How Does a Polygraph Examination Work?

A polygraph examination is a standardized test administered to a person in order to determine the probability of a person being truthful to pre-determined questions. During a polygraph examination, multiple components are used to measure physiological changes that occur inside a person while questions are being put to that person. A trained polygraph examiner can use the data collected during a polygraph examination to conclude whether a person had been truthful or not when answering questions based on the physiological changes that occur in reaction to the questions. The field is often referred to as forensic psychophysiology referencing the interplay between physiological reactions when exposed to psychological stimuli. The modern day polygraph instruments are used worldwide as the leading detection of deception instrument. Utilized by government, military, police, security, businesses and a multitude of other various organisations internationally, it remains the most accurate of truth verification system in existence.

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The Polygraph Instrument

During a polygraph examination physiological changes are monitored with the use of multiple components. The physiological data collected via the components are accumulated and visually illustrated in the form of polygraph charts through the electronic polygraph instrument. Qualified polygraph examiners can interpret the data presented in the charts with the use of a scientifically formulated normative scoring procedures such as the Empirical Scoring System (ESS) and can quantitatively determine the probability of whether a person was being truthful or not when answering a set of predetermined questions.

The components used during a polygraph examination have been specified in the regulations of the American Polygraph Association and the Southern African Polygraph Federation based on the scientific validation received for the use of these specific components. The Empirical Scoring System (ESS) has been validated as a scientifically valid and reliable scoring system based on the data collected by the specific standardized components. The following is a detailed illustration of the different components used during a polygraph examination in conjunction with the associated physiological data recorded by the individual components.

The Electronic Polygraph System

There are currently four major manufacturers of electronic polygraph systems used in modern day polygraph testing; Limestone, Lafayette, Stoelting and Axciton. All manufacturers specialize in producing medical grade monitoring components and systems for polygraph examinations. The electronic polygraph instrument consists of a mechanical box to which all the components are connected. The polygraph instrument relays the recorded physiological data through the use of a transducer to a standard computer where the data is interpreted by software designed for this unique task. The software illustrates the accumulated data in the form of graphs which can be visually interpreted by a polygraph examiner through the use of the standardized Empirical Scoring System (ESS). If you would like more information regarding the individual polygraph instrument manufacturers, please visit their sites here:

Axciton Systems Inc.       Lafayette Instruments       Limestone Technologies       Stoelting

The electronic polygraph instrument

The Electronic Polygraph Components

Lie Detector Testing in South Africa

The components used during a polygraph exam in order to record the physiological data includes two pneumograph tubes, one medical blood pressure cuff, two electro-dermal nodes and an electronic seat sensor. The electronic polygraph instrument is a mechanical box to which all the individual components are connected. The mechanical box receives the information from the pneumograph tubes and blood-pressure cuff in the form of pneumatic changes measured by an internal transducer. It also receives electronic signals from the electronic nodes and the electronic seat-pad. The data is relayed to the laptop or desktop computer on which the polygraph software is installed.

The polygraph software is designed by each individual manufacturer for the specific polygraph instrument model and is responsible for electronically quantifying the data received from the individual components. Once the data has been quantified, it illustrates the data for the examiner in the form of visual graphs which the examiner can use to visually compare the physiological changes that occurred inside the examinee while each unique pre-determined question was posed.

Pneumograph Tubes

The pneumograph tubes are placed on the thorax and lower abdomen of the examinee and is responsible for measuring and recording abdominal and thoracic breathing patterns and muscle movements. It is represented by two lines when visually illustrated in the polygraph software. As previously noted, one of the primary indicators of physiological responses are measured in respiratory variances, or changes in breathing patterns. When exposed to the test stimuli (the test question) changes in breathing patterns are visible.

How does polygraph testing works.
Electro-Dermal Activity Sensors

The electrical sensors are placed on two fingertips of one of the examinee’s hands to record autonomic activity in the palmar or distal regions. The electronic nodes applies a constant voltage to the skin, so small that it cannot be felt or perceived by the individual, and is used to measure changes in skin conductance, or the reciprocal skin resistance expressed in units called microSiemens. According to Ohm’s law, skin resistance (R) equals the voltage (V) applied between two electrodes on the skin divided by the current passed through the skin (I). The law is expressed as R = V/I.

How a polygraph examination works.
Blood-Pressure Cuff

The blood-pressure cuff is connected to a sphygmomanometer and the polygraph instrument. The bloodpressure cuff is usually placed on the upper-arm of the examinee and is inflated to a level between 50 and 80 mmHg. The blood-pressure cuff measures and records changes in the blood pressure and heart rate of the examinee. The strongest indicator of a physiological response in the cardiovascular system is an increase in the relative blood pressure. This is measured based on the amplitude of the increase in blood-pressure in response to the test stimuli.

