Chapter 3 PHYSICAL EVIDENCE

CRIMINALISTICS An Introduction to Forensic Science, 9/EBy Richard Saferstein

Chapter 3PHYSICAL EVIDENCE

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.

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

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

Chapter 3 PHYSICAL EVIDENCE

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