A comprehensive literature search in PubMed, Excerpta Medica Database, and China National Knowledge Infrastructure was carried out to get case-control studies published up to November 2013. 
PubMed
A comprehensive literature search in PubMed, Excerpta Medica Database, and China National Knowledge Infrastructure was carried out to get case-control studies published up to November 2013. 
Excerpta Medica Database
A comprehensive literature search in PubMed, Excerpta Medica Database, and China National Knowledge Infrastructure was carried out to get case-control studies published up to November 2013. 
China National Knowledge Infrastructure