The construct validity of the Perceived Stress Scale
Introduction
Stress considerably impacts the quality of life worldwide [1] and is associated with a range of adverse health outcomes, including increased risk of cardiovascular events [2], [3], metabolic syndromes [4], [5], [6] and mortality [7], [8], [9]. Stress can also lead to mental illness which is a burden for the individual but may also cause serious productivity losses with societal implications [10]. Although a stress condition carries a substantial burden, it is merely considered a ‘risk factor’. For instance, no diagnosis code for stress exists in the 10th version of the International Classification of Disease (ICD-10) and in the 5th version of the Diagnostic and Statistical Manual of Mental Disorders (DSM V) [11], [12].
The Perceived Stress Scale (PSS) is a widely used instrument for measuring stress [13]. The PSS evaluates the degree to which an individual has perceived life as unpredictable, uncontrollable and overloading during the previous month. The PSS also assesses the degree to which external demands seem to exceed the individual's perceived ability to cope [13], [14].
The original 14-item scale (PSS-14) was developed in 1983 by S. Cohen et al. [13], [15], [16], but this first version was later revised and reduced into 10-item and 4-item versions [14]. The PSS-10 was originally defined as a single construct because the ‘distinction between the two different dimensions in terms of the positively and negatively scored items, was considered irrelevant’ [16]. But exploratory factor analysis (EFA) later indicated that a two-dimensional structure was more dominant in the PSS-10 [17]. A confirmatory factor analysis (CFA) by Andreou et al. confirmed that the one-dimensional model did not provide acceptable fit, while the two-dimensional model tented to show a better fit both in the PSS-10 and PSS-14 [18]. A principal component analysis (PCA) supported the existence of two dimensions: one dimension related to perceived stress (measured by six negatively worded items), while another related to coping ability and stress resilience counter-stress (measured by four positively worded items) [17]. A Turkish study by Örücü and Demir found gender differences in a translated version of the PSS-10 [19], whereas a study by Barbosa-Leiker et al. indicated that stress and counter-stress were measured equivalently in men and women by the PSS-10 [20]. Furthermore, a study by Gitchel et al. found that women reported higher levels of perceived stress overall and on the positively worded items, but not on the negatively worded items. The study suggested that gender-related item directionality on the PSS-10 might be the primary biasing factor [21].
The psychometric properties of the different versions of the PSS have been extensively studied in many countries by classical test theory (CTT). Several studies conducted in the general population in a variety of countries have found that Cronbach's α for the total scale ranges between 0.75 and 0.91 [13], [18], [22], [23]. The criterion validity was evaluated by Mitchell et al., who found that the PSS was significantly negatively correlated with the mental component of the Short-Form Health Survey (SF-36) from the Medical Outcome Study (MOS); (p < 0.05) and (r = − 0.70) [24].
Overall the PSS-10 seems to have some unsolved issues as several of the assessment methods (including modern test theories) have indicated problems [17], [18].
The present study adopts a broader perspective by further investigating the construct validity and the dimensionality of the PSS-10 by CFA and further by applying modern test theory within the framework of a parametric Rasch analysis and a non-parametric Mokken scale analysis. In addition, we aim to investigate the fit of the Rasch model to PSS-10 data collected from the Danish National Health Survey (DNHS) for a unidimensional model and the two suggested dimensions of the scale.
Section snippets
Study population
The PSS-10 formed part of a battery of self-report questionnaires on physical and mental health in the DNHS in 2010. The DNHS was based on six random subsamples; one from each of the five Danish regions (mutually exclusive) and a national sample. In this study, we used the population-based sample of 52,400 persons from the Central Denmark Region [25].
All randomly selected individuals received an introductory letter, which briefly described the purpose of the voluntary survey and invited the
Study population
In the total sample of (N = 34,168); 5451 (15.9%) were aged 16–29 years, 4689 (13.7%) were aged 30–39 years, 6448 (18.9%) were aged 40–49 years, 6509 (19.1%) were aged 50–59 years, 6356 (18.6%) were aged 60–70 years and 4715 (13.8%) were aged > 70 years.
To achieve a more homogenous population the non-Danish or native language missing (N = 1794) was excluded and then the dataset comprised 32,374 persons with information on gender, age and responses to the PSS-10 (Fig. 1). As 57 values were missing for
Main findings
This large population-based study revealed that the Danish version of the PSS-10 did not fit the Rasch model, and no valid overall scale emerged after extensive modifications of the one-dimensional model. Performed modifications included deleting items with the largest misfit, excluding persons with extreme fit residuals, performing different combinations of subgroups, identifying challenging items and collapsing response categories. The problems primarily centred on the positively worded
Conclusion
The current version of the PSS-10 seems to have scalability problems. Our findings revealed that the PSS-10 did not fit the Rasch model as a unidimensional model, while the Mokken analysis identified a unidimensional model. We obtained a better, but not yet acceptable, fit to the Rasch model by splitting the items into the two dimensions suggested in previous research. Our CFA analysis confirmed the same grouping of items as evidenced in former studies. In conclusion, our results overall
Conflict of interest
The authors report to have no competing interests.
Ethical standards
The study was based on anonymised data and was approved by the Danish Data Protection Agency Journal Number (1-16-02-571-13).
Acknowledgements
This study was supported by an unrestricted grant from the Lundbeck Foundation (grant number: R155-2012-11280) and the Central Denmark Region Foundation for Primary Health Care Research (Praksisforskningsfonden).
We would like to thank the Centre for Public Health and Quality Improvement in the Central Denmark Region for allowing us to work with the data collected for the DNHS study.
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