The minimal important difference of

ABSTRACT: Our aim was to determine the minimal important difference (MID) for 6-min walk distance (6MWD) and maximal cycle exercise capacity (MCEC) in patients with severe chronic obstructive pulmonary disease (COPD). 1,218 patients enrolled in the National Emphysema Treatment Trial completed exercise tests before and after 4–6 weeks of pre-trial rehabilitation, and 6 months after randomisation to surgery or medical care. The St George’s Respiratory Questionnaire (domain and total scores) and University of California San Diego Shortness of Breath Questionnaire (total score) served as anchors for anchor-based MID estimates. In order to calculate distribution-based estimates, we used the standard error of measurement, Cohen’s effect size and the empirical rule effect size. Anchor-based estimates for the 6MWD were 18.9 m (95 % CI 18.1–20.1 m), 24.2 m (95 % CI 23.4– 25.4 m), 24.6 m (95 % CI 23.4–25.7 m) and 26.4 m (95 % CI 25.4–27.4 m), which were similar to distribution-based MID estimates of 25.7, 26.8 and 30.6 m. For MCEC, anchor-based estimates fo