Some study designs present higher level evidence than others. Below is the University of Illinois at Chicago's Pyramid of Levels of Evidence. In this representation, the quality of evidence increases as you move up the pyramid.
The University of Oxford's Centre for Evidence-Based Medicine offers a more detailed framework for levels of evidence, based on the type of clinical question to be answered.
A report on a series of patients with an outcome of interest. No control group is involved.
A study which involves identifying patients who have the outcome of interest (cases) and patients without the same outcome (controls), and looking back to see if they had the exposure of interest.
The observation of a defined population at a single point in time or time interval. Exposure and outcome are determined simultaneously.
Involves identification of two groups (cohorts) of patients, one which received the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.
Randomized Controlled Trial
Participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest.
A summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies and that uses appropriate statistical techniques to combine these valid studies.
A systematic review that uses quantitative methods to synthesize and summarize the results.
Glossary of EBM Terms from Evidence-Based Medicine Toolbox: https://ebm-tools.knowledgetranslation.net/resource/glossary