Confession Criterion in Case Selection Inflate Polygraph Accuracy
U.S. Department of Defense Polygraph Institute
Fort Jackson, South Carolina
Federal Bureau of Investigation
U.S. Department of Defense Polygraph Institute
Fort Jackson, South Carolina
field studies have relied on confessions as verification of ground
truth, a criterion that some critics argue creates an overestimation
of polygraph accuracy. This is because there is a relationship between
polygraph results and the likelihood that a suspect will confess.
Confessions come from interrogations, which follow failed polygraphs.
If a guilty person fails the polygraph, an interrogation is initiated,
which might yield a confession. If a guilty person passes the polygraph,
there is no interrogation, no confession, and little chance the
polygraph error will be uncovered. This would suggest that among
guilty suspects, there could be qualitative group differences between
confession and nonconfession cases. The biasing effect of this confession
criterion has not yet been resolved. In this study, a comprehensive
sample of field polygraph cases from a large U.S. government polygraph
program was examined to uncover differences in the polygraph detectibility
of guilty confessing suspects, and guilty suspects who did not confess
but were caught by other means. The present data failed to find
any differences in the groups. This manuscript does, however, correct
errors published elsewhere regarding law enforcement polygraph and
investigative practices in the field.
disciplines, none is as controversial as using the polygraph to
detect deception. The use of the polygraph to uncover criminal and
security-related behaviors now spans seven decades and has been
the center of heated debate for virtually the entire period. There
are many facets to the debate, but the most frequent issue centers
on the accuracy of the comparison question technique, the most common
polygraph technique in the field. Critics have charged that the
comparison question technique (formerly known as the control question
technique) lacks validity and argue that the empirical evidence
is, at best, incomplete. Proponents agree that more research is
needed, but argue that the preponderance of the available field
data points to an accuracy of about 90 percent.
not as comfortable with the available field studies as are the proponents.
It is well known that the method in which cases are selected for
a study affects the outcome of the study and that some methods are
better than others. Polygraph critics contend that existing research
supporting polygraphy has systematically stacked the deck in favor
of higher accuracy. The biggest culprit, according to some (Ben-Shakhar
et al. 1982; Lykken 1998; Patrick and Iacono 1991), is the confession
criterion. The confession criterion allows polygraph cases to be
selected for research based on the confession of the examinee. This
use of the confession criterion may bias the types of cases used
in a field study. The confession criterion could inflate accuracy
estimates in detecting deception by the way comparison question
technique field studies are typically conducted. This is explained
in the following paragraphs.
To test the
efficacy of the comparison question technique, it is necessary to
have confirmed cases, that is, polygraph recordings from a group
of examinees for whom ground truth has been unquestionably established.
Ground truth is easy to determine in laboratory studies because
experimenters assign examinees their roles of guilt or innocence.
In the field, on the other hand, examinees arrive for polygraph
appointments with self-assigned roles, usually not known to anyone
except themselves and their collaborators. Therefore, experimenters
must resort to other means to determine ground truth in field studies.
field research, the use of the confession criterion is fairly common.
The confessions of examinees are the most readily available confirmations,
but this is where the problem begins. Guilty examinees typically
do not spontaneously confess their crimes or deceptions to polygraph
examiners or investigators. They are far more likely to acknowledge
their acts during an interrogation. However, in standard polygraph
practice, the only examinees who are interrogated are the ones who
have failed the polygraph examination. If a guilty person manages
to pass the examination, there probably would be no confession because
there would have been no interrogation. Therefore, data sets consisting
only of confession-confirmed cases might contain merely those where
deception was most apparent in the test charts. Cases where the
polygraph was fooled would not be found in the sets. As Iacono (1991)
polygraphers seldom discover ground truth except as a consequence
of post-test confessions, and because diagnoses evaluated in
this way are almost invariably verified as correct, the typical
experienced examiner will accumulate a personal record of almost
unblemished accuracy (p. 202)."