Cardiovascular Changes
The Electronic Seat Sensor

The electronic seat-pad is a movement sensor on which the examinee sits during the examination. The sensor monitors for any physical movements made during the examination as a means of counteracting known physical countermeasures. Although there are numerous means of attempting to manipulate a polygraph examination using physical movements, professional polygraph examiners know of these techniques and actively monitor for them. Research indicated that people who attempt to manipulate an examination is more likely to inadvertently fail the examination.

Can you beat a polygraph test?

The Procedure of a Polygraph Test

The polygraph examination procedure is a standardized, structured procedure. During the administration of a polygraph examination, there are multiple differential phases that need to be applied in a standardized sequence. The sequence includes the preparation phase, the pre-examination phase, the examination phase and the post examination phase. A polygraph examination takes approximately 1 hour to complete excluding the preparation phase.

Preparation Phase

Prior to conducting the polygraph examination the examiner needs to meet with the investigating team or representative to gather information regarding the matter under investigation or reason for the polygraph examination. In the case of a pre-employment screening test or continuous integrity screening program this will only need to occur once in order for the examiner and the representative to consult prior to the implementation of the program. In the case of a specific incident examination as part of an investigation, the examiner will need gather information in order to properly prepare for the examination. The examiner will meet with the investigating team or representative to be briefed regarding the incident. The more detailed the information is that is given to the examiner, the better the questions will be. The examiner will formulate and discuss the questions with the client in order to ensure that the questions will be of maximum benefit to the investigation. Once the questions have been confirmed the examination process can begin. Whenever it is possible, the examinee should be informed beforehand that they will undergo a polygraph examination to allow them the opportunity for consultation if requested.

The Pre-Test Interview

The polygraph examination starts with a pre-test interview. During the pre-test interview the examinee meets with the examiner in a suitable venue. During this phase the examinee is adequately identified based on their full names, their position at the company as well as their company identification number if applicable.  The examinee is given an explanation of how the polygraph works, the different components used during the examination and is informed of what they can expect during the examination. The examinee is also informed of their rights with regards to undertaking a polygraph examination. Once the examinee has been properly briefed, they give their informed consent in writing to continue with the process and is afforded the opportunity to raise objections or ask any questions regarding the process. Subsequently the examinee will be asked questions regarding their medical and psychological fitness. This is to assist the examiner in determining whether a person is medically fit to undergo a polygraph examination and to identify any potential external factors that could compromise the examination. It is important to note that none of the information obtained regarding the examinee's medical fitness is reported to the investigating team or company representative unless it is relevant to the matter under investigation. If any medical information does need to be disclosed, the examinee will be informed prior to the release of the information and will be afforded the opportunity to discuss the disclosure with the examiner.

Once it has been ascertained that the examinee is fit to continue with the examination, the matter being tested is thoroughly reviewed with the examinee. The examinee is given a detailed explanation as to the reason for the examination and is afforded the opportunity to make any admissions or provide the examiner with any information relevant to the matter under investigation. Once the case has been discussed with the examinee in detail, each question that will be put to the examinee during the examination phase is discussed with the examinee. This is to ensure that the examinee understands each individual question prior to the examination being conducted. The examinee is afforded the opportunity to ask for clarity regarding any of the questions, and if necessary to make any admissions to the questions prior to the examination.

The Examination Phase

Once all the matters under investigation and case facts have been reviewed with the examinee, the examination phase will begin. During the examination phase the components are placed unto the examinee in the appropriate locations. The first step of the examination phase is conducting an acquaintance test. As prescribed in the regulations of the American Polygraph Association (APA) and the Southern African Polygraph Federation (SAPFED), an acquaintance test is administered prior to the start of the main test. The acquaintance test, often referred to as a practice test, is a short test administered to allow the examinee the opportunity to familiarize themselves with the testing process, and to afford the examiner the opportunity to calibrate the recordings based on the physiology of the examinee. This also affords the examiner the opportunity to identify any possible anomalies that could influence the examination process.

The main test is composed of three to five chart recording sequences in which all the questions are posed to the examinee during each individual sequence. The questions are rotated during the examination in accordance to a pre-validated question sequence, allowing for maximum accuracy. The examination is conducted in accordance to the test protocols stipulated for the specific question technique in use. Each individual test sequence takes approximately 5 minutes each, and the examinee is afforded a rest opportunity between each sequence. During the rest opportunity the examinee is also afforded the opportunity to indicate if they have any concerns regarding any of the questions.

The Post-Examination Interview

Once the examination is completed there is a final short interview with the examinee. The examinee is asked to give feedback on how they perceived the polygraph examination. If appropriate, the results of the polygraph examination is discussed with the examinee. Once the post-examination interview is completed, the examinee signs a 'post-examination statement' in which they declare that the examination process was completed as it was initially explained to them, and in which they agree that they were treated fairly during the process.

