Table 2 Fit parameters of the IVI (and subscales) and SF-36 scales compared to Rasch model requirements.
Parameters | Rasch model | IVI_O | IVI_F | IVI_E | SF36_C | SF-36_PF | SF-36_MH |
|---|---|---|---|---|---|---|---|
Disordered thresholds | No | Yes | No | No | No | No | No |
No. of misfitting items | 0 | 4 | 0 | 1 | 4 | 0 | 1 |
Person Separation Index | > 0.2 | 4.07 | 4.28 | 2.42 | 2.66 | 2.64 | 2.13 |
Person Reliability | > 0.8 | 0.94 | 0.95 | 0.85 | 0.88 | 0.87 | 0.82 |
Person-item mean difference | < 1 | 1.15 | 1.25 | 1.49 | 0.30 | 2.25 | 1.26 |
Variance by 1st factor | > 50% | 64.2% | 63.3% | 63.1% | 40.7% | 68.2% | 66.5% |
PCA (Eigenvalue for 1st contrast) | < 2.4 | 3.9 | 2.4 | 1.8 | 8.3 | 2.3 | 2.2 |
Differential Item Functioning | Â | Â | Â | Â | Â | Â | Â |
   Gender | < 1.0 | NA | None | None | NA | None | None |
Age group (≤50, > 50) | < 1.0 | NA | None | None | NA | None | None |
   Vision impairment (VI, non-VI) | < 1.0 | NA | None | None | NA | None | None |
Participant person measure mean ± SD | - | NA | 20.16 ± 8.16 | 8.82 ± 3.24 | NA | 19.98 ± 7.58 | 14.48 ± 6.19 |
Clinically meaningful cut-off | - | NA | < -4.1, > 4.1 | < -1.6, > 1.6 | NA | < -3.8, > 3.8 | < -3.1, > 3.1 |