Table 2. Evaluation of risk of bias in the six included studies based on the Cochrane Risk of Bias (ROB) tool[17]

study	sequence generation	allocation sequence concealment	blinding of participants and personnel	blinding of outcome assessment	incomplete outcome data	selective outcome reporting	other potential threats to validity
N/A, no information available
Zhang 2004[28]	high	N/A	high	high	low	low	low
Carrizo 2009[7]	low	N/A	low	low	high	low	low
Liu 2012[25]	high	N/A	high	high	low	low	low
Chen 2013[24]	low	low	low	low	low	low	low
Wu 2014[27]	high	N/A	high	high	low	low	low
Hebrani 2015[26]	low	N/A	low	low	high	low	low