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What a polygraph measures

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What a polygraph measures

Polygraphs chart out blood pressure, breathing rate and sweat production to help an examiner determine whether a subject may be lying. The LX4000, produced by Lafayette Instrument, displays both raw sweat production data and an adjusted chart designed to be easier to read. There are concerns that the adjusted chart, produced in "auto" mode may be introducing inaccuracies.

Sensors measure body changes as subject answers yes-no questions; data recorded on computer. Hover over the numbers below for more information.

1

Blood pressure cuff records pulse, blood pressure

2

Rubber tubes
placed over chest
and abdomen record breathing

3

Two metal plates attached to fingers record sweating

4

Laptop connects to polygraph

Click to learn more about the charting software.

X

1

Breathing
Measures both chest breathing (top) and stomach breathing (bottom)

2

Sweat duct activity

3

Blood pressure

Measurements of sweat during a polygraph test, known as electrodermal activity or EDA, can be measured using two separate modes with the LX4000. There is concern that the adjusted, "auto," mode produces less accurate data.

Learn more about the concerns

Raw data

Adjusted data from "auto" mode

Polygraph examiners look in the charts for consistent and significant responses to questions. Click through to see examples of how the raw data and "auto" mode can diverge.

Back to summary

Previous chart

More EDA charts

Sources: American Association of Police Polygraphists, American Polygraph Association, McClatchy reporting

Graphic: Melina Yingling, Judy Treible, Danny Dougherty

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