Measuring functional ability in chronic arthritis: a critical review.

Functional assessment instruments used in rheumatology are critically evaluated against five basic criteria: whether they allow for quantification, whether they have been tested for reliability, validity, and precision, and whether the data collection procedures are specified. No one instrument completely fulfills all criteria. Few adequately address the issues of measurement reliability and validity; none have sufficient measurement precision to detect subtle but clinically significant changes in function. Issues to be addressed in future research are discussed.