Re-evaluating a vision-related quality of life questionnaire with item response theory (IRT) and differential item functioning (DIF) analyses

Table 3 Item parameter estimates and fit statistics

	Item content per dimension	α	β ₁	β ₂	β ₃	β ₄	β ₅	X²	df	p
	Basic aspects
6.	Seeing moving objects	2.06	-1.21	-0.66	0.13	0.66	2.55	19.6	20	0.49
7.	Eyes getting tired	2.61	-1.25	-0.96	-0.29	0.25	2.10	17.0	17	0.45
8.	Seeing the television	2.18	-1.69	-1.18	-0.34	0.21	1.98	17.1	18	0.52
9.	Glare (dazzled by lights)	1.81	-2.17	-1.80	-1.20	-0.51	1.76	24.8	19	0.17
10.	Getting right amount light	2.12	-1.83	-1.26	-0.37	0.22	2.32	13.0	20	0.88
	Mobility
2.	Night vision inside house	1.66	-1.42	-0.88	0.32	0.85	3.16	24.6	22	0.31
3.	Seeing steps or curbs	3.56	-1.04	-0.68	-0.09	0.17	1.55	13.9	20	0.84
4.	Depth/distance perception	2.41	-0.80	-0.45	0.09	0.51	1.98	23.3	25	0.56
15.	Getting around outdoors	5.26	-1.07	-0.75	-0.14	0.22	1.72	7.3	18	0.99
16.	Crossing a road with traffic	3.35	-1.11	-0.69	-0.25	0.12	1.52	17.7	22	0.73
	Adjustment
11.	Understand eye condition	1.26	-0.94	-0.47	0.22	0.68	3.48	17.0	21	0.71
12.	Unhappy situation in life	3.23	-0.63	-0.37	0.51	0.93	3.22	10.5	21	0.97
13.	Frustration with doing tasks	3.41	-1.06	-0.69	0.02	0.38	2.44	19.3	19	0.44
14.	Visiting friends and family	1.37	-0.44	-0.13	0.52	0.89	2.99	16.5	21	0.74
	Reading and fine work
17.	Reading large print	2.14	-0.14	0.19	0.85	1.17	2.10	23.2	27	0.68
18.	Reading newspaper/books	3.98	-1.42	-1.06	-0.61	-0.26	0.92	23.7	25	0.54
19.	Reading labels	3.50	-1.79	-1.44	-0.94	-0.54	0.79	13.2	22	0.93
20.	Reading letters and mail	5.02	-1.28	-0.93	-0.36	0.02	0.98	21.9	22	0.47
21.	Finding out the time	1.91	-0.50	-0.16	0.59	1.13	2.27	25.8	29	0.64
22.	Writing	3.55	-1.04	-0.71	-0.17	0.17	1.06	21.2	27	0.78
23.	Reading own handwriting	3.17	-0.81	-0.41	0.11	0.51	1.31	24.5	26	0.55

Person distribution standard normal; α = item discrimination parameter; β = item threshold parameter;
X² = Chi-squared item fit statistic; df = degrees of freedom.

ISSN: 1471-2288