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Physical Evidence


Physical Evidence Chapter 3: Physical Evidence Examples, Identification, Comparison, Limitations, Value and Relevance Cautions and Limitations in Dealing with ... – PowerPoint PPT presentation

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Title: Physical Evidence

Physical Evidence
  • Chapter 3 Physical Evidence
  • Examples, Identification, Comparison,
    Limitations, Value and Relevance

Physical Evidence
  • Physical evidence is any and all objects that can
    establish that a crime has been committed or can
    provide a link between a crime and its victim or
    a crime and its perpetrator.

Physical Evidence
Common Types of Physical Evidence
  • Biological Blood, semen, and saliva
  • Documents handwriting, charred documents
  • Drugs Any substance seized in violation of the
    laws regulating the sale, manufacture,
    distribution and use.
  • Explosives Any device containing an explosive
    charge, as well as all objects removed from the
    scene of an explosion that may contain residues.
  • Fibers natural or synthetic
  • Fingerprints both latent (invisible) and visible
  • Firearms and ammunition any firearm, discharged
    or intact ammunition.

Common Types of Physical Evidence
  • Glass
  • Hair
  • Impressions
  • Organs and physiological fluids
  • Paint
  • Petroleum Products
  • Plastic bags

Common Types of Physical Evidence
  • Plastic, rubber other polymers
  • Powder residues
  • Serial Numbers
  • Soil and Minerals
  • Tool Marks
  • Vehicle Lights
  • Wood and Other Vegetative Matter

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

  • The process of determining a substances physical
    or chemical identity.
  • The crime laboratory is frequently requested to
    identify the chemical composition of an unknown

Examples of Identification
  • Identify the chemical composition of an illicit
    drug preparation that may contain heroin,
    cocaine, barbiturates, and so on.
  • Identify gasoline in residue recovered from the
    debris of a fire
  • Identify the nature of explosive residues,
    example dynamite or TNT.
  • Identification of blood, semen, saliva, hair and
    include determination for species origin.

  • The process of identification first requires the
    adoption of testing procedures that give
    characteristic results for specific standard
  • Once these test results have been established,
    they may be permanently recorded and used
    repeatedly to prove the identity of suspect

  • Second, Identification requires that the number
    and type of tests needed to identify a substance
    be sufficient to exclude all other substances.

Identification Putting it Together
  • A white powder gets submitted as physical
    evidence to a crime lab.
  • The forensic scientist must use standardized
    tests with known results to try to determine the
    chemical identity of the substance.
  • Suppose the white powder is identified as heroin.
  • The forensic scientist must then devise a
    specific analytical scheme that will eliminate
    all other possibilities of that substance being
    anything but heroin.
  • The forensic scientists test results must have
    been comprehensive enough to have excluded all
    other drugs or, for that matter all other
    substances from consideration.

Identification Putting it Together
Identification White Powder?
  • An officer in the field can use a simple test to
    see if the white powder might be heroin.
  • The white powder is submitted to a forensic
  • The forensic examiner then subjects the white
    powder to a series of preliminary standardized
    tests that have known results to try to confirm
    the identity of the substance.
  • Once a possible identity is found then the
    forensic examiner MUST perform a battery of tests
    to prove or confirm that all other substances
    have been eliminated.

Problems and Pitfalls
  • The forensic scientist has little or no control
    over the quality and quantity of the specimens
  • Simple rules can not be devised for defining what
    constitutes a thorough and foolproof analytical
  • Each type of evidence requires different tests
    and each test has a different degree of
  • It is left to the forensic scientist to determine
    at what point the analysis used in identification
    can be concluded and the criteria for positive
    identification satisfied.

  • 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
  • The process of ascertaining whether two or more
    objects have a common origin.

Comparison Examples
  • Comparison of a hair follicle found at the crime
    scene with a standard/reference hair from a
  • Paint chip found on a hit and run victims
    garment compared to a paint chip removed from a
    suspected vehicle

Comparison Two Step Procedure
  • First combinations of select properties are
    chosen from the crime-scene specimen and the
    standard-reference specimen for comparison.
  • Second- the forensic scientist must be prepared
    to render a conclusion with respect to the
    origins of both specimens. Do they or do they
    not come from the same source?

