Item response theory and factor analysis as a mean to characterize occurrence of response shift in a longitudinal quality of life study in breast cancer patients

Table 6 Trend τ of item easiness parameter estimated by linear logistic model with relaxed assumptions for each quality of life dimension

Dimension	Items	T1 → T2_R^a			T1 → T3_R^b			T3 → T4_R^c
Dimension	Items	N	τ	p	N	τ	p	N	τ	p
QLQ-C30
Physical functioning	1 – 4^d	100	-0.02	0.858	139	0.27	0.004	201	-0.84	<0.001
Role functioning	6, 7	84	0.36^e	0.011	118	0.71	<0.001	164	-0.60	<0.001
Emotional functioning	21 - 24	180	-0.62	<0.001	208	-0.65	<0.001	196	0.22	<0.001
Cognitive functioning	20, 25	103	-1.15	<0.001	129	-0.29	0.020	130	-0.53	<0.001
Social functioning	26, 27	75	0.07	0.791	101	0.66	<0.001	142	-0.51	<0.001
Fatigue	10, 12, 18	141	-0.19	0.280	160	0.23	0.019	183	-0.99	<0.001
Nausea and vomiting	14, 15	37	-0.60	0.022	65	0.50	0.029	98	-0.36	0.003
Pain	9, 19	96	0.11	0.410	124	0.37	0.001	148	-0.54	<0.001
Dyspnea	8	50	-0.60	0.025	55	-0.30	0.248	94	-0.42	0.014
Insomnia	11	115	-0.49	0.001	135	-0.49	<0.001	147	-0.36	0.005
Appetite loss	13	61	-0.59	0.004	85	-0.14	0.401	90	-0.51	0.003
Constipation	16	71	-0.16	0.392	89	0.17	0.295	93	-0.41	0.009
Diarrhea	17	39	-1.34	<0.001	64	-0.67	0.005	61	-0.09	0.641
Financial difficulties	28	22	0.16	0.630	33	0.25	0.322	47	-0.45	0.053
QLQ-BR23
Body image	9 - 12	68	0.10	0.489	119	0.83	<0.001	153	-0.66	<0.001
Sexual functioning	14, 15	55	0.06	0.950	71	0.33	0.078	78	0.77	0.010
Sexual enjoyment	16	13	-1.20	0.04	39	-0.43	0.14	33	0.19	0.49
Future perspective	13	122	-0.56	<0.01	135	-0.46	<0.01	144	0.01	0.95
STSE	1 - 4; 6 - 8	55	-0.40	<0.01	61	0.21	0.013	77	-0.74	<0.001
Breast symptoms	20 - 23	114	-0.11	0.07	135	0.36	0.01	190	-0.76	<0.001
Arm symptoms	17 - 19	97	0.64	<0.001	167	0.86	<0.001	156	-0.37	0.001
Hair loss	Q5	6	0.22	0.738	16	0.12	0.808	24	-0.33	0.149

^aT1 → T2_R: comparison of baseline quality of life assessment and retrospective measure performed after surgery.
^bT1 → T3_R: comparison of baseline quality of life assessment and retrospective measure performed three months later.
^cT3 → T4_R: comparison of quality of life assessment at three months and retrospective measure performed three months later.
^dQ5 was excluded because of the patients’ responses to this item at baseline: all patients chose either category 1 or 2 for this item at baseline, whereas all four categories were represented on retrospective measures.
^eτ =0.364: patients significantly chose higher response categories at the retrospective measure of baseline HRQoL performed after surgery.

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