Quality assessment depended on study design. For RCTs, modified and weighted Cochrane Review criteria were used. These comprised a 100-point scale that considered population, interventions, effect and data presentation/analysis. For observational studies, Agency for Healthcare Research and Quality criteria were used. These comprised a 100-point scale that paid special attention to study population, comparability of subjects, intervention exposure, outcome measures, statistical analysis and results. Only studies with a score of 50 or higher were included.
Clinical relevance was evaluated according to recommendations of Cochrane Back Review Group. This scale had five questions that could each be scored positive, negative or unclear.
For each study, level of evidence was determined based on United States Preventive Services Task Force criteria and degree of recommendations on Guyatt's criteria.
Two investigators assessed clinical relevance, level of evidence and degree of recommendations. A third reviewer solved discrepancies.