1. Departments of Surgery, University Medical Centre Groningen, University of Groningen, Groningen, The Netherlands
2. Department of Surgery, Groene Hart Ziekenhuis, Gouda, The Netherlands
3. Departments of Epidemiology, University Medical Centre Groningen, University of Groningen, Groningen, The Netherlands
4. Nuffield Department of Surgical Sciences, University of Oxford, Oxford, UK
5. Westfriesgasthuis, Hoorn, The Netherlands
6. Academic Medical Centre, Amsterdam, The Netherlands
7. Elkerliek Ziekenhuis, Helmond, The Netherlands
8. Streekziekenhuis Koningin Beatrix, Winterswijk, The Netherlands
9. Röpcke-Zweers Ziekenhuis, Hardenberg, The Netherlands
10. Deventer Ziekenhuis, Deventer, The Netherlands
11. Albert Schweitzer Hospital, Dordrecht, The Netherlands
12. Rijnstate Hospital, Arnhem, The Netherlands
13. Radboud University, Nijmegen Medical Centre, The Netherlands
14. Nij Smellinghe Hospital, Drachten, The Netherlands
15. Medical Centre, Leeuwarden, The Netherlands
16. Ommelander Ziekenhuis Group, Winschoten, The Netherlands
17. Beatrix Hospital, Gorinchem, The Netherlands
18. Gelderse Vallei Hospital, Ede, The Netherlands
19. Diaconessen Hospital, Meppel, The Netherlands
20. Laurentius Hospital, Roermond, The Netherlands
21. St Jansdal Hospital, Harderwijk, The Netherlands
22. Rijnland Hospital, Leiderdorp, The Netherlands
23. Bethesda Hospital, Hoogeveen, The Netherlands
24. Scheper Hospital, Emmen, The Netherlands
25. Wilhelmina Hospital, Assen, The Netherlands
26. Antonius Hospital, Sneek, The Netherlands
27. Canisius Wilhelmina Hospital, Nijmegen, The Netherlands
28. Tjongerschans Hospital, Heerenveen, The Netherlands
29. Slingeland Hospital, Doetinchem, The Netherlands
30. Diakonessenhuis, Utrecht, The Netherlands
31. Leiden University Medical Centre, The Netherlands
32. Reinier de Graaf Hospital, Delft, The Netherlands
33. Meander Medical Centre, Amersfoort, The Netherlands
34. Isala Hospital, Zwolle, The Netherlands
35. Szabolcs-Szatmár-Bereg County Hospitals, Jósa András University Teaching Hospital, Nyíregyháza, Hungary
36. Flór Ferenc Hospital of County Pest, Kistarcsa, Hungary
37. Szent Imre Hospital, Budapest, Hungary
38. Semmelweis University, First Department of surgery, Budapest, Hungary
39. Semmelweis University, Second Department of Surgery, Budapest, Hungary
40. University Medical Centre Mannheim, University of Heidelberg, Germany
41. Klinikum Rechts der Isar, Technische Universität München, Germany
42. University Medical Centre Hamburg–Eppendorf, Germany
43. Claude Huriez University Hospital, Lille, France
44. Hospital Universitario Fundación Alcorcón, Madrid, Spain