Table 1.

NOS scale for observational studies

Study	Selection				Comparability	Outcome assessment			Score
	1	2	3	4	5, 6	7	8	9
D’ Annibale 2013	*	*	*	*	*, *	*	*	*	9
Panteleimonitis 2016	*	*	*	*	*, 0	*	*	*	8
Park 2014	*	*	*	*	*, *	*	*	*	9
Kim 2012	*	*	*	*	*, *	*	*	*	9

Explanation

1: Adequate definition of the cases, study-enrolled cases with independent validation. (yes, *; no or not reported, 0)

2: Representative of the cases, consecutive or obviously representative cases. (yes, *; no or not reported, 0)

3: Selection of controls, community controls. (yes, *; no or not reported, 0)

4: Clear definition of the controls, no previous history of the same procedure. (yes, *; no or not reported, 0)

5: Comparability of cases and controls on the basis of the design or analysis, the patients baseline characteristics were similar between different groups. (yes, *; no or not reported, 0)

6: Comparability of cases and controls for other factors, the same type of procedure, the same surgical team to perform the procedure. (yes, *; no or not reported, 0)

7: Ascertainment of exposure, complete surgical records. (yes, *; no or not reported, 0)

8: Same method of ascertainment for cases and controls. (yes, *; no or not reported, 0)

9: Adequacy of follow up of cohorts (yes, *; no or not reported, 0)