Screen Calibration Tech Talk

Question Last Updated on: 5/29/2025

Raymond Nelson, Mark Handler, and Brent Smitley

Screen calibration – the practice of adjusting a computerized polygraph chart display to a fixed size – is no longer required or advised. Computerized polygraphs record and store data as time-series numerical values. That is, polygraph data consist of a sequence of numerical values recorded many times per second for each physiological sensor, across the duration of the examination.

These values are stored digitally and may be subject to scaling or other forms of signal processing for the purpose of visualization—either on a computer screen or as a printed chart. However, these graphical displays are merely representations of the data, not the data themselves. This differs from older analog polygraph instruments, where data were recorded directly as plotted lines on moving paper and were fixed at the time of recording. In analog systems, post-hoc scaling or reprocessing was impossible. In contrast, computerized polygraph data can be re-scaled and re-processed without any loss to the original signal.

Neither analog nor computerized polygraph data are designed to represent actual physiological measurements in standardized physical units. Since polygraph data are not interpreted in terms of linear physical measurements, efforts to analyze them using physical measurement units (e.g., inches, millimeters, or electrical volts) can result in misleading assumptions and erroneous conclusions. For instance, cardiovascular activity may be clinically measured in millimeters of mercury (mmHg), but polygraph recordings are not scaled to or interpreted using those units. Similarly, respiration volumes and electrodermal activity can be quantified in biomedical units, but polygraph outputs are scaled for aesthetic convenience and do not need to be displayed or scaled in absolute physiological units.

Both paper and digital polygraph formats reflect changes in physiological activity. Interpretation is based not on absolute values, but on comparisons of physiological reactivity across different stimulus conditions. Fortunately, according to the Buckingham π theorem, in any valid system of measurement and analysis, the choice of units is arbitrary or irrelevant. This means that the same analytic conclusions can be derived regardless of the measurement scale, as long as ratios or relative changes are preserved. For example, a ratio derived from data measured in inches will be identical to that derived from the same data in millimeters.

As a practical consequence, calibrating a computerized polygraph display to a fixed size has no effect on the validity of the underlying data or the conclusions drawn from them. Conversely, not calibrating the display also has no impact on analytic validity. While screen calibration may be completed for convenience or visual consistency, it should not be required. Efforts to mandate screen calibration may promote false expectations of physical linearity and invite invalid criticisms that are unrelated to polygraph validity. Analytic conclusions derived without screen calibration are just as valid as those obtained with it.

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