Exploring differential item functioning in the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)

Table 4 DIF testing of WOMAC physical functioning items

	DIF items using each criteria			Type of DIF (SRbon)	Impact (p value)
Sub-groups	Zumbo	SR	SRbon		t-tests: DIF free items^a	Effect size change^b
Age	13	2,9, 11,12,13,14	2,13, 14	Uniform (+)	0.20	<0.0005
Gender	13	6, 8,9,12,13 (2,16)	6,13	Uniform(+)	0.32	<0.0005
Social Class	none	7,16	none
Townsend	none	8,17	none
Employed	none	2,10,12,13,14	2,12,14	12 NU/U; 14 U(−)	<0.0005	0.05
Mood	none	none	none
BMI	none	2,3,13, 16,17 (9,1)	2	U(+)	<0.0005	<0.0005
No joints	none	13 (6)	none
Hip only v Knee only	none	1,2,11 (7)	1	U(+)	0.88	0.02

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 change in effect size (i.e. the difference between p-values for sub-group comparison using all items v DIF free items).

ISSN: 1471-2474