Table 4.

Tests for differential item functioning on the SF-12 mental health (MH) and physical health (PH) sub-scale items, stratified by type of joint replacement

Item	THA			TKA
	Δχ2	df	p-value	Δχ2	df	p-value
MH Sub-scale
M1	20.09	6	0.003	37.03	6	< 0.001
M2	31.85	6	< 0.001	73.22	6	< 0.001
M3	38.30	6	< 0.001	44.61	6	< 0.001
M4	99.26	6	< 0.001	83.93	6	< 0.001
M5	*	*	*	*	*	*
M6	*	*	*	*	*	*
PH Sub-scale
P1	244.32	6	< 0.001	425.16	6	< 0.001
P2	*	*	*	*	*	*
P3	27.97	6	< 0.001	53.76	6	< 0.001
P4	12.96	6	0.044	16.34	6	0.012
P5	19.78	6	0.003	20.25	6	0.003
P6	*	*	*	*	*	*

df Degree of freedom, Δχ² = Chi-square difference test obtained using the DIFFTEST procedure for WLSMV in MPlus; * = anchor item; All tests are statistically significant at α = 0.05