Polygraph Test False Negatives: All You Need to Know
People who watch a lot of television have a very limited understanding of the polygraph exams. They see the testing in bits and pieces of on law and order shows.
People forget those are shows are fictionals. What you see is a long way from the way polygraph tests are administered and read. What they call a “lie detector” is misleading according to the American Psychological Association.
So, what is a polygraph?
A polygraph administrator attaches sensors that measure the test subject’s physiological responses while being asked a series of questions.
The sensors typically measure the subject’s:
- Breathing rate
- Blood Pressure
- Arm and Leg Movements
And, contemporary polygraph machines use sophisticated sensors to record, store, and transmit data.
How does the test go?
The test is preceded by a “practice session” to establish a baseline or benchmark of the subject’s physiological status at a calm state.
A professional polygraph administrator then asks some relevant questions to see if physiological responses are recording. When the test starts, the administrator may ask control questions, relevant questions, or multiple-choice questions.
The results reveal the subject’s physiological responses. But, it takes a professional like Jack Trimarco to study the data for evidence that the responses are indicative of the emotional connection in moments of deception.
But, what about false positives?
It takes a test professional to identify “false positives,” indications that a person lied when they didn’t or didn’t lie when they did.
The following can contribute to false positives:
- Inebriants like tobacco, alcohol, and drugs will affect a subject’s responses. They may raise or lower blood pressure, increase or lower heart rate, and alter the pulse.
- Blood Pressure may or may not affect the testing. If the pressure is under management, it should remain consistent throughout the test. But, the tester must know the situation.
- Anxiety will record unfavorably. Test anxiety usually shows up as physiological responses that increase as testing proceeds and that may produce a false positive.
- Fatigue: Lack of sleep can skew physiological responses.
ABC News reported, “’Proponents will say the test is about 90 percent accurate. Critics will say it's about 70 percent accurate,’ said Frank Horvath of the American Polygraph Association.” But, those errors do not invalidate the process in the hands of a qualified, experienced, and certified polygraph administrator.
Polygraph machines do not lie. They record responses with accuracy. The machine runs a diagnostic test of those responses. What the results mean presents the challenge for the polygraph administrator.
Accuracy can differ from reliability. Many tests result in a “yes or no” conclusion. The results may have a range of scores. For instance, a blood test reports you have high cholesterol. An eye examination says you need glasses. The driving test says you may or may not drive.
The polygraph does not report that the subject lied or did not lie. It will, instead, report something like “significant response,” “no significant response,” or “inconclusive.” And, the most accomplished testers understand that false positives can be a factor in test conclusions:
- A False Positive Index (FPI) lists the ratio of false positives to true positives.
- A Positive Predictive Value (PPV) notes the probability that a test subject with a deceptive polygraph result is, in fact, being deceptive.
Today’s polygraph machines are sophisticated technologically. Given their purpose to record signs of human physiological responses, they do a very good job. Given the input and testing protocol, the machine’s results are accurate.
Their reliability as a “true” index of the test subject’s honesty or dishonesty is largely a matter of expert reading and interpretation. Testers know there is the potential for false positives and for attempts to interfere with the testing. But, their expertise can deliver valuable and useful results.