This study is the first to investigate the psychometric properties of the DASS-21 in older adults. Analyses examined the factor structure, internal consistency, convergent and discriminative validity, and diagnostic utility of the DASS-21. CFA of the DASS-21 indicated that a-three factor solution best fit the data. This is consistent with findings from populations of adult anxiety-disordered patients (Antony et al., 1998
), adult Spanish patients (Daza et al., 2002
), and adult mood-disordered patients (Clara et al., 2001
). Consistent with research on the DASS-42 (Brown et al, 1997
), the three-factor solution of the DASS-21 was superior to a two-factor solution that collapsed the Anxiety and Stress scales. The three-factor solution also produced a better fit than the two-factor solutions that collapsed the Anxiety and Depression or Stress and Depression scales. This result is in contrast to suggestions that GAD and depression are not independent constructs in older adults (Schoevers et al., 2003
, Schoevers, et al. 2005
). This suggests that a constructural distinction between depression and anxiety/stress is necessary in this sample of older adults with predominately GAD and depression. Thus, despite significant zero-order inter-scale correlations, the present results suggest the presence of three distinct factors in older adult primary care patients.
Consistent with examinations in younger adults, the DASS-21 total score demonstrated excellent internal consistency. At the scale level, the DASS-D and DASS-S had good internal consistency; whereas the DASS-A had poor internal consistency. Although a previous investigation of the DASS-21 also reported the lowest scale internal consistency for the DASS-A (α=.87) (Antony, et al. 1998
), the lower-than-expected value in this population may represent chance or a systematic difference between younger and older adults with respect to these items. Older patients present with more somatic complaints which may result in less variability in their responses than younger patients. Indeed, post-hoc exploratory factor analyses of the DASS-21 in this sample revealed that the 7 items of the DASS-A loaded onto two separate factors. Unfortunately the loadings were inconsistent across subsequent sample divisions and rotations. Regardless, caution in the interpretation of the DASS-A – and other measures of anxiety that utilize items with somatic content – is necessary in older patients, unless such effects can be ruled out. Future studies are clearly needed to examine this hypothesis.
Results strongly supported the convergent validity of the DASS-21 in older adults. The pattern of correlations between the DASS-21 total score and scale scores with associated measures was consistent with a priori predictions. Correlations between DASS scales and measures of similar constructs (i.e., BDI, BAI, and PSWQ/PANAS-NA) were consistently significantly related and nearly unanimously superior to comparison measures.
Results also provide qualified evidence for the discriminative validity of the DASS-21 in older adults. The three DASS-21 scales were tested for their ability to detect group mean differences between participants diagnosed with primary GAD, mood disorders, comorbid GAD/mood disorders, and no diagnosis. Participants diagnosed with GAD, mood, and comorbid GAD/mood scored significantly higher on each of the scales than participants without a DSM-IV diagnosis. Further, scores on the DASS-D were higher for both the pure and comorbid depression groups than the pure GAD group. Likewise, scores on the DASS-S were higher in the comorbid GAD/mood group than in the pure mood group. In contrast, no significant differences emerged across the three diagnostic groups on the DASS-A. It should be noted, however, that the patterns of means were consistent with all a priori hypotheses and may reflect Type II error. In summary, it appears as if all three DASS-21 scales are able to differentiate pathological and nonpathological samples. Moreover, the DASS-D appears to be especially adept at differentiating patients with and without depression even in the presence of comorbidity. The DASS-S, in contrast, seemed to differentiate the comorbid group from the mood only group. The performance of the DASS-A should be interpreted with caution due to characteristics of the current sample.
Using ROC analyses, we examined the diagnostic utility of the DASS-21 scales with respect to the prediction of GAD and mood disorders. These analyses indicated that the DASS-S differentiates positive versus negative GAD status equally as well as the PSWQ and PANAS-NA. Analyses also indicated that the DASS-D scale better differentiates positive versus negative mood status than the other two scales and equally as well as the BDI-II. Overall the AUC was relatively low for both scales, and especially so for the DASS-S. In comparison, the 14-item Hospital Anxiety and Depression Scale (HADS) demonstrated a larger AUC (.80) when predicting GAD in geriatric primary care patients (Wetherell, Birchler, Ramsdell, & Unüzer, 2007
). Nevertheless, these results are noteworthy in that the 7 item scales of the DASS-21 predict the diagnostic status of GAD and mood disorders equally as well as two separate measures (e.g., PSWQ=16 items and BDI-II=21 items) and therefore have the potential to reduce patient fatigue. Although short forms of the BDI and BAI exist (e.g., Mori et al., 2003
; Scheinthal et al., 2001
), most researchers use the full scales. The relative advantages of using the DASS-21 or the HADS are related to the additional information yielded by the three integrated scales of the DASS. If further research using different samples replicates these findings and extends the results to the DASS-A scale, the DASS-21 is one candidate for an economical screening questionnaire in older adults.
Several limitations must be considered when interpreting these results. First, all instruments examined in this study are self-report measures and are subject to similar sources of method error. Future studies using a multi-trait, multi-method approach might reveal different sources of variance and suggest whether the observed weaknesses in group-level discriminative validity result from construct definition, rater source, or both. A second limitation surrounds the relatively homogeneous diagnostic constellation of this convenience sample. Whereas the sample reflects both the goals of the overarching treatment study and the prevalence of mental disorders in older adults, a greater sampling of other anxiety diagnoses is needed to fully explore the psychometric properties in older adults and allow comparison with younger adults. Likewise, participants were self-referred and screened positive to the two PRIME-MD questions. It is possible that some of the results would have been stronger in the larger population of standard clinical care patients who did not answer positive to screening questions Further, the recommended cut-off scores are dependent on diagnostic aims and may need to be lowered in clinical care where a false-negative has worse consequences than a false-positive. Finally, the cross-sectional nature of these data does not allow interpretation of psychometric stability over time. Longitudinal designs across the life-span and following therapy are encouraged in future studies.
In conclusion, the DASS-21 demonstrated positive psychometric properties in a population of older primary care patients. Results indicate that the DASS-21 has overall good-to-excellent internal consistency, a three-factor structure consistent with younger samples, very good convergent validity, and acceptable discriminative validity – especially with respect the depression scale. The difficulties surrounding group- level discriminative validity are common to all measures of anxiety and depression and, based on inter-scale correlations, the DASS-21 may be better than most. These results indicate that the DASS-21 should be further examined and potentially used as a routine screening device in older adults in primary care settings.