Polygraph test

The polygraph test explained | Procedure of a professional lie detector exam

Polygraph test

It is important to understand that a polygraph test is a complete process and that the polygraph instrument is only a part of the process. Actually, the polygraph instrument should be seen as a tool used by an expert to exercise its expertise.

The polygraph test has a specific procedure that it has to follow to guarantee the reliability of the examination. It is important to understand that the polygraph test needs to follow a methodology so that the test can have a scientific reliability.

Steps of a polygraph test

A professional polygraph test needs to comply with certain conditions to guarantee the reliability of the test. First of all, it needs to use validated techniques, applying the methodology required by these techniques.

Also, there are different types of polygraph tests. Depending on the type, the content of the exam can be different, but they will normally all follow certain steps that are explained below.

Then there are certain conditions that have to be given like for example:

  • The test must be voluntary, the examinee needs to sign a written consent form
  • The location where the test is performed must guarantee privacy and free of stimuli that could distract the examinee
  • The exam must be legal, not infringing anyone’s rights
  • The examiner must make sure that the examinee is in the right condition to undergo a polygraph examination

When these basic conditions are met the polygraph examiner will then proceed with the test, guiding the examinee through the 3 phases of the exam, pre-test, in-test and post-test.


Interview polygraph test

During the pre-test interview the examiner explains the process of the exam to the examinee, obtains his or her consent, inquires about any medical conditions that could influence the test.

Then the examiner will normally explain the polygraph instrument and its functioning. Later the examiner will proceed with the pre-test interview where the case under investigation will be dealt with. This consists in the testimony of the examinee on the case with the subsequent questions of the examiner to understand the case.

After having gathered all the needed information the examiner will elaborate the questions for the polygraph examination. It is only after this interview that the examiner can decide what questions will be used to test the veracity of the examinee.

After having read the questions of the polygraph test to the examinee, the expert will connect the examinee to the polygraph and proceed to the collection of psycho-physiological data, which is known as In-test.


During In-test the examinee will be asked to answer to the questions posed by the examiner, while connected to the polygraph. This is the phase where psycho-physiological data is collected from the examinee to the different stimuli (questions) that have been formulated by the examiner to solve the issue under investigation.


During the post-test the examiner will examine the psycho-physiological data collected during the In-test phase.

Normally, the polygraph examiner will give the examinee an opportunity to add any information to complete the final report.

The polygraph report

The final product of a polygraph test is the report of the examination. This report is given in a written form to the people or person designated by the examinee in the consent form.

The polygraph report normally contains the identification information of the examinee, the testimony of the examinee, the questions used to assess his or her credibility and the final result of the test.

Normally a polygraph test will have 3 possible results:

  • DI (Deception Indicated) The examinee fails the test
  • NDI (No Deception Indicated) The examinee shows veracity
  • INCONCLUSIVE: The psycho-physiological information collected is not sufficient to arrive to a conclusive opinion on the veracity or deception of the examinee

Polygraph test questions

This is an important point, as most public has unfortunately a misconception on the use of polygraphs. As explained before the examiner will decide on the questions to use during psycho-physiological data collection.

The formulation of the polygraph questions depends on:

  • The objective of the test – What issue is under investigation? (Theft, infidelity, industrial espionage, rape, homicide, etc.
  • The testimony of the examinee about the issue under investigation

The contractor of a polygraph test must be aware that when hiring a polygraph service, it is hiring an expert in the field of psycho-physiology. Question formulation is one of the expert skills that a polygraph examiner must have to be able to exercise his profession.

A polygraph examiner that does not know what questions to ask to solve an issue should be viewed with skepticism.

If you want to know more about polygraph testing, please visit our F.A.Q. section, where we answer through videos the most frequently asked questions.

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