3-1©2011, 2008 Pearson Education, Inc. Upper Saddle River, NJ 07458

FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

PHYSICAL EVIDENCE

Chapter 3

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

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.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

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.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Examples 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 matter

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Examples of Physical Evidence

Impressions – Tire markings, shoe prints, depressions in soft soils, and all other forms of tracks. Bitemarks in skin or food, are also included.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Examining Physical Evidence

• The examination of physical evidence is done for two reasons: identification or comparison.

• Identification determines the physical or chemical identity

• Comparison subjects a suspect specimen and a standard/reference specimen to the same tests to show origin

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Identification• Identification determines the physical or chemical

identity as best as existing analytical technology will allow.

– Identification requires that the tests establish the identify a substance to exclude all other substances.

Examples: Identifying the composition of a drug Identifying blood or hair from species of origin (human,

dog, cat?)

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Evidence IdentificationThe crime laboratory is frequently asked to identify: •the chemical composition of an illicit drug.•gasoline in residues recovered from arson debris•explosive residues from bombs•blood, hair, or wood and include the species origin.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Comparison• A comparative analysis determines whether a

suspect specimen and a standard/reference specimen are the same

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

• The forensic comparison is actually a two-step procedure:

– Step One: Compare the standard/reference to the unknown specimen.

– Step Two: Give a conclusion about where the specimen came from.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Role of Probability

• Value of comparison depends on the probability of matching unknown specimens with known examples (controls/standards)

• Probability is the frequency of occurrence of an event.

Ex: When flipping a coin,

probability is easy to establish

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Classifying Characteristics

• Individual Characteristics

– Evidence that can be linked to a single source with an extremely high degree of probability.

• Class Characteristics

– Evidence linked to only a group of like objects.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Individual Characteristics

• Examples of Individual Characteristics: – the matching ridge characteristics of two

fingerprints– the comparison of random striation markings on

bullets or tool marks– the comparison of handwriting characteristics– the comparison of irregular and random wear

patterns in tire or footwear impressions– 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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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Class Characteristics

• Class characteristics are associated with only a group (such as a manufacturing lot) and not a single source (such as one individual).

• Due to mass manufacturing matching to a specific company or lot is difficult.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Determine whether the evidence is more likely to possess individual or class characteristics

a. Impression from an automobile tire

b. A fingerprint

c. A spent bullet cartridge

d. Pieces of a shredded document

e. Commercial potting soil

f. DNA

g. Fragments of silver paint recovered from a Honda Accord

Individual or Class Charact.

a.Class characteristic

b.Individual characteristic

c.Individual characteristic

d.Individual characteristic

e.Class characteristic

f.Individual characteristic

g.Class characteristic

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Crossing Over• Evidence that changes from class to individual Examples: - Number of striations are necessary to individualize a

mark to a single tool.- Performing DNA profiling on a hair sample

• These are all questions that are addressed in court .

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

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/crime scene.

• Someone can also be exonerated (or excluded from suspicion) if the physical evidence collected at a crime scene is found to be different from the reference samples collected from that suspect.

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FORENSIC SCIENCE: An Introduction, 2nd ed.By Richard Saferstein

Forensic Databases• The Integrated Automated Fingerprint Identification System

(IAFIS), a national fingerprint and criminal history system maintained by the FBI.

• The Combined DNA Index System (CODIS) enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles.

• The National Integrated Ballistics Information Network (NIBIN) allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings.

• The International Forensic Automotive Paint Data Query (PDQ) database contains chemical and color information pertaining to original automotive paints.

• SICAR (shoeprint image capture and retrieval) is a shoeprint database.