Analyzing Data and Reporting Results

Once the examination is completed the data that is recorded during the polygraph examination is analyzed by the examiner. Scoring of a polygraph examination begins with the identification of observable and therefore measurable physiological responses that have been correlated to the act of being deceptive on a statistically significant level and which can be combined into an effective diagnostic model. A small number of physiological indicators have repeatedly shown to be correlated with deception in structural decision models presently used in field polygraph programs. The physiological reactions are quantified and accumulated to provide a result. Once a result has been obtained the examiner will write a report detailing the examination process, including the pre-examination interview, the questions asked and the result of the examination. The report is subsequently released to the investigating team or representative.

The Scientific Principles of a Polygraph Examination

A polygraph examination is a procedurally administered test which measures the probability of a person being deceptive or honest based on the presence of physiological responses correlated to the act of being deceptive.

Polygraph test data is a combination of physiological proxies that have been shown to vary significantly in response to different types of test questions as a function of deception or truth telling. According to the analytical theory of polygraph testing, greater changes of physiological activity occur at different types of test stimuli as a function of deception or truth telling. The physiological mechanics of polygraph responses during comparison question tests occur in the context of the autonomic nervous system (ANS) which includes both sympathetic (S/ANS) and parasympathetic (PS/ANS) components (Bear, Barry, & Paradiso, 2007; Costanzo, 2007; Maton et al., 1993; Paradiso, Bear, & Connors, 2007; Silverthorn, 2009; Standring, 2005). The ANS regulates involuntary processes including cardiac rhythm, respiration, salivation, perspiration, and other forms of arousal. S/ANS activity is responsible for stimulation of the internal organs in response to activity demands. PS/ANS activity serves to reduce physiological activation to the minimum level necessary to ensure both longevity and adequate response to situational demands. PS/ANS and S/ANS activity are therefore in homeostatic balance with respect to real or perceived demands.

Observable and recordable physiological changes in physiological activity that are structurally correlated with deception and truth-telling during comparison question testing include specific measurable physiological reactions. These measurable reactions have been described in several publications (ASTM International, 2002; Bell et al., 1999; Department of Defense, 2006a, 2006b; Harris, Horner & McQuarrie, 2000; Kircher & Raskin, 1988; Kircher et al., 2005; Krapohl & McManus, 1999; Raskin & Hare, 1978; Raskin et al., 1988). Physiological responses for these three primary sensors (respiratory suppression, electrodermal activity, and cardiovascular activity) are easily observed and recorded.

In essence, during a polygraph examination stimuli is presented to the examinee in the form of questions. Physiological activity or changes that occur in response to these questions are recorded and analysed in order to make a conclusion on whether the person was telling the truth or not based on the presence of physiological responses to the questions.

If you would like more in depth information regarding the scientific principles and research regarding polygraph examinations, please do not hesitate to visit the site of the American Polygraph Association.

Johannesburg, Pretoria, Cape Town, Bloemfontein and Across Africa

The main offices of the Polygraph Institute of South Africa is conveniently located in Centurion from which we provide our services throughout Gauteng: Pretoria, Midrand, Brits, Johannesburg, Randburg, Sandton, Roodepoort, Heidelberg, Krugersdorp and the entire surrounding area. We also have professional affiliates assisting us across South Africa, conveniently located in Cape Town, Durban, Bloemfontein and many other regions. Our examiners are also capable of travelling to wherever they may be needed in South Africa, and have been active in Kimberley, Polokwane, Nelspruit and many other towns and cities. Essentially, our examiners can be requested to go to any location in South Africa.

Our examiners are also active and experienced in many other African countries. With successful operations in Zambia, Zimbabwe, Angola, Kenya, Swaziland, Uganda, Ghana and more. Wherever you may need our services, we are simply a phone call or email away.

How Can We Help You?

We are eager to assist you and your company and to provide you with polygraph examinations of the highest quality. If you have any questions or queries, would like us to give a presentation or if you would like more information on any of the professional services that we provide, please do not hesitate to ask. Our staff members can't wait to hear from you.

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ASTM (2002). Standard Practices for Interpretation of Psychophysiological Detection of Deception (Polygraph) Data (E 2229-02). ASTM International

Bear, M. F., Barry, W. C. & Paradiso, M.A. (2007). Neuroscience: Exploring the brain: Third Edition. Philadelphia, PA: Lippincott Williams & Wilkins.

Bell, B. G., Raskin, D. C., Honts, C. R. & Kircher, J.C. (1999). The Utah numerical scoring system. Polygraph, 28 (1), 1-9.

Costanzo, L. (2007). Physiology. Hagerstwon, MD: Lippincott Williams & Wilkins.Department of Defense (2004a). Federal psychophysiological detection of deception examiner handbook.

Department of Defense (2006a). Federal psychophysiological detection of deception examiner handbook. Reprinted in Polygraph, 40 (1), 2-66.

Department of Defense (2006b). Test data analysis: DoDPI numerical evaluation scoring system.

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Honts, C. R. & Devitt, M.K. (1992). Bootstrap decision making for polygraph examinations. Department of Defense Polygraph Institute report No DoDPI92-R-0002.

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