The need to measure the impact of treatments on health related quality of life has led to a rapid increase in the variety of instruments available and in their use as measures of outcome in clinical trials. One limitation of instruments that purport to measure health related quality of life is difficulty interpreting their results. In the past decade, investigators have progressed in making these questionnaire results interpretable. For example, we have shown that when questionnaires present response options in the form of seven point scales with verbal descriptions for each option (see box), the smallest difference that patients consider important is often approximately 0.5 per question. A moderate difference corresponds to a change of approximately 1.0 per question, and changes of greater than 1.5 can be considered large. Thus, for example, in a domain with four items, patients will consider a 1 point change in two or more items as important. This finding applies across different areas of function, including dyspnoea, fatigue, and emotional function in patients with chronic airflow limitation1; and symptoms, emotional function, and activity limitations in adults2 and children3 with asthma, parents of children with asthma,4 and adults with rhinoconjunctivitis.5 Initially, we used comparisons in the same patient to establish this difference, but more recently we have replicated this finding using differences between patients.6