others have been examined, but as yet, none of these
new measures have achieved sufficient scientific basis
for use in application.The Knowledge Approach
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In the knowledge approach, which is often referred to
as the Guilty Knowledge Test or, more correctly, as the
Concealed Knowledge Test (CKT), the subject is pre-
sented with a series of items in multiple-choice format.
The items are designed to represent some bit of knowl-
edge that the truthful person would not know. For exam-
ple, John Doe is murdered with a pistol that the police
have determined to be a .380 automatic. The media
reports indicated that the victim was shot to death, but
the police never publicized the exact type of weapon. A
suspect might be asked, If you shot John Doe, you
would know the type of weapon: Was the gun used to
shoot John Doe a .38 special revolver? A .45-caliber
automatic? A .357 Magnum? A .380 automatic? A 9mm
automatic? A .22-caliber revolver? A window of
approximately 20 seconds would follow each alternative
to allow for the suspect’s autonomic responses to take
place and then recover. Because people tend to produce
physiological responses to the first item in any series,
the critical item is never placed in the first position, and
the first item in the CKT series is never evaluated.
It is assumed that on recognition of the correct
alternative, the deceptive person will generate auto-
nomic responses. It is also assumed that the truthful
person will have no reason to produce a specific
response to the critical item and will thus be produc-
ing nonspecific responses at random. Thus, the likeli-
hood that an innocent person would produce his or her
largest response to the critical item in a 1-item CKT is
0.20. With a 2-item CKT, the likelihood that an inno-
cent person would give his or her largest response to
the critical item on both CKT series is 0.20 ×0.20, or
0.04. As the number of items increases, the likelihood
of making a false-positive error (a truthful person
appearing deceptive) is thus definable and rapidly
becomes quite small.SSttrreennggtthhss ooff tthhee CCKKTT
The CKT has two principle strengths. First, it is
possible to precisely define the likelihood of making a
false-positive error and to control that error rate by the
number of items used in the CKT. Research has con-
sistently shown that the statistical prediction holdswell in application. It is also possible to pretest the
transparency of the items in a CKT by presenting the
series of items and alternatives to persons known to be
truthful regarding knowledge of the crime. Trans-
parency refers to the ability of innocent persons to
guess the critical item from a series. While testing for
transparency of items is common in research settings,
it is not known whether or not it is a common practice
in the field.
The second great strength of the CKT is that it is a
very simple test to administer. With a few hours of
training with the equipment, an undergraduate
research assistant can administer the CKT as well as
an experienced polygraph examiner. There are exam-
ples in the literature of the CKT being completely
automated for machine administration.WWeeaakknneesssseess ooff tthhee CCKKTT
The CKT has three primary weaknesses. The first,
known as memorability, concerns the fact that for the
CKT to work, the deceptive person must remember the
details of the crime. In that regard, the extensive
research on eyewitness memory indicates that eyewit-
nesses, particularly those under stress, are prone to
make mistakes in recounting the details of a crime. The
perpetrator of a crime is an eyewitness to that crime,
and it is likely that the perpetrator will be a highly
stressed eyewitness of the crime. Moreover, many per-
petrators are also intoxicated. To date, there is no the-
ory to predict what specific details from a crime scene
are likely to be remembered. The memorability prob-
lem is avoided in most laboratory research on the CKT
by screening a number of details of the crime scene
with pilot subjects and then using only the highly
memorable items in the subsequent testing or by using
overlearned items of personal history. Such a screening
of items is not possible in real cases. Laboratory
research on the CKT has revealed a slight tendency
toward false-negative errors—that is, toward deceptive
individuals appearing truthful on the test. However, the
few existing field studies of the CKT suggest that the
false-negative rate in the field may be as high as 50%.
The second major weakness of the CKT is one of
applicability. Research conducted by the Federal
Bureau of Investigation in the United States found that
fewer than 10% of their cases were amenable to the
use of the CKT, if they had wanted to use it. In nearly
90% of the FBI case files examined, the nature of the
case was such that there were not enough items of
concealed knowledge to conduct a CKT.Polygraph and Polygraph Techniques——— 597P-Cutler (Encyc)-45463.qxd 11/18/2007 12:43 PM Page 597