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University of Washington Health Sciences Library

ENVH 583: Library Databases and Tools for Research

Guide to resources for thesis proposal research

What is 'the Literature'?

Generally, literature is 

  • Journal articles 
    • New work (e.g. Randomized Controlled Trials, Clinical Trials, etc.) 
    • Reviews (e.g. Scoping Review, Systematic Review, etc. )
    • Editorials
  • Books 

Evidence Hierarchy

See alt text for Evidence Hierarchy below

Alternative Text for Evidence Hierarchy Image 

Pictured above is a triangle with the bottom layer the biggest and top layer the smallest organized into two groupings. Layers listed top to bottom: 

  1. Grouping 1: Filtered Information: 
    1. Systematic Reviews (smallest layer)
    2. Critically-appraised topics (evidence syntheses and guidelines) 
    3. Critically-appraised individual articles (article synopses) 
  2. Grouping 2: Unfiltered Information: 
    1. Randomized Controlled Trials (RCTs) 
    2. Cohort studies 
    3. Case-controlled studies case series / reports 
    4. Background information / expert opinion (largest layer)

Source: Turner, M. (n.d.). UC Library Guides: Evidence-Based Practice in Health: Hierarchy of Evidence. Retrieved April 7, 2021, from 

About the Evidence Hierarchy 

Learn more about how to use the evidence hierarchy.

What is Gray Literature?

Generally, gray literature is anything found outside of scholarly literature. It includes the following. 

  • Conference abstracts, papers, and proceedings 
  • Government reports 
  • Pre-prints 
  • Learn more 

Why Read the Literature?

Scientists and researchers read the literature to 

  • Minimizes your chance of re-inventing the wheel
  • Gives you ideas about how to solve your problem – or perhaps what won’t work
  • Gives you a lingua franca for speaking to other researchers
  • Keeps you up-to-date (science evolves rapidly) 

Evaluating Literature

Evaluating information guide

Some examples of how to evaluate information: 


Basic guidelines:

  • Use a Citation Manager from the start of your research 
  • Search at least three databases
  • See the Writer's Guide 
  • Closely review the Author Guidelines for the journal to which you would like to submit and read others' work in that journal 
  • Closely review the syllabus and assignment instructions for an assigned research project 


Part of this page's content was adapted from Ken Rice and Lianne Sheppard.

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