Forensic Science

Forensic science generally is the application of science in the investigation of legal matters.28 In other words, scientific knowledge and technology are used to serve as witnesses in both civil and criminal matters. While science may not offer definitive solutions to the problems of society, it does provide a special investigative role, particularly in the criminal justice system. The areas of science that are exploited are diverse, but typically include the major disciplines of biology, chemistry, physics, and geology Within each discipline are many subcategories of science that may be used in a forensic science investigation. For example, within biology are the disciplines of medicine, pathology, molecular biology, immunology, odontology, serology, psychology, and entomology. The specific discipline(s) applied depends on the circumstances of the crime. Mathematics, especially statistics, are used to place weight or significance on observations or data retrieved from crime scene evidence.

The ultimate question usually is "who committed the crime?" (i.e., attribution), and crime scene evidence can play a role in answering that question. Evidence can be any material, physical, or electronic finding that can associate or exclude individuals, victim and/or suspect, with a crime. It typically comprises materials specific to the crime and to control samples for background information. Types of evidence may be fingerprints, blood, semen, saliva, hair, fibers, documents, photos, computer files, videos, firearms, glass, metals, plastics, paint, powders, explosives, tool marks, and soil.

Once a crime scene is recognized and living victims are cared for, the crime scene is secured to preserve the quality of the evidence. Then, forensic science and its practitioners take over by first recognizing and properly collecting and preserving physical evidence. Such processes commence by setting up a documented chain of custody on all evidence collected and by recording the crime scene. These actions are important for scientific and legal integrity. Chain of custody is the practice of maintaining the continuity of possession of evidence and minimizing and/or documenting all those people who have come into contact with the evidence. Importantly, if one does not properly collect and preserve pertinent evidence, crucial and reliable forensic information cannot be obtained. Improper preservation methods may lead to loss of crucial information. Thus, defining proper evidence collection and preservation procedures and the training of those individuals who carry out such activities (including first responders) are fundamental to any forensic analysis process.

Once collected and preserved, the evidence is sent to the laboratory for analysis and all chain of custody practices continue to be maintained. Within the laboratory, a cadre of analytical procedures may be used to identify the material. Often, a comparison is made of the data obtained from the evidence (for example, a DNA profile from a bloodstain) with that obtained from analysis of a reference sample (for example, a DNA profile from a buccal swab from a suspect in a murder case). Ideally, unique identification of the sample, and thus its ultimate source or origin, is desired. In many cases that may not be possible. Thus, one categorizes evidence into two types of characteristics: individual and class. When evidence can be associated with a reference, or source sample with a high degree of probability, it is considered to possess individual characteristics. Typical types of evidence that fall into the individ ual characteristics category are fingerprints and the current battery of human DNA markers. However, most evidence cannot be characterized to such a level. It cannot be associated with a high degree of confidence with an alleged source sample. Thus, the evidence can only be included within a group and hence is classified as class-characteristic evidence. There is nothing inherently wrong with class-characteristic evidence; it adds direct and/or circumstantial evidence for the fact finder to consider in the totality of the case. At times, it may be difficult to place a probability on class evidence. Yet, a failure to exclude is still valuable information. It is important, though, to recognize and convey the limited value that the analysis may comprise. Examples of evidence that fall into the class-characteristic category are ABO blood groups, mitochondrial DNA, hairs, glass, fibers, and paint. In general, while the class characteristics do not achieve the level of source attribution, they generally offer valuable information with regard to narrowing down the possible source of origin of specimens collected from the crime scene.

Interpretation of data in a comparison of an evidence sample and a reference sample is routinely carried out. Generally, there are three categories of interpretation: inclusion, exclusion, and inconclusive. An inclusion, or match, is stated when the pattern or profile from the two samples is sufficiently similar and potentially could have originated from the same source. An alternate definition of an inclusion is a failure to exclude the two samples as having a common origin or belonging to the same group. An exclusion is stated when the pattern or profile is sufficiently dissimilar such that the two samples could not have originated from the same source. Lastly, an inconclusive interpretation is rendered when there are insufficient data to provide a conclusive interpretation. Obviously, the higher the resolving power that a procedure has, the better is the chance of excluding those wrongly associated with the evidence. Note that we did not say "associated with a crime." Scientific evidence in itself does not prove guilt or innocence. It is the entirety of all evidence (science and non-science) that is used by the fact finder to determine guilt or innocence.

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