Chapter 3: Physical Evidence & Forensic Value

The goal of identification is to determine the definitive physical or chemical identity of a substance, such as confirming the composition of drugs like heroin, identifying explosive residues, or determining the species origin of biological materials like blood, requiring an extensive battery of tests to exclude all other possibilities,. Conversely, comparison analysis subjects both a known standard/reference sample and a suspect specimen to identical examinations to determine if they share a common origin,. Physical evidence is categorized based on its potential to link to a source: individual characteristics possess an extremely high degree of probability in associating evidence with a single source, such as the unique ridge patterns of fingerprints, the random wear on shoe impressions, or pieces of a broken object that can be physically pieced back together,. Class characteristics, however, can only link evidence to a particular group, not a specific single source, such as a sample of common-type paint or blood type A,. When assessing the significance of physical evidence, probability plays a crucial role. For class evidence like biological material, forensic scientists use the product rule to calculate the overall frequency of a genetic profile by multiplying the frequencies of multiple independently occurring genetic markers,. Although assigning exact probability to much class evidence remains difficult, its collective presence can dramatically increase the certainty of a suspect’s involvement, and its objectivity makes it invaluable for corroborating events and exonerating individuals from suspicion, unlike subjective human testimony,. The role of forensic science has been transformed by sophisticated computerized databases, allowing laboratories to proactively seek links. Key national systems include the Next Generation Identification (NGI) for fingerprints, which replaced IAFIS; the Combined DNA Index System (CODIS), which links DNA profiles from crime scenes and convicted offenders; and the increasingly used Rapid DNA technology, which can generate a profile from a buccal swab in (lesser than) 90 minutes for CODIS submission. Additional specialized databases include the National Integrated Ballistics Information Network (NIBIN) for linking firearm marks on casings and bullets, the International Forensic Automotive Paint Data Query (PDQ) for automotive paint analysis, and SICAR for comparing footwear impressions. The National Missing and Unidentified Persons System (NamUs) is also a vital resource for resolving missing person and unidentified decedent cases.