The Evidence Pyramid

The evidence pyramid helps provide context to the types of evidence a researcher or practitioner might find in the evidence-gathering process.   The "best" quality evidence is at the topic of the pyramid with the "lowest" quality evidence at the bottom.  The position of each type of evidence is based on the general reliability of the evidence. 

This pyramid is not a replacement for evaluating an article or study you find.  The worksheets below will help you do this.

The evidence from top to bottom

Secondary or Pre-filtered Sources (also known as summary sources):

Clinical Practice Guidelines: “Clinical practice guidelines are systematically developed statements to assist practitioner and patient make decisions about appropriate health care for specific clinical circumstances.”(Institute of Medicine, 1990) They usually involve a systematic search of the evidence, an evaluation of the evidence, and experts making recommendations based on the evidence. 

Systematic Reviews and Meta-Analyses: A systematic collection and review of empirical research to answer a specific question.  A meta-analyses which combines the statistical results from multiple studies. 

Primary Sources: 

Randomized Controlled Trial (RCT): A scientific study that attempts to control all variables.  This is done through randomized allocation and blinding. 

Primary Sources-Observational Studies:

Cohort Studies: "Cohort studies are a type of longitudinal study—an approach that follows research participants over a period of time (often many years). Specifically, cohort studies recruit and follow participants who share a common characteristic, such as a particular occupation or demographic similarity."  BMJ Journals

Case Control Studies: They're also known as retrospective studies.  Researchers look at those who have a disease and those who don't to identify factors which may be causing the disease.  

No design:
Case Report (or case study) and Case Series: A case report is the description of a patient or a small group of patients with a similar condition, often in a clinical setting.  A case series is a grouping of similar case reports. 

Non-Human Studies:

Animal and Laboratory Studies:  These studies are used in the development of drugs and to evaluate safety.  However, animal studies are poor predictors of results in humans. 

Further Reading:

Clinical Practice Guidelines
Systematic Reviews and Meta-Analyses
Randomized Controlled Trials
Case Control vs. Cohort Studies
Case Reports and Case Series
 

An understanding of the hierarchy of evidence is not a replacement for evaluating the research you find.  Here are some sheets that ask questions to help you evaluate the quality of the evidence you find.