eTable 1. Definition of patient population and stratification variables.

	Inclusion criteria	Exclusion criteria
Patient population
All inpatient cases with appendectomy as sole intervention for acute appendicitis	PD ICD-10 K35, K36, K37 and OPS 5-470, 5-455.3	OPS 5-471, 5-479
Severity
Uncomplicated appendicitis	PD ICD-10 K35.30, K35.8, K36, K37
Complicated appendicitis	PD ICD-10 K35.2, K35.31, K35.32
Surgical approach
Open	OPS 5-470.0, 5-455.31	OPS 5-470.2, 5-455.37
Laparoscopic	OPS 5-470.1, 5-455.35	OPS 5-470.0, 5-470.2, 5-455.31, 5-455.37
Conversion	OPS 5-470.2, 5-455.37
Other or undefined	OPS 5-470.x, 5-470.y	OPS 5-470.0, 5-470.1, 5-470.2, 5-455.31, 5-455.35, 5-455.37
Type of surgery
Appendectomy	OPS 5-470	OPS 5-455.3
Cecal resection	OPS 5-455.3
Indicators of complicated course
Septicemia	SD ICD-10 A40, A41, R57.2, R65
Blood transfusions, ≥ 6 units	OPS 8-800.1, 8-800.c1-cr
Postoperative ileus	SD ICD-10 K91.3
Mechanical ventilation > 24 h	Mechanical ventilation for > 24 h (separate data field)
Complex intensive care	OPS 8-980, 8-98d, 8-98f (from 2013)

ICD, International Statistical Classification of Diseases; OPS, German classification for operations and procedures (Operationen- und Prozedurenschlüssel); PD, principal diagnosis; SD, secondary diagnosis