A similar problem
exists with misdiagnosed innocent examinees. If an innocent examinee
fails a polygraph examination, he or she almost never confesses,
even when interrogated. Unless evidence surfaces that someone else
was actually guilty, the case remains unconfirmed and, therefore,
would not be selected for accuracy studies. Iacono (1991) adds that
cases are closed when an examinee has a deceptive outcome on the
polygraph, thereby cutting off the possibility of the discovery
of disconfirming information. This policy would reduce the likelihood
of an agency ever uncovering the true guilty party and discovering
the polygraph error.
was the first to investigate the possible relationship between confessions
and polygraph accuracy. He drew a sampling of verified and unverified
polygraph cases from the files of criminal suspects at a large police
agency. He used an equal number of deceptive and nondeceptive cases
from the verified and unverified categories, with a total of 112
cases used in the study. The cases were selected randomly to fill
the cells, and the criterion for verification was the confession
of an examinee. This inculpated the examinee and exculpated others
being polygraphed for the same crime. The cases were subjected to
blind analysis by ten field examiners who worked in law enforcement.
Horvath did not find any differences in the scorers' decisions with
verified and unverified cases. These results led him to cautiously
conclude that confession cases did not enjoy better discernment
by polygraph examiners, although he recommended further investigation.
Raskin et al.
(1988) evaluated all of the U.S. Secret Service polygraph cases
for a 2½ -year period and found 76 cases where ground truth
was established independently of the polygraph results. Raskin used
a two-step process in case confirmation where there was a confession
that inculpated or exculpated the examinee, and there was independent
physical evidence consistent with the confession. To investigate
the possible effects of the confession criterion, Raskin added 20
unconfirmed cases to the set. The 96 cases were then scored manually
by U.S. Secret Service polygraph examiners who did not know the
ground truth for any of the cases. Raskin reported that the average
polygraph scores of confession-confirmed guilty cases along with
unconfirmed guilty cases were different by approximately 20 percent.
In cases where examinees confessed, the scores were an average of
20 percent more in the deceptive direction than in cases that were
decided as deceptive but unconfirmed. At face value, these findings
supported the argument that the confession criterion yields inflated
accuracy estimates because the confession cases appeared easier
to diagnose. However, the Raskin conclusions were mitigated by the
findings that the unconfirmed guilty cases had scores 63 percent
beyond the threshold needed to make a conclusive decision. In other
words, the effect was statistically significant, but effectively
In their experimental
design, Raskin et al. (1988) attempted to control the sampling bias
among the innocent cases by requiring that each confirmed innocent
case be part of a multiple-suspect investigation in which the culprit
was found or that the crime be determined not to have taken place.
In that way, any false-positive outcomes could be discovered without
biasing the sample. This study has adopted Raskin's safeguard.
It is prudent
to agree with Iacono (2000) who suggested that this safeguard, by
itself, might still have two possible weaknesses. First, if a polygrapher
knew the outcome of other suspects' tests, it is not unreasonable
that this knowledge could influence how subsequent examinations
are interpreted. In the perfect field study, all of the suspects
would be polygraphed separately by polygraphers who did not know
the number of suspects or the outcomes of the other polygraph examinations.
In that way, the polygraph decisions could not be affected by examiner
expectancies, one source of variability shown to influence polygraph
scoring (Elaad et al. 1994). To control this potential scoring bias
in the present study, an automated analysis method was applied that
relies on measurements of tracing features rather than on the semiobjective
scoring system used in the field. This approach, described later,
avoids the confounding influence of examiner expectancies on chart
A second potential
source of selection bias of innocent cases, Iacono (2000) suggests,
is that polygraphers who believe so strongly in their results do
not usually test any further suspects in a case once one has failed
the polygraph examination. If the failed suspect is actually innocent,
subsequent investigative resources can be misdirected, resolution
of the case can become more difficult, and the polygraph error can
become less likely to be discovered. However, when the polygrapher
correctly identifies a suspect, the decisions of nondeception would
be confirmed for previous cases. Therefore, when the testing examiner
makes the right decision, confirmation is more likely to arise.
There are two
assumptions in Iacono's (2000) hypothesis that bear closer scrutiny.
First is the assertion that polygraphers believe in their exams
so strongly that they usually stop testing other suspects once one
has failed. It should be noted that polygraphers in the U.S. federal
government are not empowered to choose whom to polygraph or not
to polygraph. These decisions rest in the hands of investigators,
managers, and prosecutors whose distance from the polygraph makes
them less vulnerable to the errors of such blind acceptance. It
is also worthy of note that it would be quite uncommon for any state
or local law enforcement agency in the United States to delegate
the decision of whom to polygraph to its staff polygraphers. Thus,
Iacono's (2000) assumption in this regard does not apply to examinations
conducted in the United States.
