Fit parameters of the VILL-37 and VILL-33, compared with Rasch model requirements
| Parameters | Rasch model | VILL-37 reading and accessing information | VILL-37 mobility and safety | VILL-37 emotional well-being | VILL-33 reading and accessing information | VILL-33 Mobility and safety | VILL-33 Emotional well-being |
| 20 items | 13 items | 4 items | 17 items | 12 items | 4 items | ||
| Disordered thresholds | None | None | None | None | None | None | None |
| Misfitting items (outfit MNSQ) | 0 |
Item 33 (3.29) Item 32 (2.01) Item 29 (1.55) | Item 26 (1.46) | None | None | None | None |
| Person reliability | >0.8 | 0.90 | 0.86 | 0.78 | 0.91 | 0.87 | 0.78 |
| Person separation index | >2.0 | 3.07 | 2.52 | 1.90 | 3.27 | 2.58 | 1.90 |
| Difference in person and item mean | <1 | 1.80 | 1.95 | 1.51 | 2.09 | 2.08 | 1.51 |
| PCA (eigenvalue for first contrast) | <2.0 | 2.37 | 2.05 | 1.64 | 2.34 | 2.10 | 1.64 |
| DIF: item number (DIF contrast) | <0.64 | ||||||
| Gender | None | None | None | None | None | None | |
| Age group (≤70 and >70) | None | None | None | None | None | None | |
| Administration mode* | Item 33 (0.81) | None | None | None | None | None |
Bold values represent misfit to the Rasch model.
*Administration mode refers to self-administration via paper forms versus interviewer administration.
DIF, differential item functioning; MNSQ, mean-square value; PCA, principal component analysis; VILL-33, Vision Impairment in Low Luminance with 33 Items; VILL-37, Vision Impairment in Low Luminance with 37 Items.