Table 2.

Quality assessment of included studies.

Author	Selection				Comparability		Exposure			Summary
	Is the case definition adequate?	Representativeness of the cases	Selection of controls	Definition of controls	Ethnicity	Age	Ascertainment of exposure	Same method of ascertainment for cases and controls	Non-response rate
Fortunato et al.										4.1.3
Liu et al.										4.2.3
Li et al.										3.2.3