Transcript Chapter03

Chapter 3
PHYSICAL EVIDENCE
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-1
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-2
Types of Physical Evidence
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Blood, semen, and saliva
Documents
Drugs
Explosives
Fibers
Fingerprints
Firearms and
ammunition
Glass
Hair
Impressions
Organs and physiological
fluids
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
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Paint
Petroleum products
Plastic bags
Plastic, rubber, and other
polymers
Powder residues
Soil and minerals
Tool marks
Vehicle lights
Wood and other
vegetative matter
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-3
Purpose of Examining Physical Evidence
• The examination of physical evidence by a
forensic scientist is usually undertaken for
identification or comparison purposes.
• Identification 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 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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-4
Identification
• The object of an identification is to determine the
physical or chemical identity with as near absolute
certainty as existing analytical techniques will permit.
– The process of identification 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 identity of suspect materials.
– Second, identification requires that the number and
type of tests needed to identify a substance be
sufficient to exclude all other substances.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-5
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-6
Comparison
• 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.
• The forensic comparison is actually a two-step
procedure.
– First, combinations of select properties are chosen
from the suspect and the standard/reference
specimen for comparison.
– Second, once the examination has been completed,
the forensic scientist must be prepared to render a
conclusion with respect to the origins.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-7
Role of Probability
• To comprehend the evidential value of a
comparison, one must appreciate the role
that probability has in ascertaining the
origins of two or more specimens.
• Simply defined, probability is the
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-8
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-9
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 only be concluded that this
probability is so high as to defy
mathematical calculations or human
comprehension.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-10
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
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-11
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.
• Here again, probability is a determining factor.
• Nevertheless, the high diversity of class
evidence in our environment makes their
comparison very significant in the context of a
criminal investigation.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-12
Class Evidence
• One of the current weaknesses of forensic
science is the inability of the examiner to assign
exact or even approximate probability values to
the comparison of most class physical evidence.
• For example, what is the probability that a
nylon fiber originated from a particular
sweater, or that a paint chip came from a
suspect car in a hit and run?
• There are very few statistical data available
from which to derive this information, and in a
mass-produced world, gathering this kind of
data is increasingly elusive.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-13
Class Evidence
• One of the primary endeavors of forensic
scientists must be to create and update
statistical databases for evaluating the
significance of class physical evidence.
• 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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-14
Class Evidence
• The chances are low of encountering two
indistinguishable items of physical evidence at
a crime scene that actually originated from
different sources.
• When one is dealing with more than one 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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-15
Crossing Over
• 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?
• These are all questions that defy simple
answers and are the basis of arguments.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-16
Natural vs. Evidential Limits
• There are practical limits to the properties and
characteristics the forensic scientist can select
for comparison.
– Modern analytical techniques have become
so sophisticated and sensitive that natural
variations in objects become almost infinite.
– Carrying natural variations to the extreme,
no two things in this world are alike in every
detail.
– Evidential variations are not the same as
natural variations.
– Distinguishing variations of evidential use
from natural variations is not always an easy
task.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-17
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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-18
Forensic Databases
• The Integrated Automated Fingerprint Identification System
(IAFIS), a national fingerprint and criminal history system
maintained by the FBI.
• TheCombined 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.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-19
Reconstruction
• The method used to support a likely sequence
of events by the observation and evaluation of
physical evidence, as well as statements made
by those involved with the incident, is referred
to as reconstruction.
• Crime-scene reconstruction relies on the
combined efforts of medical examiners,
criminalists, and law enforcement personnel to
recover physical evidence and to sort out the
events surrounding the occurrence of a crime.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-20
The Role of Physical Evidence
• The physical evidence left behind at a crime
scene plays a crucial role in reconstructing the
events that took place surrounding the crime.
• Although the evidence alone does not describe
everything that happened, it can support or
contradict accounts given by witnesses and/or
suspects.
• Information obtained from physical evidence
can also generate leads and confirm the
reconstruction of a crime to a jury.
• The collection and documentation of physical
evidence is the foundation of a reconstruction.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-21
Summing It Up
• Reconstruction is a team effort that involves
putting together many different pieces of a
puzzle.
• The right connections have to be made among all
the parts involved so as to portray the
relationship among the victim, the suspect, and
the crime scene.
• If successful, reconstruction can play a vital role
in aiding a jury to arrive at an appropriate
verdict.
• The recognition, collection, and analysis of
physical evidence is the foundation to successful
reconstruction, but only part of the process.
FORENSIC SCIENCE
An Introduction
By Richard Saferstein
PRENTICE HALL
©2008 Pearson Education, Inc.
Upper Saddle River, NJ 07458
3-22