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).