Faculty of Psychology, University of Bergen, Bergen
Namdal Hospital, Namsos
Research Unit, Aker Hospital, Department of Psychiatry, University of Oslo, Olso, Norway
Correspondence: Alv A. Dahl, Research Unit, Department of Psychiatry, Aker Hospital, University of Oslo, Sognsvannsveien 21, 0320 Oslo, Norway. Tel: +47 22923400; fax: +47 22923971; e-mail: a.a.dahl{at}psykiatri.uio.no
Declaration of interest The Norwegian Research Council and Novo Nordisk Pharmaceuticals (Norway) supported this study.
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ABSTRACT |
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Aims To examine the psychometric properties of the HAD scale in a large population.
Method All inhabitants aged 20-89 years (n=92 100) were invited to take part in The Nord-Trøndelag Health Study, Norway. A total of 65 648 subjects participated, and only completed HAD scale forms (n=51 930) formed the basis for the psychometric examinations.
Results Principal component analysis extracted two factors in the
HAD scale that accounted for 57% of the variance. The anxiety and depression
sub-scales shared 30% of the variance. Both subscales were found to be
internally consistent, with values of Cronbach's coefficient () being
0.80 and 0.76, respectively.
Conclusions Based on data from a large population, the basic psychometric properties of the HAD scale as a self-rating instrument should be considered as quite good in terms of factor structure, intercorrelation, homogeneity and internal consistency.
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INTRODUCTION |
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The aim of the present study was to examine the factor structure, intercorrelation, homogeneity of sub-scales and internal consistency of the HAD scale based on data from a large population.
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METHOD |
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According to the Central Population Register of Norway, 92 100 individuals aged 20-89 years were eligible for HUNT. Of those eligible, 65 648 took part in the study and the PCA was based on complete HAD forms only (n=51 930).
Weighting procedures
Weighting according to the procedure used in the National Comorbidity
Survey (Kessler et al,
1994) was performed to adjust for difference in response rate
according to age and gender, and also age and gender differences between the
population of Nord-Trøndelag County and the population of Norway.
Statistics
Different sets of PCA were performed on the HAD scale, with varying
criteria for the number of factors, and on both subscales as well as the total
scale. Results using oblique rotation are reported because the sub-scales
share 30% of the variance; analyses with orthogonal rotation also were
performed, with basically the same results.
In order to test the stability of the factor structure obtained, we repeated the analyses according to gender (men: n=25 197; women: n=26 733) and in randomly split halves of the sample. We also repeated the factor analyses in the groups who reported mental problems (n=2098) and chronic somatic problems (n=10 954). The analysis was carried out also in different age strata of 18-39 years (n=18 736), 40-59 years (n=21 037), 60-79 years (n=10 804) and 80-89 years (n=1353) and according to the highest level of basic education: elementary school (n=16 210), high school (n=23 426) and college/university (n=11 184).
The internal consistency of the HAD sub-scales was calculated by Cronbach's
coefficient . Pearson's correlation coefficients were calculated for
estimation of the sub-scales' shared variance.
Ethics
HUNT was approved by the National Data Inspectorate and the Board of
Research Ethics in health region IV of Norway.
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RESULTS |
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When performing PCA with the number of factors defined by eigenvalues
1.00, a three-factor solution emerged. The first factor contained items 1,
3, 5, 9 and 13 (all anxiety items), the second factor contained items 2, 4, 6,
8, 10 and 12 (all depression items) and the third factor comprised items 7, 11
and 14. Eigenvalues in the rotated three-factor solution were 3.82, 3.60 and
1.87, respectively. In the sub-samples we found this three-factor solution
only in respondents aged 18-39 years and in the two subsamples with education
from high school and college/university. In all other subsamples the
two-factor solution emerged according to the original anxiety and depression
sub-scales.
Principal component analysis done on each sub-scale separately, with the
number of factors predefined by eigenvalues 1.00, gave only single-factor
solutions in the total sample and in all subsamples.
A one-factor solution for the total HAD scale, as suggested by Razavi et al (1990), accounted for only 35% of the total variance.
Intercorrelation of the sub-scales
The anxiety and depression sub-scales shared 30% of the variance. The
sub-scale correlation was elevated in the subsample with mental problems
(43%).
Homogeneity of the sub-scales
The anxiety items loaded more on the anxiety factor than on the depression
factor, and corresponding results were found for the depression items. This
homogeneous pattern was found in the total sample as well as in the
subsamples. However, items 6 (I feel cheerful) and 7 (I
can sit at ease and feel relaxed) diverged to some extent from this
homogeneous pattern by loading substantially also on the other sub-scales
(Tables 3 and
4).
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Internal consistency
The internal consistency of the HAD scale, as calculated by Cronbach's
, was satisfactory in all the evaluated samples (values of 0.73-0.85)
(Table 4).
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DISCUSSION |
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The factor structure of the HAD scale extracted with eigenvalues 1.00
as a criterion identified a third factor with a lower eigenvalue than the
other two factors. In accordance with Leung et al
(1993), we found support for
this dimension in non-clinical samples only and we suggest that this factor
can be named restlessness.
Intercorrelation of the sub-scales
Our results are in accordance with others, who have reported a shared
sub-scale variance of 24-36%. We find higher sub-scale correlation in the
subsample with mental problems.
Homogeneity of the sub-scales
Our finding that item 7 does not discriminate well between the two factors
is in accordance with studies of general populations
(Lisspers et al,
1997) and clinical samples
(Moorey et al, 1991)
(Table 5). In our study we also
have loadings on both factors in item 6. These findings suggest that items 6
and 7 are not unique in either anxiety or depression.
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Internal consistency
The internal consistency of the HAD scale was found to be satisfactory in
our general population and somewhat higher in sub-populations with mental
problems in our study, in accordance with others.
Significant aspects of the syndromes of anxiety and depression are not covered by the HAD scale. Somatic symptoms have been omitted in order to prevent false-positive cases in somatic hospital settings. However, other important components of depression, such as hopelessness, guilt and low self-esteem, are not assessed because the HAD-D focuses mainly on anhedonia (Silverstone, 1991; Herrmann, 1997; Bjelland et al, 2001). This somewhat narrow concept of depression may be one reason for the robustness of the factor structure in the HAD scale.
In HUNT, non-participants were more prevalent among men and among those aged 40-69 years. We presume that non-responders are more prone to have a history of mental disorders (Eaton & Kessler, 1981). However, the stability of the factor structures of the HAD scale across different subsamples indicates that selection bias due to non-attendance is of limited importance.
Psychometric analyses are more valid and reliable when based on large materials. Our results support the HAD scale as an instrument with good psychometric properties in terms of factor structure, intercorrelation, homogeneity and internal consistency.
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Clinical Implications and Limitations |
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LIMITATIONS
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ACKNOWLEDGMENTS |
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Received for publication January 2, 2001. Revision received June 20, 2001. Accepted for publication June 22, 2001.