Polygraph Test Results Explained: Clear Answers to Common Questions
What polygraphs measure: Polygraphs record involuntary physiological changes in heart rate, blood pressure, breathing, and skin conductivity to infer whether a response is truthful. Modern exams compare reactions to relevant questions against neutral control questions using validated scoring systems.
Main result categories:
NDI / NSR – No Deception Indicated or No Significant Response: stable, interpretable charts with no significant reactions; examiner concludes the examinee was truthful.
DI / SR – Deception Indicated or Significant Response: significant, consistent reactions suggest deception on one or more questions.
Inconclusive / No Opinion: charts are valid but don’t meet criteria for a definitive judgment; the examiner can’t determine truth or deception.
PNC – Purposely Non‑Cooperative: examinee fails to follow instructions or uses countermeasures; results are treated like a refusal.
Inconclusive isn’t failure: Factors such as medical conditions, fatigue, medications, or poorly worded questions may weaken responses and lead to an inconclusive result. The American Polygraph Association notes that inconclusive results aren’t counted as errors and often warrant a second exam.
Test duration and process: A professional exam lasts 1½–2½ hours for most private tests and 3–6 hours for government pre‑employment screenings. The exam has three phases: pre‑test interview, in‑test questioning, and post‑test review.
Nervousness doesn’t cause failure: Examiners look for relative changes between questions, so normal nervousness is accounted for and does not automatically lead to a deceptive result.
Choose a qualified examiner: Look for certification from reputable organizations (e.g., American Polygraph Association) and experience in the specific type of test; this ensures proper scoring, confidentiality, and ethical handling of results.
Understanding these fundamentals will help you interpret your polygraph results, prepare properly, and choose a reputable examiner.
Understanding How Polygraph Results Are Determined
A polygraph (often called a “lie detector”) does not detect lies directly. Instead, it measures changes in physiological functions—heart rate, blood pressure, respiration, and electrodermal activity—while an examinee answers a structured series of questions.
During the pre‑test phase, the examiner explains the process, establishes a rapport, and collects background information. They review all questions with the examinee, ensuring each can be answered “yes” or “no” and is free of ambiguity. This phase also includes a practice or “acquaintance” test to calibrate the equipment and establish baselines.
In the in‑test phase, sensors are attached to monitor physiology. The examiner asks three types of questions:
Relevant questions directly address the incident under investigation.
Comparison (control) questions are general, uncomfortable questions meant to elicit a stronger response if the examinee is innocent.
Irrelevant questions help the examinee relax and reset between relevant and control questions.
The examiner scores each question using numerical scoring systems (e.g., 3‑ or 7‑position scales) and compares reactions to determine whether the physiological responses indicate deception, truthfulness, or are inconclusive. The post‑test phase involves reviewing the charts, computing the scores, and issuing a written report.
Result Categories and What They Mean
NDI / NSR (No Deception Indicated / No Significant Response): The examiner observed stable, interpretable charts with no significant reactions. The evaluation criteria led to the conclusion that the examinee was truthful. In screening tests, this result is often reported as NSR (No Significant Response) because the absence of reactions is noted rather than inferring definitive honesty.
DI / SR (Deception Indicated / Significant Response): The charts show consistent, significant changes in physiological responses that meet the criteria for deception. This result does not mean the person is definitely lying; it means the data pattern is consistent with deception. Qualified examiners note which questions triggered the reactions and may advise follow‑up.
Inconclusive / No Opinion: The physiological data are valid but insufficient to draw a reliable conclusion. This can occur when responses are inconsistent or weak, or when external factors distort the charts. Inconclusive results are not failures. They simply indicate that another test or modified questioning may be necessary. The APA warns against treating inconclusive outcomes as errors because doing so distorts accuracy statistics.
PNC (Purposely Non‑Cooperative): The examinee deliberately failed to comply with instructions or attempted to manipulate their physiology. Examiners detect these countermeasures and classify the result as PNC. PNC is treated as the equivalent of refusing to take the test. Attempts to fool the polygraph are generally unproductive and can result in disqualification.
Why Results Can Be Inconclusive or False
An inconclusive result does not reflect guilt or innocence; it highlights inadequate data. Common causes include:
Medical conditions or medications: Certain illnesses and medications can affect heart rate, blood pressure, or skin conductance, producing weak or unusual responses. Examiners often advise examinees to avoid stimulants or psychoactive substances before testing.
Extreme anxiety or fatigue: Excessive nervousness may elevate all physiological measures equally, making it hard to differentiate responses. Getting restful sleep and discussing concerns during the pre‑test helps minimise this effect.
Poorly designed or misunderstood questions: Ambiguous questions or cultural misunderstandings can evoke inconsistent reactions, leading to inconclusive scores. Experienced examiners use clear, single‑issue questions and review them with examinees to avoid confusion.
Countermeasures: Attempts to manipulate results—such as biting the tongue, altering breathing, or using drugs—can distort chart readings and produce an inconclusive or PNC result.
