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) drugs 2) drugs in blood or urine 3) accelerants 4) explosives 5) blood 6) chemicals 7) semen 8) saliva 9) 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 • 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 • 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: 4,138,015Total Forensic profiles: 160,582Total 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