Table 3. Quality assessment of included studies.

Item/Study	Canbek, 2016	Chen, 2015	Gu, 2013	Jiang, 2017	Liu, 2016	Rajasekara, 2015	Rodriguez, 2008	Wu, 2014
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).