Modified Newcastle-Ottawa quality assessment scale for case-control studies included in the meta-analysis.

*Selection: (1)Is this case definition adequate? A, yes, with independent validation; B, yes, eg record linkage or based on self reports C, no description. (2) Representativeness of the cases: A, Consecutive or obviously representative series of cases; B, Potential for selection biases or not stated. (3) Selection of controls: A, Community controls; B, Hospital controls; C, No description. (4) Definition of controls: A, No history of disease; B, No description of source. •Comparability: Comparability of cases and controls on the basis of the design or analysis: A, study controls for co-morbidities; B, study controls for any additional factor (e.g., age and severity of illness). 0Exposure: Ascertainment of exposure: A, Secured records; B, Structured interview where blind to case/control status; C, Interview not blinded to case/control status; D, written self report or medical record only. Same method of ascertainment for cases and controls; A, yes; B, no. Non-response rate: A, Same for both groups; B, Non-respondents described; C, Rate different and no designation.