Statistical analysis

Statistical analysis will be performed using the SPSS statistical software package (version 18.0; SPSS, Inc., Chicago, IL, USA), and the level of significance will be established at α = 0.05. An independent professional statistician who is blinded to allocation will carry out the data analysis. Intention-to-treat (ITT) analysis will be used as the main analysis, and subordinately we will perform per-protocol (PP) analysis. The ITT population will include all participants who have been treated with at least one dose of the study drug and who record and keep a minimum of 1 day’s cough diary. The PP population will include all the participants of the study who have taken more than 80% of either the allocated investigational drug or the placebo drug and have returned their self-reported cough diary, which is the primary assessment variable, with a minimum of 34 days’ worth of assessment (more than 80% of the total 42 days of data evaluation). The cough diary will be used to compare the cough symptom scores at 6 weeks (visit 4). If the normality test is satisfied, the independent t test will be used; otherwise, the Mann-Whitney U test will be used. However, analysis of covariance will be performed if significant differences in the baseline cough diary values are found between the groups. Intergroup comparisons of cough VAS will be evaluated at weeks 2, 4, and 6. Mean differences in the LCQ-K, GSRS, and HARQ scores will be evaluated at baseline and week 6. Missing values will be replaced by the last observed value of each subject according to the last observation carried forward method. Interim analysis will not be performed for this study.