Table 2

Fit parameters of the VILL-37 and VILL-33, compared with Rasch model requirements

ParametersRasch
model
VILL-37 reading and accessing informationVILL-37 mobility and safetyVILL-37 emotional well-beingVILL-33
reading and accessing information
VILL-33
Mobility and safety
VILL-33
Emotional well-being
20 items13 items4 items17 items12 items4 items
Disordered thresholdsNoneNoneNoneNoneNoneNoneNone
Misfitting items
(outfit MNSQ)
0Item 33 (3.29)
Item 32 (2.01)
Item 29 (1.55)
Item 26 (1.46)NoneNoneNoneNone
Person reliability>0.80.900.860.780.910.870.78
Person separation index>2.03.072.521.903.272.581.90
Difference in person and item mean<11.801.951.512.092.081.51
PCA (eigenvalue for first contrast)<2.02.372.051.642.342.101.64
DIF:
item number (DIF contrast)
<0.64
 GenderNoneNoneNoneNoneNoneNone
 Age group (≤70 and >70)NoneNoneNoneNoneNoneNone
 Administration mode*Item 33 (0.81)NoneNoneNoneNoneNone
  • 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.