Abstract
Purpose::
The Daily Living Tasks Dependent on Vision (DLTV) is a visual functioning instrument which assesses the quality of life of patients with AMD. It is composed of twenty-two items and uses a four point ordinal scale. The main objective of the study was to assess whether the response scale of the DLTV was appropriate. A secondary aim was to assess if the total score across all items was linear.
Methods::
Rasch analysis was performed on data concerning a group of 186 AMD patients who completed the DLTV. A reduced version of the DLTV (DLTV-11) which employs a five point response scale was administered to a second group of patients (n = 324). Rasch analysis calculated response odd ratios per item and per person. Outfit mean square statistics for each category in the two instruments were examined to ascertain which of the two scales was optimal. For the DLTV, the average score for each individual was plotted against the Rasch person measure to assess whether the total score is linear.
Results::
Average measures were very close to the expected values and step calibrations were evenly separated for the four point scale. For the DLTV-11, step calibrations were unevenly separated suggesting that the five point scale was suboptimal and should be reduced to a four or three point scale. The total score per individual of the DLTV was non-linearly related to the Rasch person measure.
Conclusions::
Of the two scoring systems, the four point scale proved to be the most optimal for the DLTV. A double asymptotic non-linear regression of the total scores would allow for adjusted Rasch person measures which may then be utilised as a verification method for the response scale used.
Keywords: age-related macular degeneration • quality of life • clinical (human) or epidemiologic studies: biostatistics/epidemiology methodology