Chapter 3: Physical Evidence

Physical Evidence

encompasses any and all objects that can establish that a crime has been committed or can provide a link between a crime and its victim or a crime and its perpetrator, almost anything can be physical evidence

Types of Physical Evidence

* Blood, semen, and saliva, documents, drugs, explosives, fibers, fingerprints, firearms and ammunition, glass, hair, impressions, organs and physiological fluids, paint, petroleum products, plastic bags, plastic, rubber, and other polymers, powder residues, soil and minerals, tool marks, vehicle lights, wood and other vegetative matters

Purpose of examining physical evidence

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

identification

Determining the physical or chemical identity of a substance

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

Common types 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, blood semen, hair, or wood are very common

Comparative Analysis

Determines whether or not a suspect specimen and a standard/reference specimen have a common origin

Forensic comparison

First, combinations of select properties are chosen from the suspect and the standard/reference specimen for comparison. \n Second, once the examination has been completed, the forensic scientist must be prepared to render a conclusion with respect to the origins.

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Role of Probability

Probability is the frequency of occurrence of an event, probability is difficult to to define in analytical processes

individual characteristics

Evidence that can be associated with a common source with an extremely high degree of probability

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
* Matching sequentially made plastic bags by striation marks running across the bags

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Class Characteristics

Evidence associated only with a group

crossing over

Crossing over the line from class to individual does not end the discussions

Natural VS. Evidential Limits

There are practical limits to the properties and characteristics the forensic scientist can select for comparison. Evidential variations are not the same as natural variations, distinguishing variations of evidential use from natural variations is not an easy task

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

Exoneration from suspicion

If physical evidence collected at a crime scene is found to be different from standard/reference samples collected from that subject

Forensic Databases

IAFIS, CODIS, NIBIN, PDQ, SICAR

IAFIS

Integrated Automated Fingerprint Identification System

CODIS

Combined DNA Index System used by law enforcement to electronically identify DNA samples from a crime scene

NIBIN

National Integrated Ballistics Information Network, allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings

PDQ

International Forensic Automotive Paint Data Query, contains chemical and colour information pertaining to original automotive paints

SICAR

Shoeprint Image Capture and Retrieval, shoeprint database