The notion that
polygraphing stops after an examinee is found deceptive, regardless
of who decides whether or not to continue, also communicates an
incomplete understanding of law enforcement investigative practices.
At the heart is the misapprehension that law enforcement agencies
act as though all crimes have a single culprit, that there are no
coconspirators or partners that might also be on the list of suspects.
In the real world, the decision to stop polygraphing depends on
whether investigators are satisfied that all of the perpetrators
have been identified, not on whether the polygrapher caught one.
Iacono's (2000) assumption is incorrect on this aspect, as well.
Iacono (1991) also examined the sampling bias issue in a field study
carried out on police cases from Vancouver, British Columbia. Beginning
with 402 possible cases, they pared it to a sample of 89 cases where
ground truth was verified to what Patrick and Iacono characterized
as "maximum certainty"¾37 were innocent, and 52 were guilty. Among the 52 guilty, according
to the Patrick and Iacono criteria, no false negatives were found
in their exhaustive review of the evidence. They found that ground
truth as determined by examiner-verified cases did not match those
of their own strict confirmation criteria. Examiners were far more
lenient in their judgments for confirmation of their own work. For
example, an examinee was called deceptive on his polygraph examination,
and during the post-test interrogation he admitted to committing
a crime, though not the specific crime covered in the relevant test
questions. The examiner still labeled the case as confirmed. Patrick
and Iacono asserted that many comparison question technique field
studies are based on just these types of data in which accuracy
in detection of deception is inflated by the generous criteria that
polygraph examiners afford themselves. Patrick and Iacono overcame
this shortcoming through a more rigorous verification process. In
addition to the blind scoring of the charts to remove extra polygraphic
sources of information, they found that the polygraph decisions
were 98 percent correct with guilty examinees, even with their criteria.
Patrick and Iacono also reported that post-test confessions were
related to highly negative (deceptive) scores. Correct classification
of the innocent cases in the Patrick and Iacono study was near chance
levels with blind scorers. Though accuracy was far lower than that
achieved by the original examiners, the researchers proposed that
the blind scoring results of those 37 cases were representative
of polygraphy in the field.
conducted a partial replication of the Patrick and Iacono (1991)
study with a smaller data set but developed an innovative approach
to test for the biasing effects of the confession criterion. Honts
devised a scaling system that quantified the level of confirmation
for the cases. The assumptions of the confession criterion bias
would lead to the expectation that polygraph scores (and hence,
decisions) would be related to the degree in which the criminal
cases produced independent evidence. Honts' results suggested that
there was no effect on polygraph scores for the level of confirmation
of ground truth; there was no meaningful effect for the confession
criterion. His data also confirmed the high accuracy of guilty cases
that Patrick and Iacono (1991) reported but found much better accuracy
with the innocent cases than those from the Patrick and Iacono sample.
that the Patrick and Iacono (1991) study may have been an outlier
because other similar studies (Honts and Raskin 1988; Raskin et
al. 1988) found comparable accuracy for guilty and innocent examinees.
As one explanation for the discrepant findings, Honts suggested
that criterion contamination may have been an issue in the Patrick
and Iacono (1991) study, a factor Honts stated had been controlled
in the other research. In polygraph studies, criterion contamination
can take place when an examinee's intention to deceive is captured
by the examination, though not specifically to the relevant question
at hand. Honts used the example taken from the Patrick and Iacono
study where one of the polygraphers shared the following details:
a suspect had been given a relevant question¾"Did you steal the diamond ring?" The examinee was found
deceptive on the polygraph examination and was confronted. He denied
that he had stolen the ring but admitted that his brother had. The
examinee's part in the crime was only that he had sold the stolen
ring. According to Honts (1996), Patrick and Iacono reported this
as a false-positive error because the examinee was called deceptive,
even though he was not guilty of the specific relevant question.
Honts argued otherwise, pointing to the examinee's intention to
deceive about the ring theft. The issue is the subject of contentious
debate even today. Readers wanting the full flavor are directed
to the relevant chapters on polygraphy in Faigman et al. (1997).
accuracies in the Patrick and Iacono (1991) study may also have
been the consequence of how the polygraph was applied to criminal
investigation by that polygraph agency. In some settings the polygraph
is used more generally to simply determine who should remain on
the list of suspects. In other words, the appearance of unfair polygraph
outcomes in Vancouver may have been that the decision rules were
set so that no guilty examinees would escape, but that some percentage
of innocent examinees would pass through. In the end, this method
concentrates the suspect pool so that investigative resources can
be more wisely invested. Because the polygraph was not used to incarcerate
or convict the suspects, there was a relatively small cost to a
false-positive outcome that might spring from biased decision rules.
