Rasch analysis reveals problems with multiplicative scoring in the macular disease quality of life questionnaire

To evaluate validity and psychometric characteristics of the Macular Disease Quality of Life questionnaire (MacDQoL), a multiplicative rating scale designed to measure vision-related quality of life (VRQoL) in macular diseases and age-related macular degeneration (AMD).

Cross-sectional study.

We included 108 patients with neovascular AMD at baseline before ranibizumab treatment.

The psychometric properties of the MacDQoL were assessed using Rasch analysis, exploring key indices such as response category functioning, instrument unidimensionality, discriminant ability, and targeting of item difficulty to patient ability.

Measurement characteristics of the MacDQoL.

In the MacDQoL's native form, the majority of response categories were underutilized and thresholds disordered. This could not be remedied without eliminating the importance ratings owing to the ambiguous nature of the response categories. Scaling problems were resolved by using the impairment rating scale only and collapsing response categories to 4. However, the MacDQoL was multidimensional, necessitating the omission of a number of items and splitting it into an activity limitation and mobility and a socioemotional well-being scale. This improved the psychometric parameters of the revised MacDQoL, although no correlation with clinical measures such as visual acuity was found.

The multiplicative rating scale of the MacDQoL is flawed and does not provide scientific measurement of VRQoL. Measurement can be restored with a series of revisions to the instrument. This study reinforces the importance of considering rating scale design when choosing patient reported outcomes instruments for healthcare research.