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International Oncology Subject Guide: Step 2: Find the Best Evidence to Answer the Question

Step 2: Find the Best Evidence to Answer the Question

Finding best evidence is searching the literature for the best available evidence from systematic research. There are two main sources in literature:

  1.  Primary sources, they are the original studies, research, observation, or experiment on therapy, diagnosis, prognosis, or etiology, such as, clinical trials, cohort studies, and case studies.
  2. Secondary sources are studies that analyze, evaluate, and summarize the primary sources, such as, guidelines, systematic reviews, meta-analysis, and evidence-based summaries.

The 6S Hierarchy of Evidence Based Resources is a useful model for guiding clinical decision making.The model proposes 6 layers of evidence sources. The search for evidence based resources should begin at the highest possible layer. The categories of EBM resources described here are corresponding to layers of 6S model of EBM resources (Haynes, 2007).


Systems: Integrating information from the lower levels of the hierarchy with individual patient records. Systems represents the ideal source of evidence for clinical decision-making.        

Summaries: Regularly updated clinical guidelines or textbooks that integrate evidence-based information about specific clinical problems. 

Synopses of Syntheses: Summarize of information found in systematic reviews. These synopses often provide sufficient to support clinical action.

SynthesesCommonly referred to as a systematic review, which is a comprehensive summary of all the evidence surrounding a specific research question.

Synopsis of original studies: Summarize evidence from an individual, high-quality studies. 

Original Studies: studies represent unique research conducted to answer specific clinical questions.

Other Evidence Sources:

Meta-Searchers search for evidence across multiple resources. These tools return information from all levels of the pyramid:

Grey Literature is any literature that has not been published through traditional means. It is often excluded from large databases and other mainstream sources.

For more information on the grey literature resources, please see the Grey Literature Page