Table 2. Assessment of methodological quality of the selected papers.

Item	Yes	No	NA
Was an a priori design provided?	27/27
Was there duplicate study selection and data extraction?			27/27
Was a comprehensive literature search performed?			27/27
Was the status of publication (i.e., grey literature) used as an inclusion criterion?			27/27
Was a list of studies (included and excluded) provided?			27/27
Were the characteristics of the included studies provided?			27/27
Was the scientific quality of the included studies assessed and documented?			27/27
Was the scientific quality of the included studies used appropriately in formulating conclusions?			27/27
Were the methods used to combine the findings of studies appropriate?			27/27
Was the likelihood of publication bias assessed?		27/27
Was the conflict of interest stated?	7/27	20/27

NA, not applicable.