The EVIDENCE PYRAMID is often used to illustrate the development of evidence. At the base of the pyramid is animal research and laboratory studies. In some schemes, Expert Opinion is substututed or included at this level. - this is where ideas are first developed. As you progress up the pyramid the amount of information available decreases in volume, but increases in relevance to the clinical setting.
Meta Analysis - systematic review that uses quantitative methods to synthesize and summarize the results.
Systematic Review - 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.
Randomised Controlled Trial - Participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest.
Cohort Study - 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.
Case Control Study - 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.
Case Series - report on a series of patients with an outcome of interest. No control group is involved. (Definitions from CEBM)
The evidence pyramid above is just one of a number of "Evidence Levels" schemes. Other influential example include:
Joanna Briggs Institute - Levels of Evidence - One of four levels of evidence are assigned to all conclusions drawn in JBI Systematic Reviews.
Centre for Evidence Based Medicine (UK) - Levels of Evidence (March 2009)