Table 3.

Case Control Critical Appraisal

	Clement DB. 1981 [69]	Ellapen TJ. 2013 [70]	Rasmussen CH. 2013 [71]
Were the groups comparable other than the presence of disease in cases or the absence of disease in controls?	Not applicable	Not applicable	Not applicable
Were cases and controls matched appropriately?	Not applicable	Not applicable	Not applicable
Were the same criteria used for identification of cases and controls?	Not applicable	Not applicable	Not applicable
Was exposure measured in a standard, valid and reliable way?	Unclear	Yes	Yes
Was exposure measured in the same way for cases and controls?	Not applicable	Not applicable	Not applicable
Were confounding factors identified?	Yes	Yes	Yes
Were strategies to deal with confounding factors stated?	Yes	Yes	Yes
Were outcomes assessed in a standard, valid and reliable way for cases and controls?	Yes	Yes	Yes
Was the exposure period of interest long enough to be meaningful?	Yes	Yes	Yes
Was appropriate statistical analysis used?	Unclear	Yes	Yes