Table 2.

Quality assessment of included studies according to the Newcastle-Ottawa Scale

Item/study	Alshamrani et al37 2017	Chia et al 30 2003	Huang et al 29 2018	Kim et al 52 2016	Vehof et al36 2014	Wang et al35 2012
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 Newcastle-Ottawa Scale is available from (http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp).

The NOS is used to assess the methodological quality of included studies using a “star” system. 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 “*” and “**” in table 2 actually stand for the number of stars each study can get.