Table 2

Item–person fit statistics, reliability and unidimensionality of the SScQoL (country-specific data)

Country	Item fit residual		Person fit residual		Item–trait χ² interaction		Person separation reliability		Unidimensionality test*
Country	Mean	SD	Mean	SD	Value (DF)	P value	PSI	N	Proportion of independent t-tests (binomial 95% CI)
A: Analysis of the SScQoL with individual 29 items
UK	−0.232	1.043	−0.233	0.680	48.932 (29)	0.012	0.919	112	0.099 (0.060 to 0.138)
France	−0.291	1.048	−0.271	0.855	34.868 (29)	0.209	0.893	111	0.087 (0.047 to 0.127)
Germany	−0.704	2.034	−0.484	1.456	398 (116)	<0.001	0.881	263	0.063 (0.028 to 0.099)
Italy	−0.205	1.051	−0.285	0.756	88.662 (58)	0.006	0.890	125	0.053 (0.016 to 0.091)
Poland	−0.520	1.380	−0.342	0.950	187.494 (116)	<0.001	0.902	221	0.099 (0.060 to 0.138)
Spain	−0.166	0.751	−0.220	0.654	42.439 (29)	0.051	0.906	95	0.075 (0.033 to 0.116)
Sweden	−0.264	0.910	−0.273	0.728	60.886 (29)	<0.001	0.892	101	0.068 (0.026 to 0.110)
B: Analysis of the SScQoL as a 5-testlet scale
UK	0.016	1.535	−0.288	0.848	4.992 (5)	0.417	0.896	109	0.050 (0.011 to 0.088)
France	0.020	1.380	−0.280	0.895	2.368 (5)	0.796	0.826	106	0.028 (-0.013 to 0.068)
Germany	−0.698	2.795	−0.285	0.873	61.952 (20)	<0.001	0.852	263	0.030 (-0.013 to 0.073)
Italy	−0.137	1.888	−0.315	0.831	4.281 (5)	0.510	0.818	125	0.038 (0.001 to 0.075)
Poland	−0.348	2.139	−0.308	0.966	22.450 (15)	0.096	0.853	221	0.043 (0.015 to 0.071)
Spain	−0.265	1.346	−0.221	0.711	11.430 (5)	0.043	0.846	92	0.018 (-0.023 to 0.060)
Sweden	−0.136	0.793	−0.253	0.819	25.665 (44)	0.988	0.813	95	0.030 (-0.013 to 0.073)
Perfect fit	0	1	0	1		>0.05	>0.70		≤0.05 or lower-bound 95%CI≤0.05

P value, χ² probability, where >0.05 (>0.01 for Bonferroni correction) suggest adequate fit to the model.
*Unidimensionality was deemed supported if ≤5% (0.05) of independent t-tests were significant or if lower bound of a binomial 95% CI of the observed proportion overlapped 5% (0.05).
DF, degree of freedom; PSI, Person Separation Index (internal consistency) reliability; SScQoL, Systemic Sclerosis Quality of Life Questionnaire.