Validity of summing painful joint sites to assess joint-pain comorbidity in hip or knee osteoarthritis

Table 1 Rasch fit statistics of all 19 items (N=401), with hip scores removed for hip patients and knee scores for knee patients

Joint	Measure	Fit				DIF
	Logits	Infit MNSQ	Infit ZSTD	Outfit MNSQ	Outfit ZSTD	Age	Gender	BMI	Disease duration	Patient group
Lumbar spine	−2.02	1.23	3.5	1.28	2.3	−0.73	0.26	0.20	0.14	1.14
Left knee	−1.45	1.24	1.8	1.21	1.0	1.03	1.12	0.45	−0.70	N/A
Right knee	−1.11	1.02	0.2	1.06	0.3	−1.11	−0.31	0.30	−0.10	N/A
Neck	−0.90	1.00	0.1	1.04	0.4	−0.17	0.02	−0.84	0.07	−0.11
Right shoulder	−0.46	1.04	0.4	1.01	0.1	−0.17	−0.18	−0.16	0.11	0.04
Left shoulder	−0.21	0.87	−1.2	0.72	−1.6	0.21	−0.32	−0.05	−0.06	−0.15
Right hand	−0.10	0.95	−0.4	0.99	0.0	0.99	−0.53	−0.05	−0.23	−0.65
Left foot	0.06	0.94	−0.5	0.89	−0.4	0.19	−0.10	0.37	0.11	−0.45
Right hip	0.07	1.02	0.2	1.04	0.2	−0.16	0.03	0.17	0.59	N/A
Left hip	0.07	1.16	1.2	1.38	1.5	−0.84	−0.49	−0.30	0.45	N/A
Right foot	0.10	0.94	−0.5	0.77	−1.1	0.11	−0.03	−0.12	−0.46	0.08
Left hand	0.24	1.05	0.4	1.09	0.4	1.04	−0.26	−0.06	−0.28	−0.78
Thoracic spine	0.48	1.09	0.7	0.85	−0.5	−0.22	0.11	0.19	0.45	0.85
Right ankle	0.54	0.99	0.0	0.98	0.0	−0.53	0.19	−0.25	−0.60	0.69
Left ankle	0.59	0.89	−0.7	0.67	−1.3	−0.21	−0.77	0.10	−0.21	−0.69
Right wrist	0.65	0.79	−1.3	0.52	−1.9	0.38	−0.14	0.28	0.02	−0.62
Left wrist	0.71	0.87	−0.7	0.63	−1.3	1.08	−0.03	0.00	0.04	−0.19
Right elbow	1.11	1.09	0.5	1.33	0.9	−0.46	1.57	0.66	0.38	−0.99
Left elbow	1.65	0.95	−0.1	0.88	−0.1	−0.05	0.92	0.32	0.20	−1.02

Infit MNSQ: mean square Infit, Outfit MNSQ: mean square Outfit, ZSTD: standardized as a z-score, DIF: differential item functioning, BMI: body-mass index, N/A: not available. Bold values show either Infit MNSQs or Outfit MNSQs outside their specified range or show the presence of differential item functioning (DIF). DIF would have been significant when: p< (0.05/(5*19))= 0.000526 (corrected for sample size) AND when the absolute difference between the two difficulty parameters was ≥0.5 logits. However, no significant DIF results were found in this study.

ISSN: 1471-2474