Table 2. Quality assessment of included studies.

Item/study	Farrokhi Karibozorg (2018)	Lu (2015)	Menezes (2010)	Mostowska (2012)	Nikopensius (2010)	Rafighdoost (2018)	Xin (2013)
Adequate definition of cases	*	*	*	*	*	*	*
Representativeness of cases	-	-	-	-	-	-	-
Selection of control subjects	-	-	-	-	*	-	-
Definition of control subjects	*	*	*	*	*	*	*
Control for important factor or additional factor	*	-	-	**	-	**	-
Exposure assessment	*	*	*	*	*	*	*
Same method of ascertainment for all subjects	*	*	*	*	*	*	*
Non-response rate	*	*	*	*	*	*	*

A study could be awarded a maximum of one star for each item except for the item ‘Control for important factor or additional factor’.

The definition/explanation of each column of the NOS is available from http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp.