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Physical Evidence
The examination of physical evidence by a forensic scientist is
generally undertaken for the purposes of identification or
comparison.
1) Identification: The process of determining a substance’s physical
or chemical identity. Drug analysis, species determination, and
explosive residue analysis are typical examples of this undertaking in
a forensic setting. Identification has as its purpose the determination
of the physical or chemical identity of a substance as near absolute
certainty as existing analytical techniques will permit.
Identify:
1)
3)
5)
7)
9)
drugs
2) drugs in blood or urine
accelerants 4) explosives
blood
6) chemicals
semen
8) saliva
material composition
Physical Evidence
Identification of Physical Evidence
To do this requires the analysis and ultimate
identification of a specific physical or chemical
substance to the exclusion of all other possible
substances.
Physical Evidence
Identification of Physical Evidence
1) Establish a series of tests for known reference standards
a) Conduct testing on all aspects of the know reference
standards to allow for unique identification
b) Inclusion of the known standard
c) Exclusion of all other possibilities (this is the difficult one)
d) Some standards may require only a single test to establish
a unique identity
e) Some standards may require a series of tests to both
eliminate other possibilities and identify the standard
2) Apply the identity testing to your suspect sample
a) Each sample should be treated as unique and requires
exhaustive testing to establish identity
1) unique identification of all sample components
2) unique identification of some sample components
b) Different samples will require different testing strategies
Physical Evidence
Identification of Physical Evidence
3) Compare the test results of your unknown to the test results of
your know reference samples
a) compare to previously established reference sample results
b) run knowns and unknowns in side-by-side testing (this is
usually the best)
4) Expertise is gained over time and after running many sample and
reference identifications. Ultimately, the conclusion will have to be
substantiated beyond any reasonable doubt in a court of law.
NB: This sometimes can catch you if your experience and therefore
your expertise is great.
Physical Evidence
The examination of physical evidence by a forensic scientist is
generally undertaken for the purposes of identification or
comparison.
2) Comparison: The process of ascertaining whether two or more
objects have a common origin. 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 they have a common origin.
Compare:
1) Hair found at a scene to hair from a suspect
2) A paint chip found at a scene with the paint from a
suspect vehicle
3) Fibers found on a victim with fibers found in
suspect’s back seat.
Physical Evidence
Comparison of Physical Evidence
1) Forensic Comparison is a two-step procedure
a) Combinations of select properties are chosen from the
suspect and standard/reference specimen for comparison.
1)These will depend on the nature of the specimens
being compared.
2) Overriding consideration must be the ultimate
evidential value of the comparisons.
b) A conclusion must be drawn about the origins of the
specimens.
1) Do they come from the same source?
a) If different then NO.
b) If the same then…still maybe NO.
2) It depends on the types of characteristics that are
compared.
c) 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.
Physical Evidence
Comparison of Physical Evidence
2) Individual characteristics: Properties of evidence that can be
attributed to a common source with an extremely high degree of
certainty.
a) Ridge characteristics of fingerprints
b) random striation markings on bullets or tool marks
c) irregular and random wear patterns in tire and footwear
impressions
d) handwriting characteristics
e) irregular edges of broken objects
f) manufacturing marks or striations that cover consecutive
items
In each case it is not possible to state with mathematical
exactness the probability that specimens are of a common origin;
it can only be concluded that this probability is so high as to defy
mathematical calculation or human comprehension.
Physical Evidence
Striations across plastic bags;
R one found around victim’s
Head
L one found at suspect home
Matching tip of knife found in victim
With knife found in suspect’s pocket
Physical Evidence
Comparison of Physical Evidence
3) Class characteristics: Properties of evidence that can be associated
only with a group and never with a single source.
a) Here, probability is the determining factor
b) Examples include:
1) A single layer of paint
2) blood groupings (ABO, PGM, other proteins)
3) DNA loci
4) fiber types
In each case, a probability can be established for each class
characteristic. The type of paint may be unique to one car
manufacturer. The fiber type may be unique to one type of fabric.
Those distributions can be determined mathematically and you can
report the probability of finding the particular type of material or
blood protein type or DNA profile in the overall population.
Physical Evidence
Comparison of Physical Evidence
3) Class characteristics (cont’d): Properties of evidence that can be
associated only with a group and never with a single source.
c) The problem with using these probabilities is that they are
often very low. That is, the property may exist in the
population at a very high percentage
d) Examples of these low probability characteristics include:
1) blood groupings (ABO, PGM, other proteins)
2) DNA loci
In order to make the probabilities more useful, they can be
combined together using the product rule. This involves
multiplying together the frequencies of independently occurring
genetic markers to obtain an overall frequency of occurrence for a
genetic profile.
.
Physical Evidence
Comparison of Physical Evidence
3) Class characteristics (cont’d): Properties of evidence that can be
associated only with a group and never with a single source.
e) It is difficult to assign exact or even approximate
probability values to many types of class evidence
characteristics.
f) In an increasingly mass-produced world, meaningful
statistics become more difficult to generate.
g) Even so, class physical evidence can be corroborative.
1) of eyewitness and informant testimony
2) of other subjective evidence
h) As class physical evidence mounts up, it becomes
increasingly more corroborative until it can become almost
overwhelming in nature (much like the probability rule can
become overwhelming for DNA)
NB: The Wayne Williams trial
Physical Evidence
Comparison of Physical Evidence
3) Class characteristics (cont’d): Properties of evidence that can be
associated only with a group and never with a single source.
i) However, one must be careful not to allow class physical
evidence to become too important to the exclusion of
other, more subjective testimony.
j) Ultimately, as class physical evidence becomes more and
more utilized, a statistical analysis of the probability of any
certain class of physical evidence should be developed.
NB: These analyses develop into Forensic Databases
.
Physical Evidence
Comparison of Physical Evidence
4) Forensic Databases: These have been generated for all sorts of
class physical evidence types that can be statistically compared
throughout the nation and throughout the world. Computer
databases have been developed to link all 50 states and numerous
other nations.
IAFIS: Integrated Automated Fingerprint Identification System
Contains fingerprints and access to corresponding criminal
history information for over 50 million subjects
.
Physical Evidence
Comparison of Physical Evidence
4) Forensic Databases:
CODIS: Combined DNA Index System
Maintained by the FBI
Total number of profiles: 4,138,015
Total Forensic profiles: 160,582
Total Convicted Offender profiles: 3,977,433
.
NIBIN: National Integrated Ballistics Information Network
Maintained by the BATF
Currently has 236 sites over sixteen multi-state regions
Nearly 900,000 piece of evidence have been entered
The heart of the system is IBIS (Integrated Ballistics Information
System)
Physical Evidence
Comparison of Physical Evidence
4) Forensic Databases:
.
PDQ: International Forensic Automotive Paint Data Query
Maintained by the Forensic Laboratory Services of the Royal
Canadian Mounted Police
Contains information about make, model. Year and assembly plant
on more than 13,000 vehicles with a library of more than 50,000
layers of paint.
SICAR: Shoeprint Image Capture and Retrieval
Commercially available and maintained by Foster & Freeman Ltd.
Over 12,500 shoe print images plus tire tread images and others