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The pyramidal shape qualitatively integrates the amount of evidence generally available from each type of study design and the strength of evidence expected from indicated designs. Study designs in ascending levels of the pyramid generally exhibit increased quality of evidence and reduced risk of bias.
Understand the different levels of evidence
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)