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. 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. Even though a person has high bloodpressure, 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. Bloodpressure 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 occur. 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.