Comparison - Possibilities
  • If one or more properties or characteristics do
    NOT agree between the crime-scene specimen and
    the standard/reference specimen, than the
    examiner will state they are not of the same
    source or origin.
  • But . . . What (if)
  • If all the properties compare and the specimens,
    as far as the examiner can determine are
    indistinguishable. Does it logically follow that
    they come from the same source??? NOT NECESSARILY
    so. It depends upon probability, individual
    characteristics and class characteristics.

  • Probability is the likelihood that an event will
  • Probability defines the odds at which a certain
    event will occur.
  • The greater the number of similarities known as
    individual characteristics between the
    crime-scene specimen and the standard/reference
    specimen, the higher the probability that the
    objects have a similar or common origin.

Individual Characteristics
  • Properties or characteristics of evidence that
    can be attributed to a common source with an
    extremely high degree of certainty.

Individual Characteristics - Examples
  • Matching ridge characteristics of two
  • 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.

Examples of Individual Characteristics
Individual Characteristics
Probability and Individual Characteristics
  • Evidence that can be associated with a common
    source with an extremely high degree of
    probability is said to possess individual
  • In all of the prior examples of individual
    characteristics ridge patterns between
    fingerprints, tool markings, comparisons of wear
    patterns for tires, an exact probability of the
    two specimens being of common origin is not able
    to be calculated. It can only be concluded that
    this probability is so high as to defy
    mathematical calculations or human comprehension.
  • Example The probability of two humans or even
    two fingers having the same fingerprints is 1 x

Class Characteristics
  • Properties of evidence that can only be
    associated with a group and never with a single

Class Characteristics and Probability - Examples
  • Comparison of two chips of paint. One from the
    crime-scene and one from suspected vehicle. If
    each paint specimen is only one layer thick the
    chance of their having originated from the same
    car is not nearly as great as if we compare two
    paint chips having seven similar layers, not all
    of which were part of the cars original color.
  • The single layer paint chips have class
    characteristics since they could only be
    associated at best with a specific car model.
  • The seven layer paint chips have individual
    characteristics and have a high probability of
    originating from one specific car.

Class Characteristics and Probability Examples
  • A blood sample from a crime-scene and a
    standard/reference sample both are found to be
    Type A. Since 26 of the population is Type A
    then there is a high probability that they are
    not from a common origin or person.
  • BUT .. . What if the crime-scene sample and
    standard/reference sample are found to have not
    only the type by three other proteins in common?
    With the increase in the number of blood factors
    in common, the probability is higher that they
    have a common origin.

Calculating Probability- Product Rule
  • Multiplying together the frequencies of
    independently occurring events to obtain the
    overall probability of occurrence for all events
    happening at once.
  • Example Blood found at the scene of the Nicole
    Brown Simpson murder scene matched OJ Simpsons
    blood with the following blood factors and
    frequency of occurrence Type A 26, EsD 85 and
    PGM22- 2. When these are multiplied together,
    the overall probability that the blood is NOT OJ
    Simpsons is 0.44 or 1 in 200 that it was NOT
    OJs blood at the crime scene.

Individual Characteristics VSClass
  • Evidence that can be associated with a common
    source with an extremely high degree of
    probability is said to possess individual
  • Evidence associated with only a group is said to
    have class characteristics.

What is the greatest value of class physical
  • The value of class physical evidence lies in its
    ability to corroborate events with data in a
    manner that is, as nearly as possible, free of
    human error or bias.

What is the greatest weakness of class physical
  • The greatest weakness of class physical evidence
    is that it cannot relate physical evidence to a
    common origin with a high degree of certainty.
    Evidence possessing class characteristics can be
    associated only with a group and never with a
    single source.

Cautions and Limitations in Dealing with Physical
  • It is important to consider the relevance of
    scientific evidence before allowing it to be
    introduced into a criminal case because physical
    evidence is accorded great weight during jury
    deliberations. In addition, failure to take
    proper safeguards when determining the relevance
    of evidence may unfairly prejudice a case against
    the accused.

Why does a forensic scientist often opt not to
use the most sensitive analytical techniques when
comparing suspect substances?
  • A forensic scientist opts not to use the most
    sensitive analytical techniques when comparing
    suspect substances because, carried to the
    extreme, no two things in this world are alike in
    every detail. Measurements that are extremely
    sensitive may show differences between two
    samples that suggest samples came from different
    sources even though they actually came from the
    same source.

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