Item-scale correlations for the 10 global health items ranged from 0.53 (global7: rating of pain) to 0.80 (global09: satisfaction with social roles) and internal consistency reliability was 0.92. However, the single-factor confirmatory categorical factor analysis model for all 10 items was statistically rejectable (χ2 = 19,619.82, df = 15, P ≤ 0.001) and did not fit the data very well (CFI = 0.927; TLI = 0.961; RMSEA = 0.249).
The eigenvalues from a principal components analysis of the 10 global items were 6.25, 1.20, 0.75, 0.44, 0.39, 0.30, 0.22, 0.20, 0.18, and 0.05. The scree plot and parallel analysis number of factor criteria suggested two underlying dimensions for the 10 items. We performed an exploratory factor analysis and found support for a physical health and mental health factor (see Table ). Satisfaction with discretionary social activities (global05) loaded on mental health whereas satisfaction with social roles (global09) loaded on both physical and mental health (as did global02: quality of life; and global08: fatigue). The estimated correlation between the physical and mental health factors was 0.63. These results were also supported by our confirmatory categorical factor analysis, but three residual correlations were added to obtain acceptable model fit; see Table (global01 with global03 r = 0.14, global04 with global10 r = 0.14, and global08 with global10 r = 0.15; χ2 = 5,295.66, df = 17, P < 0.0001; CFI = 0.98; TLI = 0.99, RMSEA = 0.12). The estimated correlation between the physical and mental health factors was 0.69.
Two factor pattern for global health items (standardized regression coefficients)
Based on the exploratory factor analysis, we evaluated a physical health scale with the 5 items loading highest on the physical health factor. Global09 (satisfaction with social roles) was excluded because it correlated about equally with physical and mental health. Item-scale correlations for the five physical health items ranged from 0.57 (global07: rating of pain) to 0.79 (global01: rating of general health; and global03: rating of physical health). All 5 items correlated higher with the physical health scale than with the mental health scale. We fit a single-factor categorical confirmatory factor analytic model for the five physical health items and found that it was statistically rejectable (χ2 = 3,060.81, P < 0.001) and showed less than adequate practical fit according to the RMSEA index (CFI = 0.991; RMSEA = 0.220). By adding a residual correlation (r = 0.29) between global01 (rating of general health) and global03 (rating of physical health) to the initial model, we found that the fit of the model improved significantly (χ2 = 2,248.57, df = 1, P < 0.001) and the practical fit indices also improved (χ2 = 419.56, P < 0.001; CFI = 0.999; TLI = 0.998; RMSEA = 0.081).
We also evaluated a mental health scale with 4 items. Three of these items correlated most highly with the mental health scale. The fourth item, global02 (quality of life), correlated about equally with physical and mental health, but was also included because of prior evidence that it is primarily an indicator of mental health. Item-scale correlations for the 4 hypothesized mental health items ranged from 0.64 (global10: emotional problems) to 0.78 (global04: rating of mental health). One item (global09, satisfaction with social roles) had higher correlation with the global physical health scale than with the mental health scale; the 4 mental health items correlated strongest with the mental health scale. The single-factor categorical confirmatory factor analytic model we fit for these 4 mental health items was statistically rejectable (χ2 = 1,616.80, df = 2, P ≤ 0.001), and had mixed results in terms of practical fit (CFI = 0.983; TLI = 0.975; RMSEA = 0.196). When we added a residual correlation (r = 0.16) between global04 (rating of mental health) and global10 (bothered by emotional problems) to the initial model, the fit improved significantly (χ2 = 1,114.27, df = 1, P < 0.001) and the practical fit of the model improved (χ2 = 151.222, P ≤ 0.001; CFI = 0.998; TLI = 0.995; RMSEA = 0.084).
Based on these results, we formed two-four-item scales by averaging together the items scored on a 1–5 possible range. Our physical health items included global03 (physical health), global06 (physical function), global07 (pain) and global08 (fatigue). Our mental health items included global02 (quality of life), global04 (mental health), global05 (satisfaction with discretionary social activities), and global10 (emotional problems). The global physical health (GPH) scale excluded global01 (general health) because of its substantial residual correlation with global03 (physical health). We retained global03 in the scale rather than global01 to emphasize the physical nature of the construct. The GPH had an internal consistency reliability of 0.81 (mean = 3.79, SD = 0.76). We excluded global09 (satisfaction with social roles) from the global mental health (GMH) scale because of its higher correlation with the GPH scale. The GMH had an internal consistency reliability of 0.86 (mean = 3.60, SD = 0.89). The two scales were substantially inter-correlated (r = 0.63). In addition, we found that GPH correlated more strongly with the EQ-5D than did the GMH (r = 0.76 vs. 0.59). The R-square in a regression of the EQ-5D on the GPH and GMH was 0.60, indicating that the PROMIS global health composites share 60% of variance in common with the EQ-5D.
Correlations of the global health items and GPH and GMH with the nine PROMIS domain scores and the EQ-5D are given in Table . The largest correlations for global01 (rating of general health), global02 (quality of life), global03 (rating of physical health), global08 (rating of fatigue), and global09 (satisfaction with social roles) were with the fatigue domain. Global04 (rating of mental health), global05 (satisfaction with discretionary social activities) and global10 (emotional problems) correlated most strongly with the depressive symptoms domain. Global06 (carry out everyday physical activities) correlated most strongly with physical functioning whereas global07 (rating of pain) correlated highest with pain impact. The GPH correlated most strongly with pain impact (r = −0.75), fatigue (r = −0.73), and physical functioning (r = 0.71). GMH correlated most strongly with depressive symptoms (r = −0.71), fatigue (r = −0.68), and anxiety (r = −0.65).
Correlations of global items with PROMIS domains and EQ-5D
Correlations of the global items with the EQ-5D ranged from 0.51 to 0.77. The largest correlations with the EQ-5D were for the global ratings of pain, physical functioning, and satisfaction with social roles. Our regression of the EQ-5D on the global items revealed that all items except two (global03: rating of physical health; global05: satisfaction with discretionary social activities) had significantly unique associations (R-square = 0.64).
We estimated item parameters from the graded response model for the 4 global physical health items (Table ) and 4 global mental health items (Table ). The range of item threshold values indicates satisfactory coverage of the underlying latent trait from ~−4.0 to 2.0 for Physical Health and between −3.0 and 1.5 for Mental Health. Global06 (carry out everyday physical activities) had the highest slope (a parameter in Table ) and the largest information for the physical health items whereas global04 (rating of mental health) had the largest information for the mental health items. We found the lowest item information for items phrased to elicit ratings of undesirable domains of health (pain, fatigue, emotional problems).
Global physical health scale item parameters (graded response model) and item information
Global mental health scale item parameters (graded response model) and item information