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Criminalistics: Physical Evidence (Chapter 3) - Video Notes Flashcards

Physical Evidence: Overview

  • 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.
  • Almost anything can be physical evidence.
  • 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.

Purpose of Examining Physical Evidence

  • The examination of physical evidence by a forensic scientist is usually undertaken for identification or comparison purposes.
  • 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

Types of Physical Evidence (1 of 3)

  • Blood, semen, and saliva
  • Documents
  • Drugs
  • Explosives
  • Fibers
  • Fingerprints
  • Firearms and ammunition
  • Glass

Types of Physical Evidence (2 of 3)

  • Hair
  • Impressions
  • Organs and physiological fluids
  • Paint
  • Petroleum products
  • Plastic bags
  • Plastic, rubber, and other polymers

Types of Physical Evidence (3 of 3)

  • Powder residues
  • Serial numbers
  • Soil and minerals
  • Tool marks
  • Vehicle lights
  • Wood and other vegetative matter

Identification

Identification (1 of 2)

  • 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.

Identification (2 of 2)

  • 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.

Comparison

Comparison (1 of 2)

  • 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 subjected to the same tests.

Comparison (2 of 2)

  • The forensic comparison is 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.

  • For clarity: the probability of a common origin can be expressed as
    P(A) = \frac{NA}{N} where $NA$ is the number of favorable outcomes (evidence consistent with a common origin) and $N$ is the total number of possible outcomes under the stated conditions.

  • In forensic contexts, exact numeric probabilities are often unavailable or impractical to determine; reasoning relies on the strength and coherence of the pattern of evidence rather than a single probability value.

Classifying Characteristics

Individual 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 Characteristics (1 of 3)

  • 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 (2 of 3)

  • 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 Characteristics (3 of 3)

  • 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

  • Class Characteristics – Evidence associated only with a group is said to have class characteristics.
  • Examples of evidence types with class characteristics include paint, fibers, glass, drugs, and items without unique distinguishing characteristics, such as some shoe or tire imprints.

Class Evidence

Class Evidence (1 of 4)

  • 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.
  • 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 (2 of 4)

  • 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.

Class Evidence (3 of 4)

  • 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 (4 of 4)

  • 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 significance of physical evidence is ultimately determined in the courtroom.

Crossing Over

Crossing Over (1 of 3)

  • Just when an item of physical evidence crosses the line that distinguishes class from individual is a difficult question to answer and is often the source of heated debate and honest disagreement among forensic scientists.
  • The chances are low of encountering two indistinguishable items of physical evidence at a crime scene that actually originated from different sources.

Crossing Over (2 of 3)

  • 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 Over (3 of 3)

  • These are all questions that defy simple answers.
  • The task of the forensic scientist is to find as many characteristics as possible to compare one substance with another.

Natural vs. Evidential Limits

Natural vs. Evidential Limits (1 of 2)

  • 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.

Natural vs. Evidential Limits (2 of 2)

  • 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

Forensic Databases (1 of 3)

  • Integrated Automated Fingerprint Identification System (IAFIS) is a national fingerprint and criminal history system maintained by the FBI.
  • Combined DNA Index System (CODIS) enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles.

Forensic Databases (2 of 3)

  • Genealogy databases, like GEDmatch, are of emerging use to identify close relatives using DNA profiles.
  • These databases contain samples that are processed by commercial genealogy companies and uploaded by private citizens.
  • 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.

Forensic Databases (3 of 3)

  • 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.