Sample size and data analysis
This study is powered for measuring the primary outcome self-reported adherence. In order to calculate the sample size the Cochran formula was used. Assuming a percentage of patients with good adherence of 69% [7], a 10% accepted margin and a 95% confidence interval, the minimal sample size was calculated for 83 patients. Taking into account at least 10% percent non completion of the questionnaire, a convenient sample of 100 patients was strived for.
Analysis of associations between adherence, medication beliefs and side effects is for explorative purposes only. All data were analysed using STATA version 13. Descriptive statistics were provided using mean (± SD) or median (p25-p75) values depending on the (non-)parametric distribution of measured variables. P-values ≤0.05 were considered statistically significant. Differences between groups were calculated with Pearson’s chi-square test, t-test or regression testing depending on the variables (dichotomous or continuous).