What Is Amplitude in Polygraph Testing?
Amplitude in polygraph science refers to the magnitude or height of a physiological response as recorded on a polygraph tracing — essentially, how “big” the response is. Amplitude is one of the most important measurement features used in both manual and automated polygraph scoring, as it quantifies the strength of the examinee’s physiological reaction to each test question.
Amplitude Across Polygraph Channels
Amplitude is measured differently for each physiological channel recorded by the polygraph:
Electrodermal Activity (EDA) Amplitude
In the EDA channel, amplitude refers to the peak conductance change from the pre-stimulus baseline level. When an examinee responds to a question with increased sweat gland activity, the skin conductance rises — the height of this rise (measured in microsiemens, μS) is the EDA amplitude. EDA amplitude is considered the single most diagnostically powerful feature in polygraph scoring, showing the highest correlation with deception across research studies.
Cardiovascular Amplitude
In the cardiovascular channel, amplitude can refer to two distinct measurements:
- Blood pressure amplitude — The degree of sustained blood pressure elevation (baseline rise) following a question, measured via the cardio cuff
- Pulse amplitude — The height of individual pulse waves. A decrease in pulse amplitude (vasoconstriction) following a relevant question can indicate stress associated with deception
Respiratory Amplitude
In the respiratory channels, amplitude refers to the depth (height) of breathing cycles recorded by the pneumograph sensors. Respiratory suppression — a decrease in breathing amplitude following relevant questions — is a key scored feature indicating potential deception.
Amplitude in Scoring Systems
Amplitude measurements are fundamental to polygraph scoring at every level:
Manual Scoring
In manual numerical scoring, the examiner visually compares the amplitude of responses at relevant questions against the amplitude at comparison questions for each channel. The magnitude of the difference is assigned a score on either a 3-position (−1, 0, +1) or 7-position (−3 to +3) scale.
Automated Algorithms
In computerised scoring algorithms like OSS-3, PolyScore, and ESS-M, amplitude is measured mathematically as part of the Kircher feature extraction process. The algorithm precisely calculates the amplitude difference between relevant and comparison question responses and inputs these values into its statistical classification model.
Amplitude as Part of the Defensible Dozen
Amplitude-related features are well represented in the Defensible Dozen — the approximately twelve validated physiological features used in polygraph chart interpretation. These include EDA amplitude of increase, cardiovascular baseline amplitude rise, pulse amplitude decrease, and respiratory amplitude suppression.
For more on how physiological data is measured and scored in polygraph testing, visit the Polygraph Examiner Hub or explore our research database.