Antiasthma drugs: quality-of-life rating scales and sensitivity to longitudinal change.

The causal hypothesis of quality of life predicts that questionnaire items measuring the construct of problems are more sensitive to change in a clinical trial compared with those measuring the construct of evaluations. This prediction was confirmed in 2 clinical trials of patients with asthma who were administered salmeterol in comparison with another treatment. These and other data show that whether a significant effect is obtained in a clinical trial, and the size of any effect, depend on the properties of the questionnaire used to assess quality of life. Quantitative assessments of improvement in quality of life should be treated with caution in economic evaluations.