For the Public

What is Polygraph?

What a polygraph examination is in plain English: the three phases (pre-test interview, data acquisition, analysis), the sensors used, validated techniques, and what results mean.

What a Polygraph Examination Is

A polygraph examination is a psychophysiological test of deception or recognition. During an examination, the examiner records physiological responses — respiration, electrodermal activity, cardiovascular activity, and movement — while the examinee answers a structured set of questions. The recorded responses are analysed by the examiner using a validated method and are reported as a categorical result together with a margin of uncertainty.

Phases of an Examination

A BPS-standards examination has three phases:

  1. Pre-test interview — the examiner explains the process, obtains the examinee’s informed written consent, reviews medical and psychological suitability, and reviews every test question with the examinee.
  2. In-test — the examinee is connected to the polygraph instrumentation, and responses are recorded while the reviewed questions are asked in a defined order.
  3. Post-test — the examiner may give the examinee an opportunity to explain any observed physiological responses and to resolve inconsistencies.

An examination is scheduled for not less than 90 minutes and is conducted in person. Electronic communication is not substituted for in-person contact with the examiner.

What the Result Means

Results of deception tests are commonly reported using the terms:

  1. Deception Indicated (DI) — the recorded data indicate deception in relation to the test questions;
  2. No Deception Indicated (NDI) — the recorded data do not indicate deception;
  3. Inconclusive (INC) or No Opinion (NO) — the examiner cannot reach a categorical conclusion.

Results of screening tests are commonly reported as Significant Response (SR), No Significant Response (NSR), Inconclusive (INC), or No Opinion (NO).

Limits of the Polygraph

A polygraph result is not a determination of legal fact. It is an expert opinion based on the interpretation of physiological data. Validated polygraph techniques have published accuracy rates, which the BPS Standards of Practice set at 90% or greater (excluding inconclusives) for evidentiary examinations and at lower thresholds for paired, investigative, and screening testing.

A polygraph examination is not a substitute for investigation, evidence, or judicial determination. It is one input that a competent commissioner or decision-maker may consider alongside other information.

Use in UK Law

Polygraph examination has a statutory basis in England and Wales, but only as a licence condition for specified sex offenders and specified domestic abuse perpetrators — see the Legislation section. Polygraph is otherwise used on a consensual basis in pre-employment screening, corporate investigations, civil and family matters, and private contexts, subject to general UK law.