Department of Psychology, Umea University, Umea, Sweden
Correspondence: Eva C. Sundin, Department of Psychology, Umea University, 901 87 Umea, Sweden
Declaration of interest This study was granted support from the Cancer Research Coordinating Committee and the Cohn Foundation, the University of California in San Francisco, the Swedish Research Council for the Humanities and Social Sciences and the Swedish Medical Research Council.
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ABSTRACT |
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Aims To review studies that evaluated the IES's psychometric properties.
Method Literature review.
Results The results indicated that the IES's two-factor structure is stable over different types of events, that it can discriminate between stress reactions at different times after the event, and that it has convergent validity with observer-diagnosed post-traumatic stress disorder. The use of IES in many psychopharmacological trials and outcome studies is supportive of the measure's clinical relevance.
Conclusions The IES is a useful measure of stress reactions after a range of traumatic events, and it is valuable for detecting individuals who require treatment.
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INTRODUCTION |
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METHOD |
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Following the American Psychological Association's Standards for Educational and Psychological Tests (1985), reliability will be presented in terms of internal consistency and stability. In reviewing the IES' validity we discuss: theoretical formulation; content validity; construct validity; and external validity. When multiple comparisons are made, the likelihood of error-rate inflation is significant, and therefore the minimum interpretable alpha for any computed analysis is set at 0.01.
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RESULTS |
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For IES intrusion, mean =0.86 (range 0.72-0.92), for IES avoidance
mean
=0.82 (range 0.65-0.90). Using the 0.80 criterion set by Carmines
& Zeller (1979), both IES
sub-scales are consistent, which indicates that each of them measures a
homogeneous construct.
Stability
In the original report on the IES
(Horowitz et al,
1979), adequate testretest reliabilities were reported for
the two sub-scales (0.87 and 0.79); time between measurements was 1 week.
Testretest estimates were also presented by Solomon & Mikulincer
(1988), who found
testretest reliabilities of 0.56 and 0.74 respectively; time between
measurements was 1 year. Weiss & Marmar
(1997) reported
testretest reliabilities for IES sub-scales based on two different
samples. For the first sample, the average time since event was 3.1 years and
time between measurements was 6 months. The second sample completed the IES 6
weeks after the event and follow-up was 6 months later. Testretest
reliability for the first sample was 0.57 for IES intrusion and 0.51 for IES
avoidance; for the second sample, reliabilities were 0.94 and 0.89.
These estimates of testretest reliability show that the shorter time interval (<0.6 weeks) between measurements in Horowitz et al (1979) and the second sample in Weiss & Marmar (1997) contributed to higher estimates of stability compared with the estimates obtained when a longer time interval was used (>1 year).
Validity
Theoretical formulation
The IES is based on clinical studies of psychological response to stressful
events, and on Horowitz'
(1976) theory about stress
response syndrome, which offers an understanding of how people proceed through
trauma. The clinical studies revealed two common responses to stress:
intrusion and avoidance. Intrusion involved unbidden thoughts and
images, troubled dreams, strong pangs or waves of feelings, and repetitive
behavior and avoidance involved ideational constriction, denial
of meanings and consequences of the event, blunted sensation, behavioral
inhibition or counterphobic activity, and awareness of emotional
numbness (Horowitz et al,
1979). According to Horowitz
(1976), intrusions and
avoidances tend to oscillate during the same time period. Avoidant behaviour
often results from the operation of unconscious control processes, and
function to restore emotional equilibrium, prevent emotional flooding and
reduce conceptual disorganisation. These defensive efforts are disrupted by
intrusive experiences. Such dreaded states sharply contrast with a desired
state of equilibrium. To restore stability, people react with heightened
defensive control. Since individuals are not expected to report unconscious
aspects of the control processes, the term avoidance was used instead of
denial.
Content validity
We found 12 studies that examined the validity of IES' two-factor structure
based on data collected after various events. In three out of 10 studies that
successfully reproduced the intrusion and avoidance factors, a third factor
was obtained, and this factor was labeled emotional numbing
(Joseph et al, 1994; Foa et al, 1995;
McDonald, 1997). Results from
two more studies suggested an underlying structure with one factor only
(Hendrix et al, 1994;
Weiss & Marmar, 1997).
In the initial report on the IES (Horowitz et al, 1979), the correlation between IES intrusion and avoidance was 0.41. This correlation between IES intrusion and avoidance along with results from 11 more studies are shown in Table 2. Mean correlation was 0.63, which suggested that the sub-scales were relatively independent of one another, each of them representing a different type of reaction in the face of stressful events.
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Construct validity
The moderate correlation between intrusion and avoidance obtained in a
number of studies that used the IES (see
Table 2) is consistent with
Horowitz' (1976) prediction
that people tend to present an oscillating pattern wherein intrusive symptoms
are followed by avoidance. Horowitz
(1976) also postulated that
intrusive and avoidant symptoms will become less frequent over time as the
implications of the stressor event are digested. Several studies reported
results that are consistent with this assumption (e.g.
