Criminalistics and Crime Analysis Notes Chapters 3-6
Chapter 3 Physical Evidence
It would be impossible to list all the objects that could conceivably be of importance to a crime.
Almost anything can be physical evidence.
Although you cannot rely on a list of categories, it is useful to discuss some of the most common types
of physical evidence.
The purpose of recognizing physical evidence is so that it can be collected and analyzed.
It is difficult to ascertain the weight a given piece of evidence will have in a case as ultimately the weight
will be decided by a jury.
Types of Physical Evidence
-Blood, semen, saliva
-Firearms and ammunition
-Organs and physiological fluids
Purpose of Examining Physical Evidence
The exam of physical evidence by a forensic scientist is usually undertaken for ID or comparison
purposes. ID has as its purpose the determination of the physical or chemical identity of a substance with as near
absolute certainty as existing analytical techniques will permit.
A comparison analysis subjects a subject specimen and a standard/reference specimen to the same tests
and exams for the ultimate purpose of determining whether or not they have a common origin.
The objective of an ID is to determine the physical or chemical identity with as near absolute certainty as
existing analytical techniques will permit.
The process of ID first requires the adoption of testing procedures that give characteristic results for
specific standard materials
Once these test results have been established, they may be permanently recorded and used repeatedly
to prove the ID of suspect materials
Common Types of ID
The crime lab is frequently requested to ID the chemical composition of an illicit drug
It may be asked to identify gasoline in residues recovered from debris of fire
A comparative analysis has the role of determining whether or not a suspect specimen and a standard
specimen have a common origin
Both are subjected to same tests
Role of Probability
To comprehend the evidential value of a comparison, one must appreciate the role that probability has
in ascertaining the origins of 2 or more specimen
Probability – frequency of occurrence of an event
In flipping a coin, probability is easy to establish
With many analytical processes, exact probability is impossible to define
Evidence that can be associated to a common source with an extremely high degree of probability is said
to possess individual characteristics In all cases, it is not possible to state with mathematical exactness the probability that the specimens
are of common origin
It can be only concluded that this probability is so high as to defy math calculations or human
-Random striation marks on bullets or tool marks
-Irregular and random foot patterns
-Putting pieces together in the manner of a jigsaw puzzle
Evidence associated only with a group
Surprising to the inexperienced forensic scientist is the frequent inability of the lab to relate physical
evidence to a common origin with a high degree of certainty
Class Evidence(Problems and Weaknesses)
Inability of the examiner to assign exact or even approximate probability values to the comparison of
the most class physical evidence
For example, what is the probability that a nylon fiber originated for a particular sweater, or that a paint
chip came from a suspect in a hit and run
There are very few statistical data available from which to derive this information
The value of class evidence lies in its ability to provide corroboration of events with data that are, as
nearly as possible, free of human error and bias
When one is dealing with more than 1 type of class evidence, their collective presence may lead to an
extremely high certainty that they originated from the same source
Finally, the contribution of physical evidence is ultimately determined in the courtroom
Crossing over the line from class to individual does not end the discussions
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?
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.
Integrated Automated Fingerprint ID System
Combined DNA Index System
National Integrated Ballistics Info Network
International Forensic Automotive Paint Data Query
Shoeprint Image Capture and Retrieval
This field involves the investigation of sudden, unnatural, unexplained, or violent deaths
The primary role of the medical examiner is to determine the cause of death
If a cause cannot be found through observation, an autopsy is normally performed to establish the cause
Forensic Pathologist = Medical Examiner
Manner of death (5 categories)
Estimating Time of Death Rigor Mortis – immediately after death, muscles relax and then become rigid, sets within 24 hours and
disappears within 36 hours
Livor Mortis – human hearts s