Bias Khan et al 2012 Cho et al 2006 Hartmann et al 2007 Gordon et al 2007 Koinberg et al 2006 Strauss-Blasche et al 2005 Pinto e Silva  et al 2008 Random sequence generation (selection bias) - + ? + + + ? Allocation concealment (selection bias) - + ? + + + + Blinding of participants and personnel (performance bias) + + + + + + + Blinding of outcome assessments (detection bias) - + + + + + + Incomplete outcome data (attrition bias) - + + + + - + Selective reporting (reporting bias) - - + + + + + Other bias - + + + + + + Study quality rating moderate Very low Very low Very low Very low Very low Very low + = high risk; - = low risk; ? = unclear risk. GRADE = Grades of Recommendation, Assessment, Development and Evaluation

Table 2: Levels of quality of individual studies (GRADE* approach25).