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).