PHYSICAL EVIDENCE

Chapter 3

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

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.
• 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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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 matter

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

​

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

​

​

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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

​

​

​

​

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

​

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

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.
• 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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

​

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein

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.

3-*

PRENTICE HALL

©2008 Pearson Education, Inc.

Upper Saddle River, NJ 07458

FORENSIC SCIENCE

An Introduction

By Richard Saferstein