In cases where the charts are distorted by overt movements or inconsistent reactions, examiners may record a No Opinion (also called Incomplete) and recommend retesting.
How Long Does a Polygraph Test Take?
A credible polygraph exam is not a quick yes/no session. Most single‑issue tests for private clients last 1½–2½ hours, which allows time for the pre‑test interview, chart collection, and post‑test review.
The Three Phases
Pre‑Test Interview (60–90 minutes): Examiner and examinee discuss the case, medical history, and test procedures. Baseline physiological readings are taken, and questions are refined and practiced.
In‑Test Phase (30–60 minutes): Sensors monitor physiology while the examiner asks prepared, relevant, comparison, and irrelevant questions. Questions are repeated several times to ensure reliable data.
Post‑Test Review (25–45 minutes): The examiner scores the charts, compares relevant vs control responses, and explains preliminary findings. A written report may follow.
Who Interprets the Results?
Only trained, certified polygraph examiners should interpret polygraph charts. According to the American Polygraph Association, examiners must meet rigorous training standards, use validated protocols, and attend continuing education to maintain certification. The Kimbrough article notes that results are confidential and are shared only with the examinee or authorized parties.
Accuracy and Validity
Polygraph accuracy depends on proper test formats, examiner skill, and the nature of the issue being investigated. Research and field studies indicate polygraph exams, when administered by competent examiners following standardized procedures, exceeds 90%. The APA cautions that including inconclusive results as errors artificially lowers accuracy statistics and that a second exam often resolves inconclusive outcomes. Polygraphs are therefore considered one of the most accurate methods available for assessing truthfulness when used appropriately.
Uses of Polygraph Results
Polygraph examinations are used in various contexts:
Criminal and civil investigations: to focus investigations, corroborate statements, or clear suspects.
Employment screening: for sensitive positions in law enforcement, intelligence, and security. Some states and industries restrict when polygraphs can be used; always consult local laws.
Therapeutic and relationship contexts: to aid counselors in addressing trust issues, infidelity, or post‑conviction supervision.
Confidentiality
Professional examiners adhere to strict confidentiality rules. Examiners will not release results without written authorization from the examinee or an order from a court or legal authority. Government programs, such as the TSA, must also abide by the Privacy Act when handling polygraph data.
Selecting a Qualified Examiner
Choosing the right examiner is critical for reliable results. Consider the following:
Credentials and training: Look for membership in reputable organizations such as the American Polygraph Association or the American Association of Police Polygraphists. Members must complete formal training and adhere to codes of ethics.
Experience: Experienced examiners are better at formulating questions, interpreting charts, and handling factors like anxiety or medical conditions.
Transparent procedures and fees: Legitimate examiners will explain the process, provide written consent forms and be upfront about costs. Avoid “discount” or “over‑the‑phone” polygraph services, which may lack proper equipment and protocols.
Common Myths and Countermeasures
“I’ll fail if I’m nervous.” Nervousness is expected; examiners focus on changes between questions, not absolute levels of arousal. Disclosure of medical or psychological issues helps the examiner interpret data correctly.
“I can beat the test with countermeasures.” Attempts to manipulate results (physical, mental, or pharmacological) are detected and often result in a PNC or No Opinion classification. Countermeasures can include biting your tongue, altering breathing or taking drugs, but these tactics rarely produce the desired outcome and may cause retesting or dismissal of the result.
“Inconclusive means I’m guilty.” Inconclusive results simply mean the data were insufficient to draw a conclusion. A follow‑up exam with refined questions often resolves the issue.
“Polygraph results are always admissible.” Legal admissibility varies by jurisdiction. Some courts allow results by agreement of both parties; others exclude them entirely. Always consult an attorney regarding legal implications of polygraph results.
Frequently Asked Questions
How accurate is a polygraph test?
Research suggests accuracy rates exceeds 90 % for exams conducted by trained examiners. Factors such as examiner competence, test format and examinee cooperation influence reliability. Inconclusive results should not be counted as errors.
What happens if my polygraph result is inconclusive?
An inconclusive outcome means there wasn’t enough reliable data to determine truth or deception. You may be asked to return for a second exam with adjusted questions or after addressing potential factors such as medications or fatigue.
How long until I get my results?
Most examiners provide preliminary findings immediately after the post‑test review. A formal written report may follow within a few days. Government or pre‑employment processes may take longer due to additional review layers.
Can I prepare for a polygraph exam?
Yes. Get plenty of rest, take your regular medications (unless advised otherwise), eat normally and avoid alcohol or recreational drugs. Be honest during the pre‑test interview and ask questions if anything is unclear. Preparation helps reduce anxiety and improves the quality of the data collected.
Polygraph results are only as reliable as the examiner’s skill, the test format and the examinee’s cooperation. Understanding the result categories—NDI, DI, Inconclusive and PNC—and the factors that influence them will help you interpret your own exam with greater confidence. When in doubt, consult a qualified, certified examiner and seek professional legal or psychological advice as appropriate.
