Evaluating the Longitudinal Item and Category Stability of the SF-36 Full and Summary Scales Using Rasch Analysis

Table 5

SF-36 total scale Rasch analysis of summary of category structure for six waves of data collection

	Wave 1						Wave 2
Cat. Label	N	%	Average Measures	Infit MnSq	Outfit MnSq	Andrich Threshold	N	%	Average Measures	Infit MnSq	Outfit MnSq	Andrich Threshold
1	84119	19	(-3.15)	1.30	1.17	NONE	72530	19	(-3.19)	1.28	1.17	NONE
2	133566	30	-1.36	.99	1.01	-1.88	114964	31	-1.40	1.02	1.06	-1.93
3	96735	22	-.25	.66	.61	-.54	82817	22	-.26	.67	.63	-.57
4	40204	9	.48	.98	1.04	.61	34325	9	.50	.97	1.03	.61
5	44040	10	1.31	1.06	1.17	.28^b	37154	10	1.34	1.07	1.20	.34^b
6	25211	6	(2.82)	.93	1.01	1.53	23593	6	(2.86)	.92	.99	1.56
	Wave 3						Wave 4
Cat. Label	N	%	Average Measures	Infit MnSq	Outfit MnSq	Andrich Threshold	N	%	Average Measures	Infit MnSq	Outfit MnSq	Andrich Threshold
1	61757	20	(-3.14)	1.25	1.15	NONE	55809	22	(-3.06)	1.24	1.15	NONE
2	88701	29	-1.39	1.04	1.08	-1.87	72286	28	-1.35	1.05	1.10	-1.79
3	63285	20	-.28	.73	.67	-.57	50125	19	-.30	.78	.71	-.54
4	29486	9	.48	.94	.96	.48	25807	10	.45	.90	.88	.38
5	28991	9	1.34	1.09	1.16	.41^b	24560	10	1.32	1.10	1.13	.41
6	18470	6	(2.86)	.87	.93	1.55	15297	6	(2.85)	.86	.91	1.54
	Wave 5						Wave 6
Cat. Label	N	%	Average Measures	Infit MnSq	Outfit MnSq	Andrich Threshold	N	%	Average Measures	Infit MnSq	Outfit MnSq	Andrich Threshold
1	47041	24	(-3.00)	1.26	1.16	NONE	36377	25	(-2.96)	1.23	1.14	NONE
2	54905	27	-1.31	1.06	1.09	-1.72	37987	26	-1.30	1.06	1.07	-1.67
3	36216	18	-.30	.83	.76	-.49	24952	17	-.31	.88	.83	-.49
4	21172	11	.43	.87	.81	.27	15554	11	.42	.87	.79	.23
5	18847	9	1.29	1.13	1.15	.46	13560	9	1.29	1.15	1.15	.50
6	11583	6	(2.80)	.83	.88	1.48	8495	6	(2.78)	.83	.88	1.44

Notes.Andrich threshold category increase of >5; Andrich threshold category decrease where an increase is expected; values in italic for Infit or Outfit MnSq > 1.34; values underlined for Infit or Outfit MnSq < 0.64.