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Articles » Crime Scene And
Physical Evidence
Polygraph Test: Lie Detector or Liar
Sofia Mavronasios
A polygraph test usually lasts 2-3 hours and is said to have a 70% accuracy rate. The examination is broken up into 3 parts: a pretest interview, the collection of charts, and an analysis of the polygraph charts. During the pre-test phase, the examiner will give detailed instructions for the actual testing phase, define the subject's legal rights, and explain how the polygraph equipment works. The examiner will also reveal all the questions to be asked during the actual exam. The examiner then will ask the subject questions about his/her health and medical history.
A polygraph measures changes in blood pressure, pulse rate, respiration, muscle movements, and perspiration so in order to conduct an actual examination, the examiner must attach components to the examinee. Two pneumograph tubes are attached across the upper chest and abdomen, two metal plates are attached on the ring and index finger, and a blood pressure cuff is attached around the upper arm. When the polygraph test starts, the examiner will ask the examinee a couple of easy questions to establish the norm for the person's signals. Then the examiner will begin to ask the questions they reviewed during the pre-test interview.
As the person answers questions, the polygraph will measure the subject's physical reaction. Lying triggers an emotional reaction and the examiner will note and measure that reaction on the chart. Determining whether or not a person is lying is subjective and usually will take the examiner 3 or 4 charts to make a conclusion. Many times defense lawyers claim that the polygraph wouldn't be accurate because their client would be put in a very stressful situation. Errors in the polygraph test are referred to as false positives or false negatives. A false positive occurs when a truthful examinee is reported as being deceptive and a false negative is when a deceptive examinee is reported as truthful. Polygraph errors may be caused by the examiner's failure to properly prepare the examinee for the exam, by misreading the physiological data on the charts, lack of training and experience, equipment malfunction, and one's emotional and physical condition.
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