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1. What is physical evidence?

2. How is physical evidence different from testimonial evidence?

Examining Physical Evidence

Cornell NotesPage 22

EQ: What is the difference between class and

individualized evidence?

Physical Evidence The purpose of recognizing physical

evidence is so that it can be collected and analyzed.

It is difficult to determine the weight a given piece of evidence will have in a case as ultimately the weight will be decided by a jury.

The Purpose of Physical Evidence

The examination of physical evidence by a forensic scientist is usually undertaken for identification or comparison purposes.

Physical Evidence Testing Identification is the use of scientific

testing to determine the identity of a substance.

A comparison analysis subjects a suspect specimen and a standard/reference specimen to the same tests and examinations for the ultimate purpose of determining whether or not they have a common origin.

Examples of Identification The crime laboratory is frequently requested to

identify the chemical composition of an illicit drug.

It may be asked to identify gasoline in residues recovered from the debris of a fire, or it may have to identify the nature of explosive residues—for example, dynamite or TNT.

The identification of blood, semen, hair, or wood are also very common and, as a matter of routine, include a determination for species origin.

Comparative Analysis A comparative analysis has the

important role of determining whether or not a suspect specimen and a standard/reference specimen have a common origin.

Both the standard/reference and the suspect specimen are subject to the same tests.

Classifying Characteristics Individual Characteristics

• Evidence that can be associated to a common source with an extremely high degree of probability is said to possess individual characteristics.

Class Characteristics• Evidence associated only with a group is

said to have class characteristics.

Classifying Characteristics Class—common to a group of objects or

persons

Individual—can be identified with a particular person or a single source

Individual Characteristics

You cannot ever say with 100% certainty that two specimens are from a common source.

However, you can give probabilities that defy human comprehension.

Individual Characteristics

Examples: • the matching ridge characteristics of two

fingerprints• the comparison of random striation markings

on bullets or tool marks• the comparison of irregular and random wear

patterns in tire or footwear impressions• the comparison of handwriting

characteristics• the fitting together of the irregular edges of

broken objects in the manner of a jigsaw puzzle

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Class vs Individual Evidence

Which examples do you think could be individual evidence?

Class Characteristics Surprising to the inexperienced forensic

scientist is the frequent inability of the laboratory to relate physical evidence to a common origin with a high degree of certainty.

Evidence is said to possess class characteristics when it can be associated only with a group and never with a single source.

Can still be very significant in a case.

Class Characteristics Most items of physical evidence

retrieved at crime scenes cannot be linked definitively to a single person or object.

The value of class physical evidence lies in its ability to provide corroboration of events with data that are, as nearly as possible, free of human error and bias.

Individualized or Class?• How many striations are necessary to

individualize a mark to a single tool and no other? • How many color layers individualize a paint chip

to a single car? • How many ridge characteristics individualize a

fingerprint?• How many handwriting characteristics tie a

person to a signature?

These are all questions that defy simple answers and are the basis of arguments in forensic science.

Using Physical Evidence As the number of different objects linking an

individual to a crime scene increases, so does the likelihood of that individual’s involvement with the crime.

Just as important, a person may be exonerated or excluded from suspicion if physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject

Take 10 Stretch

Write your questions and summary for your notes.