Abstract
PURPOSE: This article presents a theoretical interpretation of patient-reported outcomes of low-vision rehabilitation (LVR) using rating scale questionnaires and uses previously published results of LVR outcome studies to illustrate theoretical points and validate assumptions. THEORY: Patients' judgments of the difficulty they have performing tasks are interpreted as magnitude estimates of their functional reserve for each task, which is the difference between their visual ability and the visual ability demanded by the task. We assume that improvements in functional reserve can occur by increasing the patient's visual ability with medical, surgical, or refractive interventions or decreasing the visual ability demanded by the item with activity-specific vision assistive equipment, adaptations, and environmental modifications. Activity-specific interventions cause differential item functioning (intervention-related DIF). Intervention-related DIF makes the measured size of the treatment effect dependent on the item content and the mix of responsive and unresponsive items to intervention. CONCLUSIONS: Because intervention-related DIF depends on the choice of items, the outcome measure selected should be appropriate to the aims of the intervention and the impairment level of the sample to demonstrate the full effects of an intervention. Items that are given extreme positive ratings at preintervention baseline (e.g., "not difficult") have no room for improvement. These items must also be filtered out because they will dilute the measured effect of the activity-specific interventions of LVR.