Table 2.

Risk of bias of the included observational studies (9-item Cochrane checklist for observational studies)

	Groups well defined	Selection bias	Exposure	Outcome	Blinding	Follow-up	Loss to follow up	Confounding	Generalizability
Angeli 1996 [20] 6/9	+	-	+	+	-	+	+	-	+
Baskol 2004 [21] 6/9	+	+	+	+	-	?	?	+	+
Garrison 2015 [22] 9/9	+	+	+	+	+	+	+	+	+
Garrison 2012 [2] 6/9	+	-	-	+	+	+	+	-	+
Hawke 2013 [4] 9/9	+	+	+	+	+	+	+	+	+
Hawke 2013 [23] 9/9	+	+	+	+	+	+	+	+	+
Hirai 2000 [24] 4/9	+	-	+	+	-	?	?	-	+
Naylor 1994 [25] 5/9	+	+	+	-	+	?	?	?	+
Nishant 2014 [26] 5/9	+	-	+	+	-	+	-	-	+
Oboler 1991 [27] 4/9	+	-	+	+	-	?	?	-	+

Baskol 2004 [21], Garrison 2015 [22], Hawke 2013 [4] and Hawke 2013 [24] showed low risk of bias