<qa>  What is the primary data analysis?Data analysis. The primary data analysis will be performed on the basis of the intention-to-treat principle. The investigators compare baseline data for the two groups with Students t-test, chi-square test or non-parametric tests. A large proportion of missing values for the outcome variable is expected due to drop out and non-response to the questionnaire. Data will be assumed Missing At Random MAR and therefore observed patient characteristics will be used to impute missing data, by means of multiple imputation. 