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

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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.