Responsiveness to change: an aspect of validity, not a separate dimension.

Assessment of health-related quality of life is accelerating in naturalistic observational studies, clinical trials, and clinical practice. Some researchers have argued that the ability of a quality of life instrument to detect clinically important changes over time, "responsiveness," is a distinct psychometric property from the measure's reliability and validity. We discuss the important implications of this argument and counter that responsiveness is actually one indication of a measure's validity.