how witness saw it
Research shows witnesses are better at remembering certain characteristics (hair color and sex) than others (age, height ,and specific race)
If witness is physically similar to offender, they will give a more accurate account
Victims of serious crimes sometimes have more accurate memory over long periods of time
Presence of weapon=sharpen sense/awareness
Some types of witnesses are better or worse at remembering than others
Learning disabilities and mental disorders
Alcohol and drugs
Interviewing Techniques or How information is retrieved can make a difference in accuracy of witness’s account
Witnesses are better at answering questions about what happened than answering questions regarding description of offender
Open-ended questions get better answers
Careful choice of words influence memory
Ex. Ask witness to describe what defendant was wearing rather than what color was his shirt
What would be a bad question?
Statement made under oath
What is said in court by a competent witness
Also called direct evidence or prima facie
Other important factors when weighing an eye witnesses account
Relationship of witness with the accused
How much time passed between offense and identification
Whether the witness already identified the (or failed to identify) the defendant
Whether the witness has already identified someone else
****most known cases of an innocent person being convicted happened because of a mistaken eye witness
National organization dedicated to exonerating wrongfully convicted people through DNA evidence
Eye witness misidentification is the number one cause of wrongful convictions nationwide
As of 2014, the Innocence Project exonerated 318 wrongfully convicted people
Tangible items that tend to prove some material fact
Aka real evidence
Can be any material or object
Take any form
Large as a building
Fleeting as an odor
Small as a hair
Microscopic as DNA
Much more reliable than testimonial evidence
Determining origin of a substance almost always involves a comparison of object or substance itself with something similar or with something similar that the scientist knows the origin of
Compare with a known or “control”
Types of Physical Evidence
Trace evidence refers to physical evidence that is found in small but measurable amounts, such as strands of hair, fibers, or skin cells.
Can be easily changed or lost
Usually observed by first officer on scene and must be recorded at that time
Perfume, cigarette smoke, gas
Coffee pot, car hood, water in bath tub, dead body
Footprints in sand, fingerprints in dust, teeth marks in perishable food
Produced by a specific action or event at the scene
Must be observed and recorded
Position of body
Position of furniture
Evidence that does not prove or disprove a fact in question
Evidence providing only a basis for inference about a disputed fact
May prove something like the possession of controlled substances or driving under the influence
Evidence based on suggestion rather than personal knowledge
Implies a fact or event without actually proving it
The more circumstantial evidence there is, the greater it weighs
Probability and statistics important
Blonde hair found in hand of murder victim with black hair
Size 10 sneaker print near the body
Both of these limit the pool of suspects
Material that can be related to a single source
Individualization always involves a comparison
Narrows an identity to a single person or thing
Some physical evidence
Piece of glass that fits another piece like a jigsaw puzzle
Material that can be associated with a group of items that share properties or characteristics
Object is similar to group of similar objects but not one single object
Narrows an identity to group of persons or things
We can use some individualization such as length, style, brand, shade, surface treatment but there are still thousands of jeans like that
Probability and Class Evidence
A young person is seen leaving the CHS student parking lot last week after being near a car with a broken window. The car’s CD player is missing. The suspect was identified as having blonde hair and having top with a hood, jeans, and boots. In a school of 1,900 students, how common are these characteristics?
How many students would be expected to be wearing a hood on any given day? Our class has __ students, __ are wearing hoods. How many students in the school are likely to be wearing hoods?
# in class wearing hoods/# of students in class=?
How many students is ___% of the whole student body?
Decimal form of percent x 1,900 students=____students
So if our class is a representative of the whole school, then you expect ____students to be wearing hoods today. Is this good evidence? Can you do better?
How many students would be wearing jeans? How many students in our class have jeans on today?____
What is the percentage of students in our class with jeans on?
# of students in jeans/# of students in class=?
How many students in school would be wearing jeans?
How many students is ___% of the whole student body?
Decimal form of percent x 1,900 students=____students wearing jeans
Is this good evidence? Could we do better?
How many students are wearing jeans and a hood?
(decimal) of hoods x (decimal) of jeans pants= ____ (decimal)
Multiply above decimal by 100% to get # of students in class wearing hoods and jeans
Now multiply the above # (decimal form) by 1,900 students in school
This gives us number of students in school wearing both jeans and hoods
We have narrowed the field immensely by just 2 general pieces of class evidence
Now determine how many students would be likely to have blonde hair.
# of blonde hair in class/# of students in class= # in class with blonde hair
How many students in school with brown hair?
Above decimal x 1900 students=# of students in school with brown hair
How many students with a hood, jeans, and blonde hair?
Hood decimal x jeans x blonde hair x 1900 students=____ (decimal)
So how many in whole student body meet all those?
Above decimal x 1900=___ students
We have narrowed 1900 suspects to ____ students.
How can four pieces of class evidence affect probability of nailing down one?
How many students with boots?
# of students in class with boots/# of students in class=____(decimal)
How many students in school with boots?
Above decimal x 1900 students=____students with boots
How many students in school likely to be wearing hoods, jeans, have blonde hair and boots?
(decimal hoods) x (decimal jeans) x (decimal of blonde hair) x (decimal of boots) x 1900 students=___students!
Probative: (supplying proof or evidence)
A way to increase probative value of class evidence is to find as many different types of objects as possible with which to link the suspect to the crime or the victim.
Soil and red paint on jeans each alone belong to a large class of material
But both occurring together increases the probability of linking the jeans with a certain crime even though the evidence is circumstantial