THE BASIS OF EVIDENCE: IDENTITY, CLASS, AND INDIVIDUALIZATION
All things are considered to be unique in space and time. No two (or more) objects are absolutely identical. Consider, for example, a mass-produced product like a ten nis shoe. Thousands of shoes of a particular type may be produced in any one year. The manufacturer’s goal, to help sell more shoes, is to make them all look and perform the same—consumers demand consistency. This effort is a help and a hindrance to forensic scientists because it enables them to easily separate one item from another (this red tennis shoe is different from this white one), but these same characteristics make it difficult to separate items with many of the same characteristics (two red ten nis shoes). Think about two white tennis shoes that come off the production line one after the next. How would you tell them apart? An observer might say, “this one” and “that one,” but if they were mixed up, he or she probably couldn’t sort them again. He or she would have to label them somehow, like numbering them “1” and “2.” Now consider if the two shoes are the same except for color: One’s white and one’s red. Of course, they could be easily distinguished by color but would they be put in the same category? Compared with a brown dress shoe, the two tennis shoes would have more in common with each other than with the dress shoe. All the shoes, however, are more alike than if any of them is compared to, say, a baseball bat. Forensic scientists have developed terminology to clarify the way they communicate about these issues. Identification is the examination of the chemical and physical properties of an object and using them to categorize the object as a member of a group. What is the object made of? What is its color, mass, and/or volume? The process of examining a white powder, performing one or two analyzes and concluding it is cocaine is identification. Determining that a small colored chip is automotive paint is identification. Looking at debris from a crime scene and deciding it contains hairs from a Black Labrador Retriever is identification (of those hairs). All the characteristics used to identify an object helps to refine that object’s identity and its membership in various groups. The debris has fibrous objects in it, and that restricts what they could be— most likely hairs or fibers rather than bullets, to use an absurd example. The microscopic characteristics indicate that some of the fibrous objects are hairs, that they are from a dog and the hairs are most like those from a specific breed of dog. This description places the hairs into a group of objects with similar characteristics, called a class. All Black Labrador Retriever hairs would fall into a class; these hairs belong to a larger class of items called dog hairs. Further, all dog hairs can be included in the class of nonhuman hairs and, ultimately, into a more inclusive class called hairs. Going in the other direction, as the process of identification of evidence becomes more specific, the analyst becomes able to classify the evidence into successively smaller classes of objects. Class is a movable definition; it may not be necessary to classify the evidence beyond dog hairs because you are looking for human hairs or textile fibers. Although it is possible to define the dog hairs more completely, you may not need to do so in the case at hand. Multiple items can be classified differently, depending on what questions need to be asked. For example, an orange, an apple, a bowling ball, a bowling pin, and a banana could be classified, as shown in Figure 3.3, by fruit v non-fruit, round things v non-round things, sporting goods v edible, and organic v inorganic. Notice that the bowling pin doesn’t fit into either of the classes in the last example because it is made of wood (which is organic) but is painted (which has inorganic components). Stating that two objects share a class identity may indicate they come from a common source. What is meant by a “common source” depends on the material in question, the mode of production, and the specificity of the examinations used to classify the object. A couple of examples should demonstrate the potential complexity of what constitutes a common source. Going back to the two white tennis shoes, what is their common source—the factory, the owner, or where they are found? Because shoes come in pairs, finding one at a crime scene and another in the suspect’s apart ment could be considered useful to the investigation. The forensic examinations would look for characteristics to determine if the two shoes were owned by the same person (the common source). If the question centered on identifying the production source of the shoes, then the factory would be the common source.
FIGURE 3.3 A class is a group of things with similar characteristics. The size of the class can vary widely depending on the characteristics used for definition, such as the class “all oranges” versus the class “all oranges in your refrigerator.”
Another example is fibers found on a body left in a ditch that are determined to be from an automobile. A suspect is developed, and fibers from his or her car are found to be analytically indistinguishable in all tested traits from the crime scene fibers. Is the suspect’s car the common source? For investigative and legal purposes, the car should be considered as such. But certainly it is not the only car with that carpeting. Other models from that car manufacturer or even other car manufacturers may have used that carpeting, and the carpeting may not be the only product with those fibers. But given the context of the case, it may be reasonable to conclude that the most logi cal source for the fibers is the suspect’s car. If the fibers were found on the body but no suspect was developed, part of the investigation may be to determine who made the fibers and track what products those fibers went into in an effort to find someone who owns that product. In that instance, the common source could be the fiber manufacturer, the carpet manufacturer, or the potential suspect’s car, depending on what question is being asked. If an object can be classified into a group with only one member (itself), it is said to have been “individualized.” An individualized object has been associated with one, and only one, source: It is unique. The traits that allow for individualization depend, in large part but not exclusively, on the raw materials, manufacturing methods, and history of use. Sometimes, sufficiently increasing class traits can lead nearly to individualization; for example, John Thornton’s article (1986) on the classification of firearms evidence is an excellent, if overlooked, treatment of this issue.
