Category A: randomized controlled trials reporting comparative results for specific outcomes between different interventions.
Level 1: meta-analysis of RCTs
Level 2: Multiple RCTs of which a quantitative synthesis could not be conducted.
Level 3: Single RCT.
Category B: observational studies or trials without precise comparison groups, which may allow for an inference with respect to the relationship between interventions and observed outcomes.
Level 1: non-randomized comparative studies (quasi-experimental, cohort studies, case-control).
Level 2: non-comparative observational studies with measures of association (relative risk, correlation, sensitivity, and specificity).
Level 3: non-comparative observational studies with descriptive measures (frequencies, proportions).
Level 4: case reports and case series.