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 which is assimilated, or put together, from a number of quality primary studies on a topic. It includes systematic reviews, meta-analyses, and evidence summaries.
It 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 the JBI EBP Database, 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.
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 (CENTRAL), in other trials registers, and in health and medical databases. See the next page, Where to Search for Evidence, for details of where to search for primary unfiltered evidence.
One 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. |
| 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. |
| Synopsis 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. |
When you cannot find any higher level evidence you need to look for primary research (original individual studies of various types) in databases and registers of controlled trials. Remember that the best study type might vary according to the nature of the clinical question (see Question Types and Study Types below).
Source: NSW Ministry of Health, CIAP
The different types of individual studies are:
| RCT | Randomised Controlled Trial: a carefully planned experiment that studies the effect of therapy on real patients. Includes techniques (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. |
The table below matches the different Research Question Types to the different Study Types:
| TYPE OF QUESTION | DESCRIPTION OF RESEARCH | STUDY TYPE? |
| Therapy / Intervention | Tests the efficacy of drugs, surgical procedures, therapy, or service delivery | Systematic review; randomised controlled trial (RCT); cohort study |
| Diagnosis / Screening | Tests the validity of diagnostic or mass screening tests | Cross-sectional survey |
| Prognosis | Seeks to determine the likely course of a disease or condition | Cohort study; case controlled study; case series |
| Aetiology / Causation | Seeks to determine if a harmful factor is related to the development of an illness |
Cohort study; case-controlled study Case report for a rare illness |
| Prevention | Seeks to determine how to reduce the chance of disease by identifying and reducing risk factors, and to achieve early diagnosis by screening | RCT; cohort study; case-controlled study |
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