Table 6.

DIF testing WOMAC pain items for participants with Hip OA

Sub-group	DIF items using each criteria		Type of DIF (SRbon)	Impact (p value)
	SR	SRbon		t-tests	Effect size change
Age	2,4	2	U(+)	0.007	<0.0005
Social Class	3 (5)
Employed	2,3,4,5	2,3
BMI	4	4	U and NU	0.0005	0.15

Key: Normal text: unpurified DIF items. Bold: final purified DIF items. Items in brackets: items that were added during purification. U uniform DIF, NU non-uniform DIF, + positive odds ratio, -negative odds ratio. ^a, p-value for t-tests comparing subgroups using DIF free items. ^b, p-value of ANOVA interaction term to test the change in effect size (i.e. the difference between p-values for sub-group comparison using all items v DIF free items).