Table 2. Risk of bias using Cochrane risk assessment tool for RCT.

Bias	Selection		Performance	Attrition	Outcome assessment	Reporting
Authors(year)	Random sequence generation	allocation sequence concealment	blinding	incomplete outcome data	blinding	selective reporting
Pabuccu et al.(2004)	low risk	low risk	high risk	low risk	low risk	high risk
Demirol et al.(2006)	low risk	low risk	high risk	low risk	low risk	high risk
Pabuccu et al.(2007)	low risk	low risk	low risk	low risk	low risk	high risk