Data extraction
Two reviewers independently screened all the titles and
abstracts of the retrieved studies from the searches, and
then independently assessed the full articles of selected
trials, which were possibly eligible, to confirm whether
the studies met the inclusion criteria. The results were
checked and discussed by two reviewers, to agree upon a
final list of included studies. Using a structured data collection
form, all relevant data in each included paper
were extracted by two reviewers independently, and all
the data extracted were cross-checked.