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. Once a person decides to tell the truth, the body goes about its normal patterns with no significant or consistent changes. Some of the most frequent errors may be caused by lack of training, non-functioning equipment, failure to properly prepare the examinee for the examination, poorly worded test questions, failure to assess the examinee's emotional and physical condition, improper use of testing techniques, a lack of quality control review, and misreadings of the physiological data on the polygraph charts.