Testing the measurement properties
General approach. A health status instrument that is required
to measure change over time (e.g. clinical trials
and clinical practice) must have good evaluative properties
which are responsiveness (the ability to detect important
within-patient changes, even if they are small)
and longitudinal construct validity (appropriate correlations
between changes in the new instrument and changes
in established health status measures) [9]. An instrument
that is required to distinguish between people at a single
point in time (e.g. surveys and impairment assessment)
must have good discriminative properties which
are reliability (high ratio of variance between-patients
to variance within-patients) and cross-sectional construct
validity (appropriate correlations between established
measures and the new instrument) [9]. The ACQ
was tested for both evaluative and discriminative properties.