Risk of bias
The quality of the evidence was classified as high, moderate, low or very low, according to the risk of bias in the body of evidence, openness of comparisons, accuracy and consistency in the effects of treatment. High quality of evidence has been attributed to well-designed randomized controlled trials (RCTs) with consistent results. The quality of the evidence was downgraded to moderate if 1 of the 4 criteria for the quality of the evidence had not been met and lower if 2 or more were not met. A low quality of evidence was attributed to non-randomized studies. The Cochrane instrument was be adopted to assess the quality of the included studies [12].