Table 1. Criteria for assessing risk of bias.

Risk of bias	Criteria
Low	Predetermined research question and inclusion criteria established (AMSTAR Question 1)
	At least two independent data extractors and consensus procedure reported (AMSTAR Question 2)
	At least the database MEDLINE/Pubmed used. Search strategy reported so that it can be repeated (AMSTAR Question 3)
	A list of included and excluded studies reported (AMSTAR Question 5)
	Relevant characteristics of included studies reported (AMSTAR Question 6)
	Assessment of the overall scientific quality of each included study provided (AMSTAR Question 7)
	The scientific quality of included studies used appropriately in formulating conclusions (AMSTAR Question 8)
	The rationale for combining/not combining results reported. Methods for pooling results reported (AMSTAR Question 9)
	Likely publication bias reported. This item can be omitted if publication bias was unlikely but not reported (AMSTAR Question 10)
	Any conflict of interest reported. This item can be omitted if conflicts of interest were unlikely (AMSTAR Question 11)
Moderate	A yes-answer to questions 1, 2 and 5–8 *.
High	A no-answer to any of the question listed under moderate risk of bias.

Pre-specified criteria of low, moderate and high risk of bias. Modified list of questions based on AMSTAR [5].

*List of included studies is mandatory; list of excluded studies can be absent.