chapter 3 physical evidence

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PRENTICE HALL ©2007 Pearson Education, Inc. Upper Saddle River, NJ 07458

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

Chapter 3PHYSICAL EVIDENCE

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.