Frequency of exacerbation events during the 12-month study period (from first recorded MRC dyspnoea grade from study start).

ap-value estimated, using negative binomial regression, for a distribution of exacerbations between patients with a specific grade (MRC = 1, no dyspnoea is a referent category).Abbreviation: MRC, Medical Research Council.采用负二项回归（negative binomial regression），针对MRC（Medical Research Council）分级为1级且以无呼吸困难者为参照组的患者群体的急性加重分布情况，估算ap值。缩写说明：MRC即医学研究理事会（Medical Research Council）。