Hierarchy of Evidence

Each level of evidence forms a pyramid which has the strongest evidence at its apex (level I) with the weakest at its base.

Types of evidence:

Systemic review or meta-analysis is a synthesis of randomized controlled trials which uses rigorous guidelines to make recommendations based on the available evidence (Cochrane Database of Systematic Reviews is an example).

Randomized, double-blinded control trial is a research study that randomly assigns subjects to the treatment group or control group and both the subjects and the researchers do not know which group the subjects are placed in.

Non-randomized control trial is a research study in which the subjects are assigned to a treatment group or control group without randomization.

Case-control study is a comparison between subjects that have a condition or disease with other subjects that do not have the condition or disease to determine symptoms of the disease.

Cohort study is a research study that links the cause and risk factors of a disease. 

 Qualitative  study is a research study that studies behavior and tries to explain opinions, experiences and decision-making.

Descriptive study is an observational study which describes background information on a topic in relation to variables such as person, place and time.