Statistical analysis
RECRUITMENT
Normally distributed continuous variables are presented as mean (95 per cent c.i.) and the independent The non-inferiority limit was calculated on the maximal acceptable loss of HRQoL defined by VEINES-QOL/Sym scores and was estimated at 5 per cent. Assuming that there was no difference between the standard TAP and SAP treatments, a total of 472 patients was required to achieve a power of 80 per cent with a one-sided α-level of 5 per cent. With an expected loss to follow-up of 10 per cent, a sample size of 260 patients per group was set. Owing to a much longer than expected recruitment period (4 instead of 2 years) and consequent long delay in presentation of the data, this sample size was reconsidered. At this point, 480 patients had already been included in the study, meaning that the sample size was already large enough to achieve 80 per cent power. Furthermore, there was a relatively low percentage of loss to follow-up. Therefore, it was decided by the research group that preemptive closure of the recruitment period and thus lowering the sample size was acceptable at this stage, and would not result in loss of statistical power.Statistical analysis was undertaken using SPSSVEINES-QOL/Sym scores and patient satisfaction were analysed in one multilevel repeated-measures model. The model took into account that measurements within patients could be correlated and did not apply constraints to the structure of the co-variance matrix. The explanatory variables were time (measurement at baseline (reference category), 3 months, and 1 year) and interactions of treatment and time. The estimates of the treatment effects after 3 months and 1 year were represented by the coefficients of these interaction terms. The 95 per cent confidence intervals were calculated using bootstrapping. Missing data in the primary outcome (VEINES-QOL/Sym) and in patient satisfaction outcomes were handled by the multilevel repeated-measure models.