Primary analysis
fatigue
First, to determine whether there was an effect of boosters, the primary outcome was analysed using a linear mixed model analysis, including CIS-fatigue (at both T39 and T52) as the dependent variable, booster (yes/no) and the CIS-fatigue at T20 as (fixed) covariates. To determine the effect of boosters on fatigue severity compared with no boosters at T39 and T52 separately the linear mixed model analysis was extended with time and boosters by time interaction as (fixed) covariates.All mixed model analyses were conducted with four levels: measurements were clustered within patients, patients were clustered within therapists and therapists were clustered within treatment centres.The analysis was repeated with the initial treatment condition in the non-inferiority RCT (F2F or blended CBT), and the interactions between initial treatment condition and booster condition, to investigate whether effectiveness of booster sessions differ between face-to-face CBT and blended CBT.In addition, to determine the course of fatigue severity post-treatment for the whole sample, a linear mixed model analysis was conducted, including CIS fatigue (at T20, T39 and T52) as the dependent variable, and time as (fixed) covariates. In case of a significant effect of boosters in the primary analysis, booster condition was added in the analysis.