A total of 3,851 records were analysed. Table displays the characteristics of the four component data sets, arranged by increasing mean VAS score for each set. Analysis of variance indicated that all four mean VAS scores were significantly different from one another (F(3,3848)
0.01). The index scores of the four sets differed significantly also, with the exception of those for colposcopy and surveillance (F(3,3848)
0.01). Of the four sets, those for surveillance and colposcopy displayed the greatest similarity in terms of EQ-5D profile, with the surveillance group recording slightly fewer problems at level 1 in each domain. Compared with these two data sets, those for stroke and for back pain included substantially more problems beyond level 1 in all domains. Subjects in these two sub-samples were considerably older than those in the colposcopy and surveillance sub-samples, with the stroke patients being older on average than the back pain patients. Problems with mobility and pain & discomfort were less frequently reported in the stroke sample than in the back pain sample. In the full sample 113 different health states were represented.
EQ-5D data characteristics, by data set
Table presents the regression model (adjusted R2
0.54) which possessed a number of characteristics of interest. First, neither of the coefficients for the level 2 and the level 3 variable in the “capacity for self care” domain achieved statistical significance. Second, and irrespective of intervention, the relative magnitudes of the estimated coefficients of levels 2 and 3 for the remaining domains were consistent with intuition, in that greater problem severity was associated with lower VAS score. Third, none of the coefficients on the intercept dummies (b2
) achieved statistical significance at 5 per cent. Fourth, as regards the slope dummies (b3
), moderate limitations on usual activities for women following colposcopy exerted a greater negative effect on VAS score, compared with the reference group (surveillance). For those with low back pain compared with the surveillance group, pain & discomfort at levels 2 and 3 produced significantly lower VAS scores, whereas anxiety & depression at these levels were associated with higher VAS scores. For stroke patients in comparison with the reference group, both severe pain & discomfort and severe anxiety & depression were associated with smaller negative effects on VAS scores.
Insignificant coefficients for the intercept dummies (b2) imply that the predicted mean VAS score at health state 11111 is the same for all four conditions, namely, the constant term. However, significant coefficients on at least some of the slope dummies mean that, for many other health states as described by the EQ-5D, the predicted VAS score must vary by condition. Evaluating the estimated regression equation for vector 22222 produces mean VAS scores of 50.8 for low back pain, 55.2 for surveillance and for stroke, and 60.7 for colposcopy. The predicted scores for vector 33333 are 10.5 for surveillance and colposcopy, 21.3 for back pain, and 33.7 for stroke.
Of the 243 EQ-5D states which any respondent could possibly occupy, 81 (33.3 per cent) would have a severity level of 2 for “usual activities”. Given the model’s structure, therefore, the VAS scores for the colposcopy group would differ from those of the reference group for these 81 states. By the same token, the scores for the stroke and low back pain groups would differ for 135 and 215 states (55.6 and 88.5 per cent, respectively), compared against the reference. The scores for the stroke and back pain groups would, between themselves, also differ for 215 (88.5 per cent) of states.
The estimated regression model indicates that the marginal impact of a change in health state on VAS score would, for many such changes, differ by condition. By way of example, and based on the calculation above, a change from health state vector 33333 to vector 22222 would improve VAS scores by 21.5, 29.5, 44.7 and 50.2 for stroke, back pain, surveillance and colposcopy patients, respectively. At the domain level, and according to the coefficients presented in Table , a decrease in severity from level 2 to level 1 in the pain & discomfort domain would entail an increase of 3.9 in VAS score for the surveillance group. For the low back pain group, however, the improvement would register an additional increase of 11.7, or 15.6 in total. Similarly, a pain & discomfort severity reduction from level 3 to level 2 would result in the mean VAS score being 14.6 higher amongst the colposcopy group but only 5.5 higher for the stroke group. If anxiety & depression fell from severity level 3 to level 2, the mean VAS score would rise by 19.0 according to the colposcopy patients but by 4.9 according to the stroke patients. An improvement from level 2 to level 1 would imply an increase of 4.5 in VAS score for the low back pain group but of 11.8 for the surveillance group.