Levels of evidence are a way to designate the strength of studies based on the methodological quality of their design, validity, and applicability to patient care. The lowest level is at the bottom of the pyramid, the highest is on the top. It is important to use the highest level of evidence that you can for a systematic review, recognizing that for some topics, you may need to use lower quality studies.
National Health and Medical Research Council has created a guide to help researchers perform their search. How to Use the Evidence: Assessment and Application of Scientific Evidence
The Joanna Briggs Institute offers a guide to their Levels of Evidence based on the research design of the original experiment.
Centre for Evidence Based Medicine (UK) uses another system for rating evidence used in a systematic review- Levels of Evidence (March 2009)
Meta Analysis: A systematic review that uses quantitative methods to synthesize & summarize results.
Systematic Review: A summary of the medical literature that uses explicit methods to perform a comprehensive literature search & critical appraisal of individual studies & that uses appropriate statistical techniques to combine these valid studies
Randomized Controlled Trial: Participants are randomly allocated into experimental or control groups & are followed over time for the variables/outcomes of interest. Clinicians & laboratory workers may also be blinded to participant groups.
Cohort Study: Identifies participants who currently have a certain condition or receive a particular treatment are followed over time & compared with another group of people who are not affected by the condition.
Case Control Study: Identifies participants who have a certain outcome (cases) & participants without that outcome (controls). Investigators look back over time to see if they had the specific exposure.
Case Report/Case Series: A report on one or more participants with a particular outcome. No control group is involved.