Evaluation of a self-report measure of low-vision functional ability

Abstract

The majority of data used in outcome research has until recently been based primarily on treatment provider observations of the patient. Estimating the effect of rehabilitation treatment on functional ability is increasingly relying on patient self-report ratings. Controlling for measurement bias associated with the subjective nature of this type of data was handled by putting self-report and clinician responses to items on an 11-item functional ability instrument into the same metric using a Rasch model, allowing for an accurate picture of whether and how their perceptions differ. For this study, the traditionally clinician-rated instrument was modified to accommodate self-report ratings. The questionnaire was verbally administered during individualized discharge interviews to 112 visually-impaired geriatric veterans whose responses were then paired with clinician ratings. Clinician ratings replicate previous findings which provided a stable foundation against which the self-report ratings were compared. Fit statistics for self-report ratings demonstrate unidimensionality of the assumed functional ability construct. Construct validity, or to what extent clinicians and patients agree on what it is that is being measured (i.e., functional ability), was supported by a moderate correlation between person ability estimates. More informative, however, was the agreement on item difficulty estimates. While six of the 11 items had roughly the same difficulty estimates across the two rating samples, 5 others lacked agreement. Two possible reasons for this incongruity are (1) patient expectations about what they should be able to do by the end of rehabilitation are not realized and (2) exemplars used in eliciting self-report ratings are not as specific as those used by clinicians. The single-factor structure was also analyzed by a confirmatory factor analysis. Although the model did not meet conventional criteria of .95 for the CFI, it was the most parsimonious solution for both sets of ratings as demonstrated by a multisample analysis. These analyses indicated that self-perceived functional ability ratings may be used in place of clinician ratings on the majority of the FAST items. Until the remaining items are modified, at this point, self-report ratings may, at best, supplement, but not be used in place of, clinician ratings.