Sloan et al, 1994;
Kelly et al,
1995).
According to Horowitz (1976), strong avoidance of painful thoughts may reduce dreaded states; however, it may also prevent adaptation to traumatic experiences. This assumption was supported by several researchers, for example McFarlane (1988) found that individuals who developed PTSD at 8 months after trauma had reported more avoidance on the IES at 4 months after the event as compared with those without PTSD.
Convergent validity
Table 3 presents studies
that assessed the convergent validity of the IES, grouped according to the
variable assessed in the study, for example, anxiety, depression, and general
symptoms. In Table 4,
correlations between the IES sub-scales and PTSD as diagnosed with six
different instruments are presented. The correlations indicated that these
relationships were moderate, indicating that IES intrusion and avoidance
contribute information that is not captured with other symptom inventories and
measures of PTSD.
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Clinical validity
The review of studies that examined the reliability and/or validity of the
IES suggested that the IES is a psychometrically sound measure, and thus it is
appropriate to explore the measure's clinical validity: is the information
obtained with the IES relevant to clinical practice? For instance, can
self-rated symptom severity serve a screening purpose, and enhance
decision-making about treatment options? Several studies showed that the IES
discriminates between people with severe and mild stress reactions. For
example, a study of psychological responses to testing for the breast cancer
gene BRCA1 reported that gene carriers manifested higher levels of
intrusion and avoidance than non-carriers
(Croyle et al, 1997).
Women who considered genetic testing reported significantly higher
breast-cancer-specific distress but similar levels of general psychological
morbidity when compared with a group of matched controls
(Lloyd et al, 1996).
Studies of other groups, such as bereaved individuals (e.g.
Horowitz et al, 1984)
and war veterans (e.g. Solomon &
Kleinhauz, 1996), have showed that the IES can aid the clinician
in identifying individuals who need treatment.
Furthermore, the IES has been used in many psychopharmacological trials (e.g. Frank et al, 1988; Davidson et al, 1993; Brady et al, 1995; Rothbaum et al, 1996), and outcome studies (Horowitz et al, 1984; Tunis et al, 1994; Chemtob et al, 1997; Grisaru et al, 1998), which yield additional evidence of the measure's clinical relevance.
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DISCUSSION |
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IES two-factor structure
Twelve studies examined the IES' dimensionality and 10 of these replicated
the intrusion and avoidance scales despite considerable differences between
the samples and elapsed time since the event. Three of these studies reported
that the avoidant factor was split in two: one avoidant and a second, labelled
emotional numbing. Foa et al
(1995) maintained that this
finding contributes to the understanding of trauma victims' coping strategies:
when dreaded states involving intrusive experiences cannot be warded off with
avoidant behaviour, emotion is stifled (i.e. emotional numbing). Two more
studies that examined the factorial structure of the IES obtained one
meaningful factor only. The authors of one of these studies,
Hendrix et al, 1994,
interpreted the result to mean that over time, the distinction between
intrusion and avoidance blurs, and the two merge into one over-all pattern of
stress reactions or general level of distress. This general distress appears
to contain both intrusive and avoidant symptoms as measured with the IES.
Stability of IES intrusion and avoidance
Both studies that used the original IES and the ones that used slightly
altered sub-scales reported internal consistencies of a similar magnitude; all
of them indicated that intrusion and avoidance sub-scales have good
reliabilities and thus each sub-scale measures a relatively homogeneous
construct. The fact that the correlation between the two sub-scales when
averaged over 11 studies was moderate (0.63) suggested that intrusion and
avoidance are separable constructs. The original intrusion and avoidance
sub-scales shared approximately the same amount of variance as obtained when
slightly altered scales were used, which indicates the stability of the
IES.
IES a measure of PTSD?
It has been suggested that the IES is a valid measure of post-traumatic
stress symptoms but should not be used as a measure of PTSD. One reason is
that the IES does not measure the hyperarousal symptoms included in the
criteria for the diagnosis in the most recent version of the DSM.
The results summarised here add to the support of IES' reliability and validity. Particularly, the high correlation between IES intrusion and avoidance and PTSD diagnosis obtained in a number of studies validates the usage of the subject-rated IES as a screening measure for PTSD. Since the IES is a short self-report measure, it provides a low-cost measure to detect PTSD (Rothbaum et al, 1992). Moreover, a number of the studies summarised here reported that the IES is well suited to assessing outcome from various types of treatment, and its sensitivity for drugplacebo differences has been confirmed.
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Clinical Implications and Limitations |
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LIMITATIONS
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Received for publication June 15, 2000. Revision received May 9, 2001. Accepted for publication May 17, 2001.