Proper timing is one of the fundamental principles for conducting a reliable polygraph test.
Unknown to many aspiring polygraphers, there are industry benchmarks on how long a lie detector test should last. Observing these standards can minimize the chances of false positives and false negatives during polygraph testing, enhancing the accuracy of each exam.
Here’s a guide to the average duration of lie detector tests and what typically happens during the period.
How Long Should Polygraph Tests Last?
Polygraph exams should last between 1.5 and 2.5 hours.
According to industry experts, that’s sufficient time to prepare the examinee, administer the actual test, and compile a detailed report.
But while 1.5 – 2.5 hours is the average length of a lie detector test, the actual duration would depend on factors like the matter under investigation and screening type.
Why Do Polygraph Tests Take This Long?
Contrary to popular misconceptions, a lie detector test doesn’t start and end with the questioning.
First, the would-be polygraph examinee must be prepared psychologically for the evaluation before scheduling a lie detector test. This constitutes the pre-test phase.
Next comes the in-test phase, during which the examiner asks the examinee a series of polygraph questions as the polygraph machine records each response.
The final stage is the post-test phase. Here, the examiner reviews the examinee’s responses to the in-test questions before compiling a report.
What Happens At Various Polygraph Stages?
1. Pre-Test Phase
During the pre-test phase, a polygraph examiner collects relevant background information on the matter under investigation. This typically requires engaging the examinee in a candid discussion to get their side of the story. It’s often the most time-consuming stage of the entire lie detector test.
Another critical event at the pre-test phase is to establish the examinee’s suitability for testing. Individuals with mental illness and those still reeling from traumatic experiences may be exempted from taking a polygraph test.
Note that polygraphs measure changes in certain involuntary physiological reactions controlled by the autonomic nervous system (ANS), including heart rate, skin conductivity, and breathing. Getting an examinee’s baseline is critical, as the final results will be gauged against these measurements.
The next step in the pre-test stage is laying down the ground rules for a polygraph test.
Finally, the polygrapher will design appropriate questions and feed them into a computer. They’ll then review each question with the examinee before proceeding to the next stage.
The entire pre-test phase takes 60-90 minutes.
2. In-Test
At the in-test phase, a polygrapher will attach a series of sensors to the examinee’s body. These include electrodermal sensors, blood pressure cuffs, pneumographs, etc., each designed to track fluctuations in specific physiological reactions.
Next, the previously recorded polygraph questions would need to be asked one at a time.
Each response triggers measurable reactions in the ANS-controlled physiological functions, which are captured in real-time.
To conduct an accurate polygraph test, the questions would need to be split into three categories:
- Relevant Questions – Address the matter at hand directly
- Control Questions – Designed to evoke a greater response by referencing minor but related past incidents
- Irrelevant Questions – Help the examinee relax between the questioning
The in-test phase takes 30-60 minutes, depending primarily on the number of questions asked.
3. Post-Test
Once the examiner is done with the questioning, they’ll analyze the data captured for each question.
Note that a single question may be asked multiple times during the in-test phase.
To draw an accurate report, the examiner would need to aggregate the numerical scores recorded for each query. They’ll then interpret the findings by comparing the responses to relevant and control questions.
An inference of guilt is deduced if there’s a stronger pattern of response to relevant questions than control. If the examinee reacted more strongly to control questions than relevant questions, that indicates they’re likely being truthful.
The post-test phase is the shortest polygraph stage, often lasting 25 – 45 minutes.
When Might The 1-5 – 2.5 Hours Duration Be Untenable?
As hinted, the actual length of a polygraph test depends on multiple factors. Of noteworthy mention is the screening technique.
One specific screening methodology is known to take the shortest time, namely the Single Issue Exam.
Single issue exams seek to resolve one issue. It often involves asking just one question. While the sole question may be reworded and asked several times, single issue exams usually take a shorter period in the in-test phase.
Single-issue exams are widely regarded as the most accurate type of polygraph testing due to their narrow focus. While they are more likely to meet scientific validity standards such as those outlined in the Daubert ruling, the admissibility of polygraph results ultimately depends on the specific court and jurisdiction.
Enhancing Polygraph Credibility Through Proper Timing
A credible polygraph exam should last anywhere from 90 minutes to 2 ½ hours. The actual length of a lie detector test depends on several factors, notably the screening technique deployed and the test’s complexity.
The inclusion of an interpreter can also impact the duration of a polygraph evaluation.
Besides the sheer length of polygraph exams, an examiner must uphold other basic question design techniques to enhance the accuracy of each test. These include asking brief and unambiguous questions, framing questions that require a flat Yes or No answer, and assuming an objective, non-accusatory tone.
