TABLE 1.

The Newcastle–Ottawa scale (NOS) of studies included.

Study	Selection				Comparability		Outcome assessment			Total score
	1	2	3	4	1	2	1	2	3
Barakat et al. (23)	+	+	+	+			+	+	+	7
Mercier et al. (11)	+		+		+		+	+	+	6
Lin et al. (24)	+	+	+	+			+	+	+	7
Asenov et al. (25)	+	+	+		+		+	+		6
Harputluoglu et al. (26)	+	+	+	+	+		+	+	+	8
Wen et al. (27)	+	+	+	+			+	+	+	7
Zeng et al. (28)	+	+	+		+		+	+	+	7
Kohoutova et al. (29)	+	+	+				+	+	+	6
Czubkowski et al. (13)	+		+	+	+		+	+	+	7
Rosen et al. (30)	+	+	+	+	+		+	+	+	8
Giefer and Kozarek (31)	+	+	+		+	+	+	+	+	8
Dechêne et al. (32)	+	+	+		+		+		+	5
Kargl et al. (33)	+	+	+				+	+	+	6
Agarwal et al. (34)	+	+	+		+		+	+	+	7
Oracz et al. (35)	+	+	+				+	+	+	6
Steen et al. (36)	+	+	+		+		+		+	6
Tsuchiya et al. (16)	+	+	+				+			5
Limketkai et al. (37)	+	+	+		+		+	+		6
Enestvedt et al. (38)	+	+	+				+	+	+	6
Otto et al. (39)	+	+	+	+	+		+		+	7
Berquist (40)	+	+	+		+		+			5
Otto et al. (41)	+	+	+	+			+	+	+	7
Jang et al. (42)	+	+	+		+		+	+	+	7
Li et al. (18)	+	+	+	+	+		+	+	+	8
Vegting et al. (43)	+	+	+		+		+		+	6
Issa et al. (17)	+	+	+				+	+	+	6
Rocca et al. (1)	+	+	+		+		+	+	+	7
Cheng et al. (44)	+	+	+	+	+		+	+	+	8
Varadarajulu et al. (45)	+	+	+	+			+	+	+	7
Pfau et al. (46)	+	+	+				+	+	+	6
Poddar et al. (47)	+	+	+		+		+	+	+	7
Hsu et al. (48)	+	+	+	+			+	+	+	7
Guelrud et al. (49)	+	+	+				+	+	+	6

Selection:

1: Representativeness of the exposed cohort.

2: Selection of the non-exposed cohort.

3: Ascertainment of exposure.

4: Demonstration that outcome of interest was not present at start of study.

Outcome assessment:

1: Assessment of outcome.

2: Follow-up long enough for outcomes to occur.

3: Adequacy of follow up of cohorts.

Comparability: study controls for the most important factor or any additional factor.