Hierarchy of evidence

The hierarchy of evidence (1) is illustrated below:

Systematic Review: A review created after reviewing and combining all the information from both published and unpublished studies (focusing on clinical trials of similar treatments) and then summarizing the findings.

RCT: A study design that randomly assigns participants into an experimental group or a control group. As the study is conducted, the only expected difference between the control and experimental groups is the outcome variable being studied.

Cross-sectional Studies: In a cross-sectional study, data are collected on the entire population at a single point in time to examine the relationship between disease and exposure. Therefore, they provide a snapshot of the frequency of a disease in a population at a given point in time.

Case-control Studies: In a case-control study the study group is defined by the outcome disease, not by exposure to a risk factor. The study starts with the identification of a group of cases (individuals with a particular health outcome) in a given population and a group of controls (individuals without the health outcome) under investigation.

Non-randomised studies: A study different from RCT in a sense that the selection of subjects and controls is not randomised. Therefore, despite the fact of being experimental studies, these feature lower on the evidence hierarchy.

Cohort Studies: Cohort studies evaluate a possible association between exposure and outcome by following a group of exposed individuals over a period of time (often years) to see whether they develop the disease or outcome of interest. A cohort is a group of individuals who share a common characteristic, such as workers from the same factory.

Ecological Studies: Sometimes known as geographical or descriptive studies, can be used to demonstrate patterns of disease and associated factors in a population.

Below is a summary table (2) illustrating the respective pros and cons of each study type in the evidence hierarchy:

References:
1. NHMRC additional levels of evidence and grades for recommendations for developers of guidelines,  National Health and Medical Research Council (Australia), 2009

2. Public Health Action Support Team (PHAST) [Internet] 2011. [cited 2013 June 5] . Available from: http://healthknowledge.org.uk/public-health-textbook/research-methods/1a-epidemiology