Polygraphs have emerged as one of the most prominent tools used by a private investigator to collect truthful information from subjects during investigations. There are numerous instances where polygraph has been used to shed more light into the investigation by helping private detective to discern the potential culprits and the innocent parties.

With the above in mind it important to note that the use of polygraphs depends on the nature of the case in hand. The test has to be tailored to fit every particular investigation, especially with regards to the types of questions to be asked, and the subjects to be questioned.

However, the basic principles and procedures of administering polygraphs are the same across many of the private investigator agencies. In order to achieve the necessary accuracy, the proper laid down procedures must be used. It therefore, becomes paramount for the private investigator administering the test to be sufficiently experienced and skilled. This not only refers to the questioning but also in reading and analysing the polygraph’s results.


The Guiding Principles

The basic guiding principle in the working of the polygraph is that humans have psychological reactions every time they are about to respond to a question. This reaction creates physiological changes such as blood pressure and pulse, temperature, breathing and perspiration. By measuring and continuously monitoring these changes, the machine is able to predict with considerable accuracy when a person is lying or when a person is telling the truth. This is the principle that private investigator rely on when they administer polygraphs.


Procedures In Polygraph Admiration

Polygraphs are based on two elements; the first of which is to establish a base level of vital signs responses; and the second part is measuring and monitoring the vital signs responses during the interview. In this regard, equipment must be used for both these elements to be achieved and at least three of the mentioned vital signs must be monitored.

To administer a polygraph, the private investigator begins with a pre-test phase. It is the part of the interview where the interviewer asks several questions intended to determine the levels in the vital signs that are associated with telling the truth and the vital signs associated with lying. The questions in this part of the interview are basically “Yes” or “No” questions.

After completion of the first phase, the next stage involves asking questions that the private investigator deems as relevant to the investigation at hand. While asking the questions, the examiner also monitors the vital signs being monitored and recorded. In doing so, the seasoned private investigator conducting the investigation is able to identify when the interviewee is lying.

Since most of the polygraphs are based on measuring changes of multiple physiological elements, it is very difficult to cheat, thus yielding a high degree of accuracy.