Class versus individual evidence
Class versus individual evidence is a foundational concept in forensic science that distinguishes between two types of evidence found at crime scenes. Class evidence refers to material that can be associated with a broader category or type of item, such as glass fragments that can indicate a particular source but not pinpoint a specific origin. This type of evidence is prevalent in forensic investigations and can help establish connections but lacks the specificity needed to link directly to individuals. In contrast, individual evidence can be attributed uniquely to a single source, such as fingerprints, which are unique to each person.
While class evidence forms the majority of forensic findings, it is often the individual characteristics that emerge from wear and usage that provide crucial links to suspects. For example, shoes, tools, and vehicles can develop unique patterns over time that can be traced back to their specific user. Even biological evidence, like DNA, has limitations in individualization, particularly in cases involving identical twins. The interplay between class and individual evidence is significant in forensic analysis, as experts strive to transform general evidence into distinctive identifiers that can lead to solving cases. Understanding this distinction is essential for anyone interested in forensic science and criminal investigations.
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Class versus individual evidence
Definitions: Class evidence is evidence that can be linked to a type (or class) of items; individual evidence is evidence that can be linked to a specific individual or item.
Significance: Because the majority of evidence at crime scenes is class evidence and not individual evidence, it would be difficult to link objects at crime scenes to specific suspects were it not for the fact that many objects pick up individual characteristics.
Class evidence makes up the vast majority of all evidence in forensic cases. For example, a glass fragment can be analyzed to determine its refractive index and chemical makeup. The resultant laboratory report can tell investigators that the fragment’s properties are consistent with a certain type of glass, such as that from windowpanes or car headlights. What the analysis usually cannot reveal is from which particular window or which particular car headlight the fragment comes.
By contrast, individual evidence can be linked to specific objects, such as fingerprints, no two of which have ever been found to be exactly alike. For this reason, any fingerprint that is found must have been made by one, and only one, person. Other examples of individual evidence include human lip prints, ear prints, and sole prints. Researchers have also found through X-ray analyses of skulls that sinus prints—the unique patterns of bone and space in sinus cavities—are also individual. Forensic anthropologists can use this information to identify bodies of long-dead people. Surprisingly, DNA (deoxyribonucleic acid) evidence is not individual, as identical twins have identical DNA.
Many objects that would otherwise fall under the heading of class evidence pick up individual characteristics. For example, the soles of all shoes of a specific model and size come out of their factories looking exactly the same. After they have undergone some substantial wear, however, they develop distinctive wear patterns. The ways in which different users distribute their weight, the feet that they favor, and many other factors, including chemical and biological materials on which they step, contribute to making the soles of their shoes take on individual characteristics.
The individuation of characteristics can be seen in many different types of evidence. For example, tools can develop distinctive wear patterns and leave distinctive marks when they are used. Vehicle tires develop distinctive wear patterns over time, just as shoes do. Firearms can leave distinctive marks on the bullets they discharge. Glass may fracture in ways that make it possible for investigators to reassemble the broken pieces, much like a jigsaw puzzle. In forensic investigations, considerable time is devoted to looking at how class evidence becomes individualized.
Bibliography
Beavan, Colin. Fingerprints: The Origins of Crime Detection and the Murder Case That Launched Forensic Science. New York: Hyperion, 2001.
Platt, Richard. Crime Scene: The Ultimate Guide to Forensic Science. New York: Dorling Kindersley, 2003.