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Although polygraph examiners often test suspects and witnesses in criminal cases, the applications of the polygraph are not limited to police work. For example the armed forces government agencies employ polygraph examiners to screen prospective civilian employees.
Before polygraph examiners meet the subject they will test, they gather information about the individual and the circumstances involved. They research the subject's childhood, medical history—including use of medications, possible emotional illnesses, or drug/alcohol abuse—and inquire if the subject has a police record. In criminal cases, they may visit the police station, the crime scene, or the morgue for information.
After gathering this information, polygraph examiners spend at least an hour with the test subject to obtain information about background, current health, and knowledge of the circumstances that led to the polygraph examination. They try to calm the subject's fears about the test by explaining how the polygraph instrument works and explaining the test procedure.
Next, polygraph examiners develop test questions that are easy to understand and are not ambiguous. Before they administer the test, they read the questions to the subject to assure the subject that there will be no surprise questions. Then the examiners attach the apparatus to the individual to measure changes in certain nonvoluntary body responses.
The examiner fastens a tube around the subject's chest to measure the rate of the subject's respiration. A cuff similar to that found on a blood pressure meter is wrapped around the subject's arm in order to record cardiovascular activity. Special sensors are placed on the subject's skin to measure skin reflex. All these sensors are connected to the polygraph machine, which contains pens that respond to changes in the subject's breathing, heart rate, and perspiration. An electric motor moves a roll of graph paper while the pens record the subject's responses on the paper.
The actual testing session is rather brief; a 10-question test takes about four minutes. The test may consist of control questions, which are not likely to cause stress, and key questions, which are likely to produce a strong reaction if the subject is lying. Another form of the test includes a number of similar questions, with only one question containing the correct details. Because only the guilty subject knows which question has the right details, a reaction to that question can indicate their guilt or innocence.
Some people believe they can affect their reactions by taking drugs. But a drug that reduces an individual's reactions to key questions also will reduce the individual's reactions to the control questions. Thus the test results will still show a difference between the subject's reactions when answering truthfully and when lying.
After administering the test, polygraph examiners evaluate the subject's recorded responses and then discuss the results with the individual. If it appears that the subject has been untruthful, examiners try to give individuals a chance to explain the reasons for their reactions and may even retest them later.
In addition to administering and evaluating polygraph tests, polygraph examiners keep records and make reports on test results. They may appear in court as witnesses on matters dealing with polygraph examinations, and some also teach classes in polygraph operation and interrogation techniques.