Patients should be provided the best care with medicine based on evidence. Clinical studies are the basis for forming this evidence. They are classified into three categories: interventional studies, observational studies, and systematic reviews that integrate the results of these studies. The level of evidence is determined by the study design, and the more reliable results are produced by studies of the higher level. However, even if the level of evidence is high, studies with concerns about the research methods themselves may not produce results that reflect the truth which should be revealed. We need to pay attention not only to the level of evidence but also to its quality. The quality of evidence is evaluated using tools like Risk of Bias 2.0 for randomized controlled trials and ROBINS-E for observational studies. Medical researchers are expected to conduct high-quality studies and publish the results in journals, while medical professionals who use evidence in their practice are expected to interpret research papers carefully, considering both the level and quality of evidence.