According to WHO handbook for Guidelines we used the GRADE (Grading of Recommendations, Assessment, Development and Evaluation) approach to assess the quality of a body of evidence, develop and report recommendations. GRADE methods are used by WHO because these represent internationally agreed standards for making transparent recommendations. Detailed information on GRADE is available through the on the following sites: 

Table 1: Quality of evidence in GRADE

Table 2: Significance of the four levels of evidence 

Table 3: Factors that determine how to upgrade or downgrade the quality of evidence.