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Evidence-Based Practice: Levels of Evidence

Levels of Evidence - Secondary and Primary

There are two broad types of evidence: secondary and primary.

We list secondary first because in Evidence-Based Practice it is the higher level of evidence and will probably be what you seek first in answering a clinical or research question.

Secondary evidence (filtered, pre-appraised)

This is evidence assimilated, or put together, from a number of quality primary studies on a topic. It includes systematic reviews, meta-analyses, and evidence summaries.

This is sometimes referred to as filtered or pre-appraised evidence.

You can find it in specialised EBP sources such as The Cochrane Library (notably in the Cochrane Database of Systematic Reviews) and JBI COnNECT+, and in health and medical databases such MEDLINE, PubMed and CINAHL Plus with Full Text. See the next page, Where to Search for Evidence, for details of where to search for secondary, filtered, pre-appraised evidence.


Primary evidence (unfiltered)

This body of evidence consists of original individual studies such as controlled trials, cohort studies, and case studies.

This is sometimes referred to as unfiltered evidence.

You can find primary studies in specialised EBP resources such as The Cochrane Library, notably in the Central Register of Controlled Trials, and in health and medical databases. See the next page for details of where to search for primary unfiltered evidence.

Levels of Evidence - What's the Best Evidence?

Haynes' hierachy of evidenceOne system that is often used as a guide to the different levels of evidence is Brian Haynes' six-level pyramid, also known as the 6S pyramid.

The quality of evidence tends to be higher at the top of the pyramid. When researching a clinical question, the idea is to start at the top to see if the body of evidence has already been appraised, then work down the pyramid through the different levels of secondary pre-appraised research, with primary single studies as the final resort.

Source: NSW Ministry of Health, CIAP

The different levels are:


System Known as a Computerised Decision Support System (CDSS), this matches information from individual patients with the best available evidence. Futuristic!?
Synopsis Integrates the best available evidence from lower pre-appraised levels of the hierarchy (especially from syntheses/systematic reviews) to provide evidence for the management of a given health problem. This level includes Clinical Practice Guidelines (CPGs).
Synopsis of synthesis Summarises the findings of a high-quality systematic review.
Synthesis Better known as a systematic review, this brings together and distils the best evidence to answer a clinical question. Generally, this will pool the results of several RCTs or meta-analyses on the same clinical problem. CATs (Critically Appraised Topics) are a form of systematic review: for more details, see the CATs page in the Library’s guide on Systematic and Systematic-Like Reviews.
Synopis of study Provides a brief, but sometimes quite detailed, summary of an individual, high quality, clinically relevant study.
Single study An individual study which constitutes original primary research.

Primary sources for evidence-based research

When you cannot find any higher level evidence you need to look for primary research - original individual studies of various types - that can be found in databases and registers of controlled trials. Remember that the best study type might vary according to the nature of the clinical question (refer back to Introduction and PICO).

Source: NSW Ministry of Health, CIAP

The different types of studies are:

RCT A carefully planned experiment that studies the effect of therapy on real patients. Includes methodologies (randomisation and blinding) that reduce bias, and that allow for comparison between an intervention group and a control group (no intervention). Can provide sound evidence for cause and effect.
Cohort Study Follows patients who have a particular condition or receive a particular treatment over time, and compares them with another group who have not been affected by the condition or treatment being studied. Not as reliable as an RCT since the two groups might differ in ways other than the variable being studied. 
Case Control Study A study in which people who already have a condition are compared with those who do not. The researcher looks back over time to identify factors that might be associated with the condition. Often relies on medical data or patient recall, and is less reliable than an RCT or cohort study because cause and effect is not necessarily established.
Cross Sectional Study A study where a disease or condition and other related factors are measured at a specific point in time for a given population. Often referred to as a "snapshot" of the frequency and characteristics of a condition.
Case Report / Case Study A report on the treatment of an individual patient. Because there is no control group for comparison, there is no statistical validity. Where there are a number of case reports, this is known as a case series.

Question Types and Study Types

The table below matches the different Research Question Types to the different Study Types:

Therapy / Intervention Testing the efficacy of drugs, surgical procedures, or service delivery Randomised controlled trial (RCT)
Diagnosis / Screening Testing the validity of diagnostic or mass screening tests Cross-sectional survey
Prognosis Determining the likely course of a disease or condition     Longitudinal survey
Aetiology / Causation Determining if a harmful factor is related to the development of an illness

Cohort study or case-controlled study

Case report for a rare illness

Prevention Determining how to reduce the chance of disease by identifying and reducing risk factors, and how to achieve early diagnosis by screening RCT or cohort study or case-controlled study