Statistical methods
Statistical analysis was performed with a multi-level logistic regression model for patency when data for patency was given in 2-by-2 tables stratified by type of technique (on-pump or off-pump) and harvesting technique. The main predictor variable for patency was the two different techniques, on- or off-pump. Most patients had more than one graft, with a maximum number of three grafts per patient. Multiple grafts per patient might introduce dependencies between the observations. We corrected for clustered observations in the calculation of confidence intervals (CIs), with each patient forming a separate cluster.
With the aim of investigating a possible equivalence between on- and off-pump, the graphical approach recommended by Fleming11 and by Blackwelder 12 was used. An essential point in this approach is to define, based on experience, limits of what can be regarded as equivalent outcomes with respect to patency. Therefore, no formal statistical hypothesis testing was conducted, and instead results are shown in graphics with CIs for the differences in patency rates between the two techniques. Through these CIs, possible equivalences between on- and off-pump can be visualised. When the CI does not cover the point of no difference in patency between the methods (the ‘0-point’), equivalence between the techniques is rejected. If, on the other hand, the CI covers the ‘0-point’ and in addition the CI does not stretch outside the equivalence limits there is an evidence for equivalence. Finally, if the ‘0-point’ is covered by the CI but at least one of the two limits of CI stretches outside the equivalence limits the result is inclusive, possibly due to lack of power. Computations were performed with STATA version 15 (www.stata.com).