Defining primary and secondary forms of evidence

It is important to understand the difference between primary and secondary sources of evidence, and how they may each help to answer clinical questions.

Primary evidence

A primary source of evidence presents a record of the original events. In the case of healthcare, primary evidence usually refers to single research studies; such as randomised controlled trials or observational studies that investigate an aspect of clinical interest. These are the ‘building blocks’ that gradually construct clinical knowledge and understanding about a topic.

Single studies can provide recent research findings, but aspects of the results may be inconsistent with other studies, and it is the reader's responsibility to appraise the study and make a decision about its clinical relevance in the light of alternative findings.

Secondary evidence

Secondary evidence provides an interpretation or analysis of several studies (primary evidence) that share a common focus. This type of evidence is often called ‘pre-appraised’. In secondary forms of evidence, experts have selected high quality studies, appraised and compiled the findings, and often commented on their clinical relevance or implications. Pre-appraised evidence uses an explicit, reproducible process to evaluate the scientific merit of its source evidence. It also usually evaluates clinical relevance.

Secondary sources of evidence remove the need for individuals to undertake the time consuming and complex task of reading and assimilating individual studies, and are presented in a way that helps clinicians to make decisions. Increasingly, clinicians are using these forms of evidence in their day to day practice, in preference to single study searches, because of their high quality analysis and the time saving benefits that they offer. Evidence summaries and systematic reviews are examples of secondary evidence.