Purpose:  Characterization of minimally important difference (MID) requires comparison with an external criterion for health change. Our was to compare MIDs obtained for several preference-weighted measurement systems using alternative approaches to defining change among Spine Patient Outcomes Research Trial (SPORT) participants.

Methods: Health state values obtained from EuroQol with US and UK scoring (EQ5D-US, EQ5D-UK), the Health Utilities Index Mark 2 and 3 (HUI2, HUI3), and SF-36-derived Quality of Well Being Scale (eQWB) and SF-6D at study entry and one year  were used to compute change scores (follow-up minus baseline ) for each system. SPORT participants who completed both surveys are included in the analysis. MID was defined using the mean change score for those who had minimal change according to the following criteria: ODI, symptom satisfaction (SATIS), transition rating (TRANS), and self-perceived health (HEALTH), where minimal was defined as one level change among five, except for ODI where it was defined as 10 points.  We computed MID stratified by those with minimal improvement and minimal worsening.

Results: Relationships between systems was generally consistent regardless of the criterion used. MIDs tended to be smaller for those who worsened than for those who improved. Satisfaction and self-perceived health provided larger estimates of MID than other criteria.  Selected MID results are as follows:

	Mean Score Change (95%CI)	Effect Size	SRM	MID
EQ5D-UK	0.308(0.290, 0.326)	0.952	0.818	0.096
EQ5D-US	0.205(0.193, 0.216)	0.967	0.820	0.063
HUI3	0.258(0.244, 0.272)	0.972	0.872	0.100
HUI2	0.206(0.195, 0.218)	0.977	0.865	0.072
SF-6D	0.158(0.150, 0.165)	1.338	0.972	0.041
eQWB	0.094(0.090, 0.099)	1.730	1.013	0.024