The Visual Analogue WOMAC 3.0 scale - internal validity and responsiveness of the VAS version

Table 2 Rasch analysis WOMAC subscales (pre data)

Analysis number	Item fit residual		Person fit residual		χ² interaction		PSI	Uni-dimensionality Independent t-test (95% CI)
Pain subscale	Mean	SD	Mean	SD	Value (df)	P*
Pre-data
Analysis 1	0.538	1.101	-0.433	1.224	5.65 (10)	0.844	0.86	6.3% (3.5 to 9.2)
Analysis 2	-0.065	2.811	-0.538	1.194	8.15 (8)	0.419	0.85	4.5% (1.7 to 7.4)
Physical Functioning subscale
Analysis 3	0.478	1.511	-0.557	1.896	63.37 (34)	0.002	0.96	9.5% (6.6 to 12.4)
Analysis 4	0.757	1.393	-0.469	1.823	44.81 (32)	0.066	0.96	7.7% (4.8 to 10.6)
Stiffness subscale
Analysis 5	-0.546	0.181	-0.499	0.722	8.61 (4)	0.072	0.81	2.3% (-0.6 to 5.2)

Pain subscale: Analysis 1: fit statistics of Pain subscale; Analysis 2: in this analysis we have combined biased items 2 and 4 into a testlet, the remaining items are unchanged.
Physical Functioning subscale: Analysis 3: fit statistics of Physical Functioning subscale; Analysis 4: in this analysis we have combined biased items 1 and 5 into a testlet, the remaining items are unchanged.
* Pain subscale Bonferroni adjusted (0.05/5 items, p-value considered significant if <0.01); Physical Functioning subscale Bonferroni adjusted (0.05/17 items, p-value considered significant if <0.003); Stiffness subscale Bonferroni adjusted (0.05/17 items, p-value considered significant if < 0.025)

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