Chapter 3: Physical Evidence

1. Chapter 3:Physical Evidence “You can learn a lot by just watching.” —Yogi Berra, former New York Yankees catcher and sage

2. Please Do Now What is evidence? State 5 examples.

3. Locard’s Exchange Principle Edmond Locard(1877-1966) • “Locard's Exchange Principle” states that whenever two objects come into contact there is a transference of material between them.

4. Types of Evidence Two general types: • Testimonial—a statement made under oath; also known as direct evidence or Prima Facie evidence • Physical—any object or material that is relevant in a crime; also known as indirect evidence. Examples are hair, fiber, fingerprints, documents, blood, soil, drugs, tool marks, impressions, glass. Note: This chapter will focus on physical evidence

5. Reliability of Eyewitness Factors: • Nature of the offense and the situation in which the crime is observed • Characteristics of the witness • Manner in which the information is retrieved Additional factors: • Witness’s prior relationship with the accused • Length of time between the offense and the identification • Any prior identification or failure to identify the defendant • Any prior identification of a person other than the defendant by the eyewitness

6. Eyewitness • A police composite may be developed from the witness testimony by a computer program or forensic artist. • “Perception is reality.” • As a result of the influences in eyewitness memory, physical evidence becomes critical. Faces—a composite program by InterQuest

7. Value of Physical Evidence • Generally more reliable than testimonial • Can prove that a crime has been committed • Can corroborate or refute testimony • Can link a suspect with a victim or with a crime scene • Can establish the identity of persons associated with a crime • Can allow reconstruction of events of a crime

8. Physical Evidence • It would be impossible to list all the objects that could conceivably be of importance to a crime. • Almost anything can be Physical Evidence. • Although you cannot rely on a list of categories, it is useful to discuss some of the most common types of physical evidence. • The purpose of recognizing physical evidence is so that it can be collected and analyzed. • 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.

9. Common Types of Physical Evidence • Blood, semen, and saliva • Documents • Drugs • Explosives • Fibers • Fingerprints • Firearms and ammunition • Glass • Hair • Impressions • Organs and physiological fluids • Paint • Petroleum products • Plastic bags • Plastic, rubber, and other polymers • Powder residues • Serial numbers • Soil and minerals • Tool marks • Vehicle lights • Wood and other vegetative matter

10. Blood, semen and saliva Documents Physical Evidence

11. Drugs Explosives Physical Evidence

12. Fibers Fingerprints Physical Evidence Silk fibers

13. Firearms and ammunition Glass Physical Evidence Refractive index

14. Hair Impressions Physical Evidence Dyed human hair Impression foam Hair shaft comparison

15. Organs and physiological fluids Paint Physical Evidence Car paint layers 300X

16. Petroleum products Plastic bags Physical Evidence

17. Plastic, rubber and other polymers Powder residues Physical Evidence Powder residue test

18. Serial numbers Soil and minerals Physical Evidence Serial number restoration

19. Tool marks Vehicle lights Physical Evidence Honda Tail lights

20. Wood and other vegetative matter Physical Evidence Mixed pollen Diatoms

21. Common Types of Physical Evidence • Blood, semen, and saliva • Documents • Drugs • Explosives • Fibers • Fingerprints • Firearms and ammunition • Glass • Hair • Impressions • Organs and physiological fluids • Paint • Petroleum products • Plastic bags • Plastic, rubber, and other polymers • Powder residues • Serial numbers • Soil and minerals • Tool marks • Vehicle lights • Wood and other vegetative matter

22. 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 • Also 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 suspect

23. Why Examine Physical Evidence? • The examination of physical evidence by a forensic scientist is usually undertaken for identification or comparison purposes. • Identification has, as its purpose, the determination of the physical or chemical identity of a substance with as near absolute certainty as existing analytical techniques will permit. • 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 origin.