Those innocent examinees were on the suspect list before being polygraphed,
and the polygraph examination merely failed to remove them from
that list. In view of the potential harm to the community that can
arise from a false-negative decision, especially when speaking of
violent offenders, it may be that some police agencies adjust the
decision rules to ensure those examinees are correctly classified,
even when it means retaining some innocent examinees on the list.
However, very different decision rules may be appropriate for other
circumstances where there are more dire consequences for false-positive
outcomes. For example, a far more balanced approach is warranted
when polygraph evidence is used in courts of law.
suspected sampling problems has not been easy, and to date no mutually
satisfying solution has been reported in the literature. Lykken
(1998) proposed a novel approach to the investigation of accuracy
of field polygraphy. He suggested that the FBI could use its own
polygraph examiner staff, employing the polygraph in its usual manner,
but to set aside the results of the examinations, and take no action;
that is, no interrogations. At some later date, a panel would try
to verify ground truth from all available evidence and compare it
to blind scorings of the polygraph data.
Though it is
interesting from an academic viewpoint and would help answer the
question of polygraph accuracy, Lykken's is not a practical proposal
because the FBI is not likely to be persuaded to ignore one of its
forensic tools when there are serious crimes to solve. A less intrusive
approach is proposed here, at least with regard to deceptive examinees,
and it begins with this assumption: if the confession criterion
causes a bias in the sampling of field cases, there should be qualitative
group differences in scores and decisions between guilty confessors
and guilty nonconfessors. Guilty confessors are those who would
collectively have their deceptions more apparent on the polygraph
charts. This would be consistent with Patrick and Iacono's (1991)
report that confessions corresponded with more deceptive scores
in most cases. If there are no significant differences in polygraph
scores or results between confessors and nonconfessors, the impact
of the confession criterion is likely to be relatively small and
support the conclusions of Raskin et al. (1988), Horvath (1977),
and Honts (1996). The present study was designed to test these two
from the U.S. Army Criminal Investigation Detachment Polygraph Division
were used in this study. Criminal Investigation Detachment cases
were selected because of the uniform procedures, high standards,
and multiple levels of quality control implemented by that organization.
Examiners in the Criminal Investigation Detachment have conducted
polygraph exams throughout the United States and the world, wherever
U.S. Army service members are assigned. About 20 field examiners
and two quality control supervisors staff the Criminal Investigation
Detachment Polygraph Division at any given time. All have field
investigative experience, have at least a four-year college degree,
are federally trained and certified, and meet continuing education
There are two
important features of the U.S. Army Criminal Investigation Detachment
investigative practices that merit comment. In that system, only
those suspects who are the focus of the investigations are asked
to submit to the polygraph examination. The polygraph is not used
in a dragnet fashion. Also, all suspects are routinely confronted
and interrogated by a Criminal Investigation Detachment criminal
investigator a number of days before the polygraph examination is
scheduled. Those who acknowledge the crime to the investigator are
usually not polygraphed. It is these two pre-polygraph processes
that might cause an increase of the proportion of guilty examinees
in that polygraph population, and a decrease of the proportion of
those predisposed to confess, more than in other systems with less
1996 through March 1998, U.S. Department of Defense Polygraph Institute
researchers reviewed all of the Criminal Investigation Detachment's
polygraph cases for which ground truth confirmation could be found,
beginning with cases conducted after January 1, 1995. The time period
for the sampling was January 1, 1995, through February 3, 1997,
when the last case meeting selection criteria was available to the
researchers. During this period 3,349 polygraph examinations were
conducted in criminal cases. Of these, 2,010 (60.0 percent) were
calls of deception indicated, 884 (26.4 percent) were no deception
indicated, and 455 (13.6%) were no opinion (inconclusive). There
were 1,146 cases of examinee confessions, and no reports of false
were the investigative files for those polygraph cases that are
maintained separately from the polygraph files and include details
of all of the investigative and laboratory findings. Confirmation
of the polygraph cases required at least one of the following: an
unrecanted confession of the examinee, an unrecanted confession
from someone who exculpated the examinee, evidence that the crime
under investigation was never committed such as when missing property
was discovered to have been innocently misplaced instead of stolen,
forensic evidence such as urinalysis or surveillance tapes that
substantiated the truth, or suspects led investigators to where
they had hidden evidence or the stolen property. Eyewitness testimony,
prosecutorial decisions, or judicial outcomes did not rise to the
level of sufficient confirmation. Because, in the Criminal Investigation
Detachment system, polygraph and other investigative measures were
conducted concurrently rather than sequentially, discovery of evidence
was somewhat more independent of the polygraph outcomes than in
a system where the polygraph is used either very early or very late
in the investigative process.
