Unveiling the Precision of Polygraph Tests

A polygraph test consists of several different phases: a pretest interview, the collection of charts, and an analysis of the polygraph charts. The average polygraph test will usually last two to four hours from beginning to end. The longest part of the polygraph test will be the pretest interview, which normally lasts anywhere between 45–90 minutes.

During the pretest interview, the polygraph examiner will explain your legal rights, explain the polygraph instrument and how it works, discuss the issue, develop and review all questions asked on the polygraph test, cover general background information, and will provide instructions for the actual testing phase.

The polygraph consists of three or more components: the pneumograph component, which records respiration and movement; the galvanograph or GSR component, which records your galvanic skin response and changes in skin resistance; and the cardiosphygmograph component, which records your relative blood pressure and pulse rate. Once the pretest interview has been completed, the polygraph examiner will place four attachments on the subject's person: two rubber pneumograph tubes across the upper chest and abdomen, two metal finger plates across the ring and index finger, and a blood pressure cuff around the upper arm. During the collection of charts, the examiner will ask the reviewed questions two to three times and run a minimum of two to three separate charts before rendering an opinion.

It is expected that anyone who takes a polygraph test will be nervous; however, nervousness does not normally interfere with the test. Even though a person has high blood pressure, diabetes, etc., their body has a set of normal patterns on a daily basis. When a person decides to lie, however, physiological changes begin to take place in the body. Blood pressure begins to increase or decrease. Heart rate can increase or decrease. A person's heart can skip a beat. Blood volume begins to change. These are just a few of the types of physical changes that can occur. In the case of deception, more than one type of physiological change usually occurs. The person taking the test must decide whether or not they are going, to tell the truth, or whether or not they are going to lie and withhold information about the relevant issues. Once a person decides to tell the truth, the body goes about its normal patterns with no significant or consistent changes.