Functional performance measures: are they superior to self-assessments?

BACKGROUND: It has been suggested that performance measures of functional status have several advantages over self-report measures for both clinical and research purposes, including: greater patient acceptability, interpretability, reproducibility, sensitivity to change, and the focus on actual ability rather than presumed capability. This article challenges this assumed superiority of "objective," "behavioral" measures by directly comparing self-assessments and blindly rated performance assessments on a specific item by task basis, using an identical rating format.
METHODS: A set of 14 performance tasks, consisting of a range of functional abilities (including simulations of cooking and sweeping), was administered to 99 community-dwelling older adults (aged 60-92) who had previously completed a 50-item instrumental activities of daily living (IADL) questionnaire. A subsample was retested 2 weeks later, and reassessed at 1 year.
RESULTS: Of 182 subjects willing and able to complete the IADL questionnaire, only 99 attempted at least one of the performance tasks. Tasks that took longer to complete were not necessarily associated with a greater number of errors, nor did accuracy ratings correspond well with difficulty ratings. Good correspondence (greater than 80% agreement) between observed and perceived difficulty was found for only one-third of the item/task matchings. Generally, the rater tended to underestimate difficulty relative to subjective assessments.
CONCLUSIONS: Relative to questionnaires, performance measures were not found to be psychometrically superior, more acceptable to respondents, easier to administer, or easier to interpret. Neither type of measure by itself distinguishes between motivation and capability, reflects adaptations made in everyday living, or accounts for personal preferences or reasons for difficulty.