24. Identification • to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit • 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 • Second, the number and type of tests needed to identify a substance be sufficient to exclude all other substances

25. Common Types of Identification • The crime lab is often requested to identify: • Chemical composition of an illicit drug • Gasoline in residues recovered from the debris of a fire, or the nature of explosive residues— for example, dynamite or TNT • Blood, semen, hair or wood - including a determination for species origin

26. Comparison • 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 subject to the same tests.

27. Forensic comparison • 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 draw a conclusion about the origins of the specimens

28. Role of Probability in Forensic Comparison • 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 • Probability: the frequency of occurrence of an event • Easy to establish the probability of flipping a coin • Exact probability is impossible to define with many analytical processes

29. Probability For example: Blood Factors Frequency A 26% EsD 85% PGM 2+2— 2% What is the probability of someone having all three factors? Using the product rule: 0.26 X 0.85 X 0.02 = 0.00442 which is ONLY 0.442% or 1OO or 1 out of 226 people .442

30. Classifying Characteristics • Individual Characteristics • Properties of evidence that can be attributed to a common source with an extremely high degree of certainty • Class Characteristics • Properties of evidence that can only be associated with a group and never with a single source

31. Individual Characteristics • 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.

32. Evidence Characteristics • Class characteristics—Evidence common to a group of objects or persons • Individual characteristics—Evidence can be identified with a particular person or a single source with an extremely high degree of probability Fingerprints Blood DNA Typing

33. Examples of Individual Characteristics • 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 • 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

34. Class vs Individual Evidence Which examples do you think could be individual evidence?

35. Class vs Individual Evidence • The large piece of glass fits to the bottle—it is individual evidence • These fibers are class evidence—there are millions like them.

36. Class vs Individual Evidence • This tape is class evidence—but it could be individual evidence if the striations of the tear match. • it is individual evidence— the dental work on the teeth can be used to identify the person

37. Class vs Individual Evidence • This tire track is class evidence, as it can be linked to a class of tire —but it could be individual evidence if there are unique impressions.

38. Class Characteristics • Frequently forensic scientists cannot relate physical evidence to a common origin with a high degree of certainty • Evidence is said to possess class characteristics when it can be associated only with a group and never with a single source. • Again, probability is a determining factor. • Nevertheless, the high diversity of class evidence in our environment makes their comparison very significant in the context of a criminal investigation.

39. Class Evidence • A 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. • For 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.

40. Class Evidence • 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. • 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.

41. Class Evidence • The chances are low of encountering two indistinguishable items of physical evidence at a crime scene that actually originated from different sources. • 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 contribution of physical evidence is ultimately determined in the courtroom.

42. Crossing Over • Crossing over the line from class to individual does not end the discussions. • 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? • These are all questions that defy simple answers and are the basis of arguments.

43. Natural vs. Evidential Limits • 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. • 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.

44. Forensic Databases • The ultimate contribution a criminalist can make is • The crime lab is on the forefront of the investigation seeking to identify perpetrators • computerized databases link all 50 states and those that link police agencies throughout the world to link a suspect to a crime through comparative analysis

45. Forensic Databases • Integrated Automated Fingerprint Identification System (IAFIS) • a national fingerprint and criminal history system maintained by the FBI 60 million prints on file

46. Forensic Databases • Combined DNA Index System (CODIS) • enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles - maintained by the FBI

47. Forensic Databases • National Integrated Ballistics Information Network (NIBIN) • Allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings • maintained by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF)

48. Forensic Databases • International Forensic Automotive Paint Data Query (PDQ) • contains chemical and color information pertaining to original automotive paints • maintained by the Forensic Laboratory Services of the Royal Canadian Mounted Police Cross section of Automobile paint

49. Forensic Databases • Shoeprint image capture and retrieval (SICAR) • shoeprint database • computer retrieval system - not maintained by a government agency

50. Reconstruction • The method used to support a likely sequence of events by the observation and evaluation of physical evidence, as well as statements made by those involved with the incident, is referred to as reconstruction. • Crime-scene reconstruction relies on the combined efforts of medical examiners, criminalists, and law enforcement personnel to recover physical evidence and to sort out the events surrounding the occurrence of a crime.