What the machine actually records
A polygraph is a set of sensors connected to a recorder. The record it produces is called a polygraph chart. It tracks three things:
| Channel | What it picks up | Usually worn as |
|---|---|---|
| Respiration | Breathing rate and depth | Two stretch bands, chest and abdomen |
| Cardiovascular | Heart rate and blood pressure | A cuff on the upper arm |
| Electrodermal | Skin conductance (sweat) | Small sensors on the fingertips |
These are signs of arousal, not signs of lying. Stress, surprise, pain and anger can move them too, which is why the human part of the exam matters as much as the equipment.
The four phases
-
1
Pre-test interview
- What happens
- The examiner explains the procedure, asks about your health, medication and sleep, covers background details, and reviews every question that will be asked so that you both understand them the same way. Some examiners demonstrate the instrument first.
- What the examiner is doing
- Forming an impression of you and choosing the wording of the comparison questions. The National Research Council notes that examiners often know background information about you before the test, and that their impressions of your demeanor can affect the outcome.
- What helps you
- Ask about any question that is unclear or too broad and agree its exact wording. Tell the examiner about health conditions and medication. Treat this phase as part of the test, not as small talk.
-
2
Chart collection
- What happens
- The sensors are attached. You sit still and answer each question with yes or no. The examiner asks the reviewed questions in a set order with pauses in between, and the questions are usually repeated across more than one chart.
- What the examiner is doing
- Recording your reactions to relevant, comparison and neutral questions and noting movement or anything unusual. If a reaction stands out, the examiner may pursue it, or clear up a misunderstanding and ask the question again in a later chart.
- What helps you
- Sit still, breathe normally and answer the way you agreed in the pre-test interview. Don't try to manage your body: deliberate attempts to change your reactions are called countermeasures, and examiners are trained to look for them.
-
3
Scoring the charts
- What happens
- The examiner, a second examiner or software compares your reactions to the relevant questions with your reactions to the comparison questions, usually following a scoring protocol.
- What the examiner is doing
- Reading the pattern. Stronger reactions to comparison questions generally point toward truthfulness. Stronger reactions to relevant questions often mean more questioning is needed, not necessarily that you lied.
- What helps you
- Nothing to do here. The result is the examiner's reading of the charts, which is also the step an independent review can re-check later.
-
4
Post-test interview
- What happens
- The examiner talks through how the charts looked. If some questions drew stronger reactions, you may be asked to explain them. Misunderstandings can be cleared up, and sometimes another chart is run.
- What the examiner is doing
- Looking for an explanation or an admission. Admissions can come out both before the charts and in response to questions about them.
- What helps you
- Stay calm and factual, and explain what is true. Don't invent an explanation or admit to something that did not happen just to end the session. If you feel pressured, ask for a break.
The three kinds of questions
You will hear the same three types in most exams. Knowing them explains why the questions feel the way they do.
| Type | Example | How it is read |
|---|---|---|
| Relevant | A specific question about the matter being tested, for example whether you did a particular thing on a particular date | A stronger reaction here is read as a possible sign of deception |
| Comparison ("control") | A broad question about your past, for example whether you have ever broken a minor traffic law | Truthful people are presumed to react more to these than to the relevant questions |
| Neutral (irrelevant) | Something harmless, such as whether today is Monday | A baseline for your normal reactions |
This design is also why honest people sometimes fail. If you are very afraid of a relevant question, your reaction can outweigh your reaction to the comparison questions even though your answer is true. We explain what to do if that happens in our guide to failing a polygraph while telling the truth.
A different format, the concealed information test, asks about details that only someone involved would know. It needs a specific event, so it is not used for general screening.
How long it takes
Allow plenty of time. The total depends on the test type and the organization, and with the interviews it often runs from one to a few hours. Ask for the schedule in advance, and don't plan anything stressful right after the exam.
Before you go: a practical checklist
- Find out the basics. Who requested the test, what topics it covers, and who receives the result.
- Look after yourself. Sleep well, eat normally and take prescribed medication as usual unless your doctor says otherwise. Tell the examiner what you take.
- Arrive early with ID. A rushed arrival raises your stress before the exam even starts.
- Think through the topics. If a topic could expose you legally, get legal advice before the exam, not during it.
- Use the pre-test interview. Ask questions, and have every unclear or over-broad question reworded until you are sure what it asks.
This guide is general information. Procedures differ between examiners, test formats and countries.
Frequently asked questions
How long does a polygraph test take?
It depends on the type of test and the organization. Including the interviews before and after the charts, it often takes from one to a few hours. Ask the examiner or the organization for the schedule in advance.
Will I know the questions in advance?
Normally yes. In the pre-test interview the examiner explains the procedure and goes over the questions so that you both understand them the same way. If a question is unclear, this is the time to say so.
What do the sensors feel like?
Usually a cuff on the upper arm, similar to a blood-pressure cuff, two stretch bands around the chest and abdomen, and small sensors on the fingertips. They are not painful, but you are asked to sit still.
Will I get the result straight away?
It varies. Some examiners discuss how the charts looked in the post-test interview, while the formal result goes to the organization that requested the test. Ask beforehand who receives the result.
Is the procedure the same everywhere?
No. Examiners use several test formats, and screening tests, criminal investigations and specific-incident tests are structured differently. The four phases are the common frame, but the details change.
Sources
Want to walk through the exam before you take it?
In a private 1-on-1 session we go through the phases for your specific test, the questions you are likely to face, and how to keep your answers clear and calm.