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Chapter 3 – Physical Evidence

Chapter 3 – Physical Evidence. List the common types of physical evidence encountered at crime scenes Explain the difference between the identification and comparison of physical evidence Define individual and class characteristics

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Chapter 3 – Physical Evidence

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  1. Chapter 3 – Physical Evidence • List the common types of physical evidence encountered at crime scenes • Explain the difference between the identification and comparison of physical evidence • Define individual and class characteristics • Discuss the value of class evidence to a criminal investigation • Explain the purpose physical evidence plays in reconstructing the events surrounding the commission of the crime scene.

  2. Common Types of Physical Evidence • Blood, semen, and saliva - identity and possible origin • Liquid or dried • Human or Animal • Documents – authenticity or source • Handwriting • Typewriting • Drugs – • Any seized substance

  3. Common Types of Physical Evidence • Explosives • Devices containing explosive charge • Objects suspected to contain residues of explosive • Fibers – transfers establish relationships • Natural • Synthetic • Fingerprints • Latent • Visible

  4. Common Types of Physical Evidence • Firearms & Ammunition • Firearm • Discharged or intact ammunition • Glass • Glass particle or fragment • Windowpanes w/ holes made by bullet • Hair • Animal • human

  5. Common Types of Physical Evidence • Impressions • Tire markings • Shoe prints • Depressions in soft soils • Glove, fabric impressions • Bitemarks in skin, food • Organs and physiological fluid • Test existence of drugs or poisons • Blood analyzed for alcohol or drugs • Paint • Liquid or dried • Transferred from one surface to another

  6. Common Types of Physical Evidence • Petroleum products • Gasoline – residues from arson • Grease and oil stains • Plastic Bags • Polyethelene disposable bags • Plastic, rubber, other polymers • Remnants of man-made materials • Powder Residues • Item suspected of containing firearm discharge residues

  7. Common Types of Physical Evidence • Serial numbers • Stolen property for restoration of erased I.D. # • Soil and Minerals • Imbedded in shoes • Safe insulation in garments • Tool Marks • Screwdriver or crowbar impressed into another object • Vehicle Lights • Light on or off at time of impact

  8. Common Types of Physical Evidence • Wood & other vegetative matter • Wood • Sawdust • Shavings • Vegetative matter • On shoes, clothing, or tools

  9. Identification • Ultimate Goal in identifying a specific physical or chemical substance is exclude other substances • Requires adoption of testing procedures that have characteristic results for specific standard materials • Once established, procedure can be used repeatedly to prove identity • Requires that # and type of tests needed to identify a substance be: • Sufficient to exclude all other substances • I.E. eliminate all but one substance

  10. Identification • Determination of physical or chemical identity of a substance with as near absolute certainty as possible • Illicit drug preperation – heroin, cocaine, barbiturates • Gasoline residue from debris of a fire • Nature of explosive residues – dynamite or TNT • Species of Origin – blood, semen, hair, wood

  11. Identification • Each type of evidence requires a different test; each test has a different degree of specificity • Substance A – one test • Substance B – 5 or 6 tests • No control over quality or quantity of specimens • Forensic scientist determines at what point analysis is concluded • Has to be beyond reasonable doubt for court of law

  12. Comparison • Suspect specimen and a standard/reference specimen run through same tests, examinations to determine common origin • Hairs at crime scene to hairs in suspect head • Paint chip from victim’s clothing to car in hit-and-run

  13. Comparison • Procedure: • Combinations of select properties chosen from suspect and standard/reference sample • Which properties and how many properties depends on type of material (more details later….) • Once testing complete, forensic scientist concludes on origins of specimen • One or more properties doesn’t agree – not same origin • All properties agree – is it always the same origin?

  14. Comparison • Not necessarily always the same • Evidential value – probability in ascertaining origins of two or more specimens • Probability • Frequency of occurrence of an event • Odds at which a certain event will occur • What does this mean? • Variation of characteristics of each specimen need to be taken into account • Individual vs. Class Characteristics

  15. Class Characteristics • The properties that all the members of a certain group of objects or substances have in common • Frequent problem: inability of lab to relate physical evidence to a common origin with a high degree of certainty • Probability is a determining factor

  16. Class Characteristics • Initial categories are broad and then narrowed down as more information is obtained • Example • Blue substance • Blue paint • Blue car paint • Blue car paint from Ford Manufacturing Company • Blue car paint from Ford produced 1998-2000 • Blue car paint from Ford produced 1998-2000, used on Mustang and Explorer

  17. Class Characteristics • The previous “paint” example was a refined class evidence identification • The “paint” was identified by comparing its class characteristics with those of known standards or previously established criteria. • The “paint” is considered CLASS EVIDENCE

  18. Other Examples… • Single layered paint • Soil • Glass fragments too small to fit back together • Hairs • Fibers

  19. Class Characteristics • Probability • Two paint chips with one layer of similar color • Two paint chips with 7 similar paint layers, not all part of car’s original color • Which has high probability of same origin?

