How Accurate Is the Lie Detector Test?
How accurate is the polygraph test? The answer depends on
who you ask. Studies show results all over the board for this
supposed lie detector test. In a recent Department of Defense
Polygraph Institute study, one experiment found that less than
37% of test takers who were non-deceptive were classified as
such. The remainder were either classified as "deceptive"
(false-positives) or "inconclusive." (And people think
a polygraph will "prove" their innocence?)
The American Polygraph Association (APA) says on their web
site that the problem of accuracy is one of differing methods
of measuring it. Critics, they say, "who often don't understand
polygraph testing, classify inconclusive test results as errors."
This APA says an inconclusive result isn't an error, but I imagine
that if you are accused of murder, and you are innocent, you
might want a more accurate result than the suspicion-arousing
"inconclusive." (Interestingly, they will not call
it a lie detector test, even though detecting lies is what it
is supposed to accomplish.)
They explain the problem: "If 10 polygraph examinations
are administered and the examiner is correct in 7 decisions,
wrong in 1 and has 2 inconclusive test results, we calculate
the accuracy rate as 87.5% (8 definitive results, 7 of which
were correct.) Critics of the polygraph technique would calculate
the accuracy rate in this example as 70%, (10 examinations with
7 correct decisions.)" Their argument is not entirely unreasonable.
There are many ways to measure things.
On the other hand, what is more interesting, is that even
in an argument from the biggest promoters of the polygraph, the
example given is of of 87.5% accuracy, and 20% "inconclusive"
results. That might sound good until you realize that of a 100
people tested in a murder case, about 10 innocent people would
be found to be "lying," and 20 with "inconclusive"
results. The latter might include both innocent people and murderers.
Looked at another way, of a 100 murderers, 10 would be found
to be telling the truth, and 20 would have inconclusive results.
Out of 100 murders, 30 would not be identified, according to
the accuracy assumed in the example.
But let's look at their method again, with a new example.
Suppose 100 innocent people were questioned about a crime, and
one was found to be telling the truth, while the other 99 tests
were "inconclusive." This would appear to be a relatively
useless test, right? It correctly identifies just 1 out of 100
innocent people, leaving a cloud of suspicion over the other
99. Yet, measuring the results the way the American Polygraph
Association does, the accuracy would be 100%.
The lie detector test isn't considered science by most scientists,
although somewhere on the APA web site, you can find a little
about scientific evidence. Here is one small excerpt: "Researchers
conducted 41 studies involving the accuracy of 1,787 laboratory
simulations of polygraph examinations, producing an average accuracy
of 80%. Researchers conducted 16 studies involving the reliability
of independent analyses of 810 sets of charts from laboratory
simulations producing an average accuracy of 81%."
80% is supposed to be accurate enough?! Is a test that would
identify 200 out of a thousand innocent job applicants as liars,
and many more as possible liars ("inconclusive") really
something that should be promoted? Maybe this helps explain item
number 7 from their "Checklist for the Polygraph Examiner":
"Carry a minimum of $50,000 or equivalent professional liability
The site How
to Beat the Lie Detector Test has specific countermeasures
for passing a polygraph test.