Table 1.

Quality assessment according to the Newcastle-Ottawa Quality Assessment scale

Author	Year of publication	Case Definition Adequate	Representativeness Cases	Controls Selection	Controls Definition	Design and Analysis	Outcome Assessment	Equal Ascertainment Cases and Controls	Non-Response Rate	Total Stars/Total Possible Stars	Quality
Dudda et al	2010	+	+	−	+	++	+	+	−	7/9	Good
Fujishiro et al	2016	+	+	−	−	+	+	+	−	5/9	Fair
Jolles et al	2002	+	+	−	+	++	+	+	−	7/9	Good
Kawarai et al	2017	+	-	−	?	+	-	+	−	3/9	Poor
Komeno et al	2006	+	+	−	+	++	+	+	−	7/9	Good
Min et al	2008	+	+	−	+	++	+	+	−	7/9	Good
Nishii et al	2004	+	+	−	+	++	+	+	−	7/9	Good
Pierchon et al	1994	+	+	−	−	+	+	+	−	5/9	Fair
Reina et al	2017	+	+	−	+	++	+	+	−	7/9	Good
Vrelisovic et al	1994	+	-	?	?	+	?	?	?	2/9	Poor
Yoshitani et al	2018	+	+	−	+	++	+	+	−	7/9	Good

The two studies with ‘poor’ quality were excluded for further analyses