polygraph examinations using a common testing format were selected
for this study. The cases had to be single-issue examinations in
which the U.S. Department of Defense Polygraph Institute zone comparison
technique (U.S. Department of Defense Polygraph Institute 1992)
was employed. Single-issue examinations are those in which a lie
to one relevant question means the examinee lied to all relevant
questions, or if truthful to one, was truthful to all. By U.S. Department
of Defense Polygraph Institute standards, a minimum of three repetitions
(charts) of the questions is required. If more than three charts
were collected, only the first three complete charts were used in
the study. By limiting the data in this fashion, the inconclusive
rate for the samples was likely to have increased (Senter et al.
submitted for publication), but it was seen as necessary to standardize
the quantity of data from each case.
There were 704
examinations that met the criteria for polygraph format, scope (single
issue), and a minimum number of charts. From that group, the authors
obtained an in-depth sampling of 177 confirmed guilty cases where
a confession was obtained from the examinee and 61 cases where the
examinee did not confess, but other evidence established guilt.
Of the 177 confessor cases, 28 had other supporting forensic evidence,
and 149 were confession-confirmed only.
complete review of the archived Criminal Investigation Detachment
cases included a search for confirmed innocent cases meeting these
criteria. For this study, an additional criterion was imposed on
the innocent cases consistent with Raskin et al. (1988)¾ innocent cases had to come from multiple-examinee investigations
in which the guilty party was discovered, or it was proven that
the crime did not take place. Sixteen innocent cases were found
to satisfy the multisuspect, scope, polygraph format, minimum chart,
and ground truth criteria. Of these, five were theft cases in which
the missing items were later discovered not to have been stolen,
and the remaining cases were confirmed by the confession of someone
other than the examinee. Examinee demographics for all cases meeting
the selection criteria are found in Table
Investigation Detachment polygraph program during this period used
the Axciton computer polygraph (Axciton Systems, Incorporated, Houston,
Texas) to record the traditional polygraph channels. There are two
pneumographic sensors to register breathing, a standard blood pressure
cuff for changes in blood volume, and finger electrodes for electrodermal
activity. Data are digitized and available for offline analysis.
study avoided the original examiners' scorings and decisions. They
may have been prejudiced to some unknown extent by extra polygraphic
sources of information such as case facts or the examinees' gestures
and verbal behaviors (Iacono and Patrick 1987). The interest was
in determining just how diagnostic the physiological data were when
these extra polygraphic sources of information were excluded. A
scoring method developed at the U.S. Department of Defense Polygraph
Institute was chosen for this type of polygraph format, called the
objective scoring system (Dutton 2000; Krapohl and McManus 1999).
The objective scoring system uses physiological tracing features
previously shown to be most diagnostic: respiration line length,
electrodermal response amplitude, and blood volume amplitude (Kircher
and Raskin 1988). Feature sizes for the relevant and comparison
questions were converted into ratios where the measurement of each
relevant question was divided by the measurement taken of the matched
comparison question. The resultant ratios were compared to a chart
of empirically developed thresholds for score assignment (Table
2). The scores were summed, and the totals were used for making
a veracity decision.
scoring system scores for a three-chart polygraph examination have
a potential range of -108 to +108. This system allows users to set
their own cutting scores based on their tolerance for risk. The
U.S. Department of Defense Polygraph Institute cutting scores of
±6 were used here: +6 or greater were categorized as no deception
indicated, and -6 or lower were categorized as deception indicated.
