Quality assessment of studies according to Newcastle-Ottawa Scale.

S1: Selection1-is the case definition adequate; S2: Selection2-representativeness of the cases; S3: Selection3-selection of controls; S4: Selection4-definition of controls. C1: Comparability1-comparability of controls for most important factor; C2: Comparability2-comparability of controls for other factors. E1: Exposure1-ascertainment of exposure; E2: Exposure2-same method of ascertainment for cases and controls; E3: Exposure3-non-response rate. Quality assessment of studies according to Newcastle-Ottawa Scale.