AJ150 Chapter 3

CriminalisticsCriminalisticsAn Introduction to Forensic ScienceAn Introduction to Forensic Science

CHAPTER

Criminalistics: An Introduction to Forensic Science, 11eRichard Saferstein

Copyright © 2015 by Pearson Education, Inc.All Rights Reserved

ELEVENTH EDITION

Physical Evidence

3



Physical EvidencePhysical Evidence• Physical evidence encompasses any and all

objects that can establish that a crime has or has not been committed or can provide a link between a crime and its victim or a crime and its perpetrator.

• If the investigator cannot recognize physical evidence or cannot properly preserve it for laboratory examination, no amount of sophisticated laboratory instrumentation or technical expertise can salvage the situation.



Physical EvidencePhysical 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.



Physical EvidencePhysical 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 EvidenceTypes of Physical Evidence• Blood, semen, and saliva• Documents• Drugs (including drug paraphernalia)• Explosives (pre and post blast materials)• Fibers (natural and synthetic)• Fingerprints (latent and visible)• Firearms and ammunition• Glass• Hair (animal and human)



Types of Physical EvidenceTypes of Physical Evidence

• Impressions (tire, shoe, fabric, bite marks)• Organs and physiological fluids• Paint (liquid or dried)• Petroleum products• Plastic bags• Plastic, rubber, and other polymers• Powder residues (gunshot residues)• Serial numbers (all stolen property)• Soil and minerals



Types of Physical EvidenceTypes of Physical Evidence• Tool marks• Vehicle lights (headlights and taillights)• Wood and other vegetative matter



Purpose of Examining Physical Purpose of Examining Physical EvidenceEvidence

• 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.



Purpose of Examining Physical Purpose of Examining Physical EvidenceEvidence

• 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.



IdentificationIdentification 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.



Common Types of IdentificationCommon Types of Identification• The crime laboratory is frequently requested to

identify the chemical composition of an illicit drug. (i.e. cocaine, meth, heroin)

• 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).



Common Types of IdentificationCommon Types of Identification• The identification of blood, semen, hair, or wood

are also very common and as a matter of routine, would include a determination for species origin.

• Note: a substance can be identified in one test or a series of tests to arrive at an identification. Analysts have no control over the quality and quantity of evidence received.



ComparisonComparison• 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.



ComparisonComparison

• 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.



Role of ProbabilityRole 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.



Classifying CharacteristicsClassifying Characteristics

Individual vs. Class 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.



Individual CharacteristicsIndividual 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.



Individual CharacteristicsIndividual 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



Individual CharacteristicsIndividual Characteristics

• Examples: 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



Class CharacteristicsClass Characteristics

• Class Characteristics Evidence associated only with a group is said

to have class characteristics.

Probability is the determining factor.



Class EvidenceClass 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?



Class EvidenceClass Evidence• 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.

• One of the primary endeavors of forensic scientists must be to create and update statistical databases for evaluating the significance of class physical evidence.



Class EvidenceClass 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.



Class EvidenceClass 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.



In The CourtroomIn The Courtroom• The contribution of physical evidence is

ultimately determined in the courtroom by the jury.

• Scientifically evaluated evidence is weighted heavily among jurists.

• It is impossible to determine at the crime scene what evidence will ultimately be crucial to a case.



Crossing OverCrossing 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?



Crossing OverCrossing Over• These are all questions that defy simple answers

and are the basis of arguments.

• It is the forensic scientist’s job as many characteristics to compare as possible and determine their significance based on quality/composition of the evidence, the case history, and the examiner’s experience.



Natural vs. Evidential LimitsNatural 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 even 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.



Natural vs. Evidential LimitsNatural vs. Evidential Limits Distinguishing variations of evidential use from

natural variations is not always an easy task.



Using Physical EvidenceUsing 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 DatabasesForensic Databases• The crime laboratory is on the forefront of the

investigation seeking to identify perpetrators.



Forensic DatabasesForensic Databases• The Integrated Automated Fingerprint

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

• Operational in 1999.• Contains fingerprints and access to corresponding

criminal history information for nearly 75 million subjects.





Forensic DatabasesForensic Databases• The Combined DNA Index System (CODIS)

enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles.

• Fully operational in 1998.• Creates investigative leads from three indexes:

the forensic, offender, and arrestee indices.



Forensic DatabasesForensic Databases• 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.

• Integrated Ballistic Identification System (IBIS) captures images of bullets or cartridge casings. Gives list of potential candidates to the examiner to manually compare.



Forensic DatabasesForensic Databases• The International Forensic Automotive Paint

Data Query (PDQ) database contains chemical and color information pertaining to original automotive paints. Contains information about make, model, year, and assembly plant on more than 13,000 vehicles w/ a library of more than 50,000 layers of paint.

• SICAR (shoeprint image capture and retrieval) is a shoeprint database. Images are entered by digital camera or scanner. Includes 22,000 footwear entries.

