3.1 Participants’ characteristics
The participant’s characteristics are shown in Table 1. The sample was predominantly made up of young adults (18 to 25 years: 74%), females (60%), people living in urban areas (68%), and mostly students (51.3%).
3.2 Classical Test Theory (CTT)
Table 2 shows the item response frequency of each item along with CTT statistics. The respondents endorsed all four of the response categories. Over 40% of the respondents agreed that they find it sometimes “hard to wind down”; about half of the participants (47%) had never “experienced breathing difficulty” in the past week; and about 42% of the respondents had never “experienced trembling” in the past week. Item 4 (“I experienced breathing difficulty”) on the anxiety sub-scale, had the least frequently endorsed “most or all of the time” category, with 9% of the total respondents.
3.2.1 Reliability analysis:
Overall, Cronbach’s alpha of the DASS-21 was 0.959, which indicates an excellent internal consistency. The Cronbach’s alpha value of each sub-scale for anxiety, depression, and stress were 0.87 (95% CI 0.86 to 0.89), 0.92 (95% CI 0.91 to 0.93) and 0.89 (95% CI 0.88 to 0.90) respectively. Alpha for these data would remain consistent if items were deleted, staying within the 95% confidence interval range of 0.95 to 0.97.
The average inter-item correlation was 0.739, and corrected inter-item correlations for each of the sub-scales indicated acceptable discrimination (0.69 – 0.80, depression; 0.44 – 0.78, anxiety; 0.61 – 0.80, stress). Item 2 (“I was aware of dryness of my mouth.”), on the anxiety sub-scale, was the least discriminating item (0.49); Item 11 (“I find myself getting agitated.”), on the stress sub-scale, had the highest item discrimination (0.82).
3.3 Testing assumptions:
CFA confirmed the one-dimensional structure of the overall DASS-21 scale: CFI of 0.90, TLI of 0.89, RMSEA of 0.088 (90% CI 0.084 to 0.092) and SRMR of 0.043 suggested that the model fit the data . The assumption of unidimensionality for the anxiety and stress sub-scales were also empirically supported by CFI, TLI, RMSEA, and SRMR (see Table 3). The depression sub-scale, however, had worse RMSEA (0.112; 90% CI 0.098 to 0.127) but acceptable CFI and TLI.
3.3.2 Local independence
The effect size of the model fit (MADaQ3) and test of global model fit (max aQ3) statistics, and SRMR and Standardized Root Mean Square Root of Squared Residuals (SRMSR) were estimated for assessing the local independence [40, 41]. MADaQ3 and max aQ3 were 0.0851 and 0.4388 respectively; SRMR and SRMSR were 0.081 and 0.101 respectively, supporting the assumption of local independence.
3.4 IRT calibration:
3.4.1 Item properties
GRM had better empirical fit to sub-scale response data when considering AIC, BIC, -2loglikelihood values, and the LRT (see Supplementary tables). Therefore, the GRM was used to calibrate and examine item-level parameters for each of the sub-scales. In addition, we assessed the performance of the GRM, GRSM, GPCM, and the Partial Credit Model (PCM; a Rasch model) for the overall DASS-21. The GRM also had the best model fit for the overall DASS-21 response data. DASS-21 sub-scale item characteristics are displayed in Table 4. Standard error of measurement for each of the items is minimized in the theta range of -3.0 to 3.0, with the anxiety sub-scale having the best reliability across the entire range of theta. Test reliability, standard error, and information functions for each sub-scale are provided in Figure 1. Individual item function, Item Categorical Response Curves, and Operation Characteristic Curves for each sub-scale are in Supplemental Figures 1-9.
On the depression sub-scale, item information was maximized for Item 21 (‘life is meaningless’), Item 10 (‘nothing to look forward to’), and Item 16 (‘unable to become enthusiastic about anything’), while Item 5 (‘difficult to work up the initiative’) provided the least amount of information. Item 5 also had the lowest mean threshold, indicating that people with lower levels of the latent trait of depression had higher probability of endorsing the item. Items with higher mean thresholds – Item 16 and Item 3 (‘couldn’t experience any positive feeling’) – were more related to the symptoms of feeling pleasure.
The anxiety sub-scale had one item with item discrimination over 3.0: Item 15 (‘close to panic’), and the item providing less information was Item 2 (‘dryness in mouth’). The lowest mean thresholds on the anxiety sub-scale were observed for Item 9 (‘worried about situations I might panic’) and Item 2. Higher mean thresholds were indicated for items with stronger physiological symptoms of anxiety: Item 4 (‘difficulty breathing’), Item 7 (‘trembling’), and Item 15.
The stress sub-scale had two items with relatively larger item information functions: Item 11 (‘getting agitated’) and Item 12 (‘difficult to relax’). Item 6 (‘tend to overreact’) and Item 8 (‘nervous energy’) had the lowest mean thresholds. Item 14 (‘intolerant of anything that kept me from getting on’) and Item 12 had the highest mean thresholds.
3.4.2 Person properties
Person fit estimates indicated the majority (> 95%) of participants within each sub-scale had Zh statistics between the range of -1.96 and 1.96, indicating strong person fit. Latent scores (θ) indicated the participants had latent scores between -6.309 to 3.035 with a mean of 0.174 (see Table 5). Raw summated and latent (IRT estimated) scores of the Depression, Anxiety, and Stress sub-scales, and overall DASS-21 were strongly correlated (rs = 0.985, 0.978, 0.985 and 0.980, respectively).