  20. Class Characteristics • Blood – 2 samples, both human origin, both type A • Type A is 26% occurrence in population – not enough for comparison -BUT- • Use more blood factors to compare – probability increases • Blood proteins, growth factors, clotting factors, enzymes

  21. Class Characteristics • Product Rule – calculates overall frequency of occurrence in a population • Factor A % occurrence * Factor B % occurrence= probability that both events will occur simultaneously • Applies when using factors that occur independently of one another

  22. Class Characteristics • Product Rule Example: Blood Factors Frequency A 26% EsD1 85% PGM2+2- 2% • Multipy (.26) (.85) (.02)= .44% or 1 in 200 • Probability that a blood sample will contain all three types - .44% or 1 in 200 people

  23. Value of Class Evidence • Many lawyers try to discredit class evidence because it cannot be limited to just one possible source • However Class Evidence DOES HAVE VALUE. • Look around room, most have different clothes on. If I were to pick a fiber from one persons clothes in this room, that fiber could possibly pick out the single source or at least narrow the field • Some class evidence holds little forensic value such as fiber from jeans or white cotton shirts…they are too common

  24. Individual Characteristics • Properties of evidence that can be attributed to a common source with extremely high degree of certainty • Examples: • Matching ridge characteristics of 2 fingerprints • Comparison of random striation markings on bullets or tool marks (figure 3-1) • Comparison of irregular & random wear patterns in tire or footwear impressions

  25. Individual Characteristics • Handwriting characteristics • Fitting together of irregular edges of broken pieces like a jigsaw puzzle • Matching sequentially made plastic bags by striation marks running across the bags (figure3-2)

  26. Individual Characteristics • Not possible to state with mathematical exactness probability that specimens are of common origin • Conclusion must be made by practical experience of the examiner • probability of 2 individuals having same fingerprints is 1 x 1060

  27. Individual Characteristics • Individual characteristics can be used to distinguish members of the same class • Forensic scientists try to individualize a piece of physical evidence by some type of comparison process • Only a few types of physical evidence (primarily physical pattern evidence) can be truly individualized

  28. The Ultimate Goal • Ultimate goal would be to move all class evidence to individualistic which is not very likely • Hair is trying to make the move...something like paint probably never will. • For human evidence to be individualistic, the odds of two people matching the same piece of evidence must be 1 in about 7.5 billion, which is the population of the earth.

  29. Class vs. Individual • When does evidence cross the line that distinguishes class from individual evidence? • This is debated and disagreed among many forensic scientists. • 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?

  30. Class vs. Individual • How many handwriting characteristics tie a person to a signature? • Up to forensic scientist to find as many characteristics as possible to compare one substance to another. • Significant to consider: • Quality • Composition of evidence • Case history • Examiner’s experience

  31. Functions of the Forensic Scientist • Analysis of Physical Evidence 1. Apply scientific techniques to analysis of evidence 2. Be aware of demands and constraints of legal system

  32. Functions of the Forensic Scientist B. Provision of expert testimony 1. Expert witness – an individual whom the court determines possesses knowledge relevant to the trial that is not expected of the average person 2. Competency shown by degrees held, professional organizations belong to, professional articles published, years of experience, participation in special courses

  33. Expert Witness 3. Opposing attorney may cross-examine the witness & point out weaknesses in background or knowledge 4. Laypersons testifying may not state their opinion, but an expert witness can

  34. Functions of the Forensic Scientist C. Furnishing Training in the Proper Recognition, Collection, & Preservation of Physical Evidence 1. often have Evidence Technicians on 24 hour call to aid criminal investigations in retrieving evidence 2. where patrol officers or detectives gather the evidence, forensic scientist should train all officers engaged in fieldwork

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