Scores between +/-6 were called inconclusive. These cutting scores
produced decision accuracy at about 90 percent with the U.S. Department
of Defense Polygraph Institute zone comparison technique (Krapohl
and McManus 1999). The proportion of agreement between the trichotomous
decisions of the objective scoring system and human blind scorers
averaged 0.69 in that study.
Though the objective
scoring system was designed to be performed manually in the field,
the process was automated here to assure reliability. The three
diagnostic features were measured automatically by a software package
called Extract, version 3.0, developed for the U.S. government (Harris
1999). All had been conducted two years prior to the development
of the objective scoring system; therefore, this scoring method
had no influence on polygraph decisions by the original examiners
or quality control personnel.
accuracies for each of the four groups are found in Table
3. Tests of proportions were conducted for each group to determine
whether their accuracies exceeded chance expectancy of 0.50. In
the first evaluation, decision errors and inconclusive decisions
were both counted as errors. Each of the guilty groups produced
detection rates above chance levels: confession only (z=5.49, p<.01),
confession plus evidence (z=4.91, p<.01),
and evidence only (z=4.23, p<.01).
The detection rate for the 16 innocent cases was not greater than
chance (z=1.50, p>.01).
Tests of proportions that excluded inconclusive decisions found
all four groups to have detection accuracy greater than chance:
confession only (z=7.78, p<.01),
confession plus evidence (z=4.91, p<.01),
evidence only (z=5.63, p<.01),
and innocent (z=3.33, p<.01).
1 Mean Scores with the
Standard Error of Measurement Bars for Confession Only, Confession
Plus Evidence, Evidence Only, and Innocent Cases Click
objective scoring system scores were evaluated for the three guilty
groups, and a one-way ANOVA was calculated as a function of the
group using scores as the dependent measure. The group effect was
not significant (F[2,
235] = 0.58, p>.01).
Figure 1 displays the mean scores, along with the standard error
of measurement bars, for the three guilty groups and the one innocent
group. The mean scores and standard deviations for the four groups
are found in Table 4.
were no differences among the scores of the three guilty groups,
those data were combined and a point-biserial correlation was conducted.
Innocence was coded as 1 and guilt as 0. The correlation (r=0.43)
was significant (t
 = 7.65, p<.01).
findings are consistent with the conclusions of Horvath (1977),
Raskin et al. (1988), and Honts (1996). A liberal estimation with
these datas' effect size, based on the one-way ANOVA, is quite small
and negative due to the small value of the F ratio
ω2 = -.015 (Keppel 1991). Taken in context with most
of the other literature on the issue, this evidence should offer
some reassurance to those who wish to undertake field research on
the polygraph and the comparison question technique. However, the
present conclusions are restricted to data that came from sources
with practices similar to those of the U.S. Army Criminal Investigation
in the present data are at odds with the Patrick and Iacono (1991)
findings. Both the present study and that of Patrick and Iacono
(1991) used extensive field samples taken from law enforcement agencies,
high-confirmation criteria, and independent analysis of the polygraph
recordings, although there were significant methodological differences
that limit what could be said about the discrepant findings. Patrick
and Iacono relied on a semiobjective field-scoring system performed
by human blind scorers, while the present study used an objective
and automated method of scoring the data not available to Patrick
and Iacono when their work was published. Also, the polygraph was
not used as a last-ditch method of solving cases with the agency
this study sampled, as Patrick and Iacono described the practice
in their report. Therefore, it may have been easier to uncover ground
truth for a larger proportion of cases in this study. The present
study had the benefit of larger and possibly more homogenous samples,
a more consistent polygraph testing protocol that had been monitored
by quality control oversight, and digitized physiological data.
And, while it should be noted that Patrick and Iacono's (1991) polygraph
examiners used state-of-the-art examination procedures in the early
1980s when their data were collected in Vancouver, this study acknowledges
that the practices of the more dispersed U.S. federal polygraph
program in the 1990s are probably different.
The goal of
this study was to determine whether there were differences in scores
and decisions attributable to the confession criterion. Though none
were found in this study, the confession criterion remains a potential
source of contamination in undercontrolled studies. The present
data demonstrate, however, that it is an overstatement to broadly
assert that the confession criterion is a contaminant in a study.
It is more defensible to state that the confession criterion is
suspected when it leads to samples of cases with non-representative
data, such as those with scores more extreme than the population
as a whole. It should be relatively straightforward for researchers
to collect and report such evidence as others have done so that
skewed data can be recognized.
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