Control samples (Forensic Science)
Control samples (also called controls, known samples, or knowns) provide a level of quality control that can verify laboratory test results. When a control sample is not used, it is possible for a laboratory result to be a false positive (a result that indicates something is true when, in fact, it is false) or a false negative (a result that indicates something is false when, in fact, it is true).
Forensic laboratories may use a variety of control samples to ensure accurate results. For example, they may use known combustibles to verify that particular combustibles are present in arson cases and known drug samples to verify that particular drugs are present in drug cases. Known DNA (deoxyribonucleic acid) samples are used to compare with unknown DNA samples (for example, in the comparison of a suspect’s DNA with DNA found at a crime scene).
In many cases, forensic laboratories acquire the known samples they use as controls from reliable outside sources. For example, the forensic science Service in England, an internationally recognized leader in applied forensic technology, is a widely respected source of reliable control samples for fibers and paints. Crime labs around the world use control samples from such sources to ensure that they are meeting the quality standards necessary for their results to be accepted in courts of law.
Another type of control sample is a blank, or a control sample that is known to contain nothing....
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Further Reading (Forensic Science)
Evans, Colin. The Casebook of Forensic Detection: How Science Solved One Hundred of the World’s Most Baffling Crimes. Updated ed. New York: Berkley Books, 2007.
Fisher, Barry A. J. Techniques of Crime Scene Investigation. 7th ed. Boca Raton, Fla.: CRC Press, 2004.
Genge, N. E. The Forensic Casebook: The Science of Crime Scene Investigation. New York: Ballantine, 2002.
James, Stuart H., and Jon J. Nordby, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques. 2d ed. Boca Raton, Fla.: CRC Press, 2005.
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Control Samples (World of Forensic Science)
Control samples are any type of well-known forensic samples used to assure analyses are properly performed so that results are reliable. Also called controls, known samples, and knowns, these control samples are fully known to the forensic community with respect to composition, identification, source, and type. Examples of control samples include known combustible substances used for arson cases, known drug samples for suspected illegal drug samples, known blood types in violent crime investigations, and known DNA types for trace evidence cases.
Control samples are an important part of quality control and assurance procedures that forensic scientists use to eliminate the inaccuracy of laboratory results. Without control samples such scientific results could yield false positives (any result that is true when in reality it is false) and false negatives (any result that is false when in reality it is true). For example, a forensic scientist tests a control sample along with a suspect sample when conducting DNA analysis. The control sample is collected before the suspect sample to reduce the possibility of contamination.
Control samples are acquired through any source that is considered completely reliable and whose identification has been verified through proper authorities. These sources include commercial vendors and manufacturers for such items as ammunition, fibers, and paints. The Forensic Science Service (FSS) in England, for example, examines fibers and paints recovered from crime scenes with microspectrophotometers. These sophisticated devices measure the spectra of a single suspect sample for comparison with the spectra of a control sample. Because the FSS is recognized internationally as a leader in applied forensic technology, its complex comparisons of suspect (or crime scene) samples and control samples are regularly used as evidence in courts of law.
Another often used type of control sample is one that contains nothing, a blank. In these cases, the control sample is known not to contain whatever substance is being considered. The idea behind a blank sample is to verify that the test instruments, equipment, or other implements are not contaminated with the substance being considered (which would lead to a false positive result). As an example, if an instrument has been used to test an illegal drug, then it is necessary to make sure that the instrument has been sterilized. When a blank sample is tested on the instrument, one that is known to be free of that illegal drug, a positive result will identify a contaminated instrument and require that the instrument be sterilized before proceeding with analysis.
SEE ALSO Forensic Science Service (U.K.); Quality control of forensic evidence.