Learning curve
The RA-CUSUM chart is displayed in Figure 1 . The RA-CUSUM+ chart shows a peak at 61 procedures, where after a consistent decrease is observed and the RA-CUSUM+ chart moves towards zero indicating satisfactory results with respect to the predicted recurrence rates. Hence, the first 61 procedures comprise the learning phase for the surgical team. In the 20 procedures thereafter, representing the first part of the experienced phase, surgical performance is still improving as the chart moves further negatively, indicating a decrease of surgical failure as presented by the RA-CUSUM- chart. From procedure 81 onwards the RA-CUSUM- chart stabilises, indicating the plateau of the learning curve has been reached.
As a sensitivity analysis, RA-CUSUM plots were constructed for the different risk models as described earlier, all showing an identical learning phase of 61 procedures.
Based on the RA-CUSUM plot the patients were divided into two groups: group 1 (the learning phase; procedure 1-61) and group 2 (the experienced phase; procedure 62-165).