Agreement between the EQ-5D-5 L and SF-6D

When we repeat measurements by each of two methods on the same subjects, agreement analysis is essential to see whether they agree sufficiently for one method to replace the other one [33]. Both EQ-5D-5 L and SF-6D are measures for health utility, even though the EQ-5D-5 L has a possible range of − 0.39 to 1.00, while the SF-6D has a range of 0.315 to 1.00. Hence, it is necessary for us to know to what degree these two utility measures agree and if it is possible to use these two measures interchangeably in the context of LBP patients in China. Agreement was assessed by intra-class correlation coefficients (ICCs) and Bland-Altman plots. The ICCs were calculated with two-way random effects model using average measures and absolute agreement. The ICCs can range between 0 and 1. An ICC < 0.4 suggests poor agreement, 0.4–0.59 fair, 0.6–0.74 good, and 0.75–1 excellent agreement [34]. Bland-Altman plots were also performed to explore the agreement between these two measures. In this method, the differences between the scores of the two instruments were plotted against the average utility scores [35].