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.