CUSUM analysis of postoperative complications. Each point represents a binary outcome (presence or absence of complication), with deviations from a predefined reference value (6%) cumulatively summed. The initial rising curve up to case 42 reflects a higher-than-expected complication rate during the learning phase. The inflection point at case 42 (red dot and dashed line) marks the transition to improved surgical performance. After this point, the consistent downward trend indicates reduced complication rates.