Table 1.

Quality assessment of six case-control studies.

References	Selection				Comparability	Exposure			Overall score
	Is the case definition adequate?	Representativeness of the cases	Selection of controls	Definition of Controls	Comparability of cohorts on the basis of the design or analysis	Ascertainment of exposure	Same method of ascertainment for cases and controls	Non-response rate
Emamat et al. (24)	1	1	1	1	2	-	1	-	7
Giraldi et al. (32)	1	1	1	1	2	-	1	-	7
Noureddin et al. (29)	1	1	1	1	2	-	1	1	8
Tutunchi et al. (30)	1	1	1	1	2	-	1	1	8
Guo et al. (26)	1	1	1	1	2	-	1	1	8