1. 3-3 The Examination of Physical Evidence
Unit 3: Physical Evidence

2. Homework Questions
What is the process of determining a substance’s physical or chemical identity?
What is the process of determining whether two or more objects have a common origin?
What are class characteristics?

3. Physical Evidence
Physical evidence is examined by a forensic scientist for examination and comparison.
There are two methods of examination
Identification
Comparison

4. Identification
Identification is the process of determining a substance’s identity to the exclusion of all other substances.
By comparison to a known substance
Chemically
Physically

5. Comparison
A comparison analysis determines whether a suspect specimen and a standard/reference specimen have a common origin.
Micrograph of hair

6. Class Characteristics
Class characteristics are general characteristics that define a category of items or objects, but are not alone sufficient to define individuality.
These define the shoe, not the individual that wore them.
. An example would be a shoe print in the dirt. The print is lifted for comparison. There are hundreds of different shoes in the world, with hundreds of different sole patterns. Class characteristics allow the scientists to narrow the pattern down to specific shoes. The FBI has created a shoe and tire database for such purposes. Once the laboratory has narrowed the pattern to one shoe pattern, they are able to address the issue of whether or not it was the print found at the crime scene.

7. Individual Characteristics
Evidence that can be associated with a common source with an extremely high degree of probability is said to possess individual characteristics.
Fingerprints
Bullet striations from gun barrels
Individual characteristics – properties of evidence that can be attributed to a common source with a high degree of certainty. > Marks on an object produced by the random imperfections or irregularities on the surfaces of the tools used to manufacture the object. > Produced incidental to the manufacturing process and typically seen at microscopic level. > Can be produced on an object by use, abuse, and/or corrosion. > May be uniquely identifiable with a source. Examples: > Fingerprints – the odds of two people having the same prints are one out of 1 x 10 to the 60th power. This is greater than the known number of humans on the earth. > Bullet striations from gun barrels, tool marks, bite marks and irregular edges that fit together like a jigsaw puzzle. > Striated and/or impressed marks produced by the action of the tip of a screwdriver on an object.

8. Product Rule
The overall frequency of occurrence of an event or trait can be obtained by multiplying the frequencies of all independently occurring instances related to that event.
Heterochromia is a condition
best recognized by the
different coloring of one's eyes,
hair or skin. It is a great
example of examining the
frequency of a trait.