Table 3 Multivariate relationship between BASDAI and mSASSS with BASFI and DFI separately, established by generalised estimating equations in the OASIS database in 188 patients who had a BASDAI ⩽6

Independent variable	Dependent variable:
	BASFI		DFI
	Parameter estimate (SE)	p Value	Parameter estimate (SE)	p Value
BASDAI (units)	0.73 (0.051)	<0.001	3.8 (0.43)	<0.001
mSASSS (units)	0.057 (0.0049)	<0.001	0.29 (0.045)	<0.001
ESR (mm)	0.016 (0.011)	0.142	0.12 (0.064)	0.065

Models were adjusted for age, sex, disease duration, HLA-B27 and hip involvement. These variables did not have a significant independent contribution in the model. Results were similar with CRP instead of ESR in the model.
The parameter estimate, like a regression coefficient, describes the independent relationship between the explanatory variable (eg, mSASSS) and the dependent variable (eg, BASFI).
BASDAI, Bath Ankylosing Spondylitis Disease Activity Index; BASFI, Bath Ankylosing Spondylitis Functional Index; CRP, C-reactive protein; DFI, Dougados Functional Index; ESR, erythrocyte sedimentation rate; mSASSS, modified Stoke Ankylosing Spondylitis Spine Score; OASIS, Outcome in Ankylosing Spondylitis International Study; SE, standard error.