The GRADE system categorizes the quality of evidence into four levels: high, moderate, low, and very low. This classification is based on several factors, including study design, consistency of results, directness of the evidence, precision of estimates, and the risk of bias. Randomized controlled trials (RCTs) typically start as high-quality evidence but can be downgraded based on these factors. Conversely, observational studies usually start as low-quality evidence but can be upgraded if they meet certain criteria.
