What is Physical Evidence? Physical evidence is any object that can establish that a crime has been committed or can link a crime and its victim or perpetrator. Almost anything can be physical evidence, to list the objects that could be used as physical evidence is impossible.
Common Types of Physical Evidence There are several common types of physical evidence that are found at a crime scene and can be used in cases. However, the weight of a given piece of evidence is ultimately decided by a jury. The types of physical evidence include but are not limited to; blood, semen, saliva, documents, hair, fingerprints, paint, fibers, and drugs. The collection of evidence must be thorough enough to include as many pertinent clues as possible, but selective enough not to delay the laboratory. (Saferstein, R. 2009)
Handling Evidence When handling evidence it must be done to where contamination does not take place. Contamination occurs when the transfer of extraneous matter between the collector and the evidence or multiple pieces of evidence, producing tainted evidence that cannot be used in the subsequent investigation. The use of latex gloves or disposable forceps when touching evidence can prevent contamination. Evidence should not be moved until investigators have
3 documented its location and appearance in notes, sketches, and photographs. Blood, hairs, fibers, and soil particles should not be removed from garments, weapons, or other articles that bear them. Instead, package the items separately and send the entire object to the laboratory for processing. (Saferstein, R. 2009)
The Examination of Physical Evidence Physical evidence is usually examined by a forensic scientist for identification or comparison. Identification is the process of determining a substance’s physical or chemical identity, whereas comparison is the process of ascertaining whether two objects have a common origin. The process of identification first requires the adoption of testing procedures that give characteristic results for specific standard materials.
When testing has been completed and results have been established the results may be permanently recorded and used repeatedly to prove the identity of suspect materials. Identification requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances.
Forensic comparison is normally completed in two steps. Combinations of select properties are chosen from the suspect and the standard/reference specimen for comparison. Once the examination has been completed, a conclusion must be determined by the forensic scientist about the origins of the specimens. The forensic scientist has to determine whether or not the specimens come from the same source. (Saferstein, R. 2009)
4 Classifying Characteristics Physical evidence has two classifying characteristics, Individual Characteristics and Class Characteristics. Individual characteristics are properties of evidence that can be attributed to a common source with an extremely high degree of certainty. Examples of individual characteristics include but not limited to; the matching ridge characteristics of two fingerprints, the comparison of handwriting characteristics or the comparison of random markings on bullets or tool marks.
Class characteristics are properties of evidence that can be associated only with one group and not with a single source. Testimony that pubic hairs found on a rape and homicide victim came from a Caucasian male or that shell casings found at the scene came from a certain make and model of firearm are two examples of class characteristics. (Saferstein, R. 2009)
The Significance of Physical Evidence One of the primary functions of a forensic scientist is to be able to create and update databases in order to evaluate the significance of physical evidence. The value of physical evidence lies in the ability to provide corroboration of events with data that are, as nearly as possible, free of human error or bias. An example of physical evidence having a great significance is the case of Georgia v. Wayne Williams. Williams was charged with the murders of two individuals in the Atlanta, Georgia area. He was also linked to the murders of multiple boys and young men.
5 Before Wayne Williams was developed as a suspect it was discovered in the laboratory that each victim had matching hairs and fibers on them, suggesting that had come into contact with the same person or person’s environment. The fiber had very unusual properties, a yellowish-green nylon fiber with one leg of three lobes much shorter than the other two lobes. Shortly after William’s was developed as a suspect, it was determined the fibers were manufactured by the Wellman corporation.
The next step was to ascertain, if possible, how much carpet like William’s bedroom carpet had been sold in the Atlanta area. It was confirmed that the Wellman Corporation had, in fact manufactured the fiber in William’s carpet and that no other fiber manufacturer was known to have made a fiber with a similar cross section. After examination the fibers from the victims were able to be matched to the carpet in the apartment belong to William’s.
But regardless of the apparent validity of the fiber findings, it is during the trial that its true weight would be determined. Because of the significance of the fibers found on the victims a Supreme Court jury was able to come to a conclusion that Wayne Williams was indeed guilty of the murders on February 26, 1982. (www.teacherweb.com; Saferstein, R. 2009)
- Saferstein, R. (2009) Forensic Science: From the Crime Scene to the Crime Lab. Upper Saddle River, New Jersey: Prentice Hall www.teacherweb.com