Forensic Chapter 3

  • Physical Evidence- encompasses any and all objects that can establish that a crime has or has not been committed or can provide a link between a crime and its victim or a crime and its perpetrator.

    • If the investigator cannot recognize physical evidence or cannot properly preserve it for laboratory examination, no amount of sophisticated lab instrumentation or technical expertise can salvage the situation.

    • 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 decided by a jury 


  • Types of Physical Evidence

    • Blood, semen, and saliva

    • • Documents

    • • Drugs

    • • Explosives

    • • Fibers

    • • Fingerprints

    • • Firearms and ammunition

    • • GlassHair

    • • Impressions

    • • Organs and physiological fluids

    • • Paint

    • • Petroleum products

    • • Plastic bags

    • • Plastic, rubber, and other polymers

    • Powder residues

    • • Soil and minerals

    • • Tool marks

    • • Vehicle lights

    • • Wood and other vegetative matter


Purpose of Examining 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.


Identification


  • The object of an identification is to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit

    • The process of identification first requires the adoption of testing procedures that give characteristic results for specific standard materials.


  • The object of an identification is to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit.

    • Once these test results have been established, they may be permanently recorded and used repeatedly to prove the identity of suspect materials.


  • The object of an identification is to determine the physical or chemical identity with as near absolute certainty as existing analytical techniques will permit

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


Common Types of Identification


  • The crime laboratory is frequently requested to identify the chemical composition of an illicit drug.

  • It may be asked to identify gasoline in residues recovered from the debris of a fire, or it may have to identify the nature of explosive residues—for example, dynamite or TNT.

  • The identification of blood, semen, hair, or wood are also very common and as a matter of routine, include a determination for species origin.


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.

  • The forensic comparison is actually 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 be prepared to render a conclusion with respect to the origins.


Role of Probability


  • 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.

  • Simply defined, probability is the frequency of occurrence of an event.

  • In flipping a coin, probability is easy to establish.

  • With many analytical processes exact probability is impossible to define.


Classifying Characteristics


  • Individual Characteristics

    • Evidence that can be associated with a common source with an extremely high degree of probability is said to possess individual characteristics.


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.

  • Examples:

    • 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


  • Examples

    • 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


Class Characteristics


  • Class Characteristics

    • Evidence associated only with a group is said to have class characteristics.


*Class Evidence 


  • One of the current 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.

  • 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.

  • 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.


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.


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.


  • There are practical limits to the properties and characteristics the forensic scientist can select for comparison.

    • 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.


  • There are practical limits to the properties and characteristics the forensic scientist can select for comparison.

    • Distinguishing variations of evidential use from natural variations is not always an easy task.


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.

  • Just as important, 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 subject.


*Forensic Databases


  • The Integrated Automated Fingerprint Identification System

  • (IAFIS), a national fingerprint and criminal history system

  • maintained by the FBI.

  • The Combined DNA Index System (CODIS) enables federal, state, and local crime laboratories to electronically exchange and compare DNA profiles.

  • The National Integrated Ballistics Information Network (NIBIN) allows firearm analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings.

  • The International Forensic Automotive Paint Data Query (PDQ) database contains chemical and color information pertaining to original automotive paints.

  • SICAR (shoeprint image capture and retrieval) is a shoeprint database.