Strength of evidence5
Category A	Category A evidence is based on data derived from meta-analyses of randomized controlled trials with homogeneity about the directions and degrees of results between individual studies. Multiple, well-done randomized clinical trials involving large numbers of patients.
Category B	Category B evidence is based on data derived from meta-analyses of randomized controlled trials with conflicting conclusions regarding the directions and degrees of results between individual studies. Randomized controlled trials that involved small numbers of patients or had significant methodological flaws (e.g., bias, drop-out rate, and flawed analysis). Nonrandomized studies (e.g., cohort studies, case–control studies, and observational studies).
Category C	Category C evidence is based on data derived from expert opinion or consensus, case reports, or case series.
Category D	No evidence

Note: Adapted from MICROMEDEX SOLUTION 2017 version (Healthcare Series Thomson Reuter, Greenwood, CO).⁵