PubMed: Types of Medical Literature

Guide to searching PubMed

Levels of Evidence

When searching for evidence-based information, one should select the highest level of evidence possible--systematic reviews or meta-analyses. Systematic reviews, meta-analyses, and critically-appraised topics/articles have all gone through an evaluation process: they have been "filtered". 

Information that has not been critically appraised is considered "unfiltered".

As you move up the pyramid, however, fewer studies are available; it's important to recognize that high levels of evidence may not exist for your clinical question.  If this is the case, you'll need to move down the pyramid if your quest for resources at the top of the pyramid is unsuccessful.

  • Meta-Analysis  A systematic review that uses quantitative methods to summarize the results.
  • Systematic Review    An article in which the authors have systematically searched for, appraised, and summarised all of the medical literature for a specific topic.
  • Critically Appraised Topic     Authors of critically-appraised topics evaluate and synthesize multiple research studies.
  • Critically Appraised Articles  Authors of critically-appraised individual articles evaluate and synopsize individual research studies.
  • Randomized Controlled Trials  RCT's include a randomized group of patients in an experimental group and a control group. These groups are followed up for the variables/outcomes of interest.
  • Cohort Study  Identifies two groups (cohorts) of patients, one which did receive the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.
  • Case-Control Study  Involves identifying patients who have the outcome of interest (cases) and control patients without the same outcome, and looking to see if they had the exposure of interest.
  • Background Information / Expert Opinion   Handbooks, encyclopedias, and textbooks often provide a good foundation or introduction and often include generalized information about a condition.  While background information presents a convenient summary, often it takes about three years for this type of literature to be published.
  • Animal Research / Lab Studies  Information begins at the bottom of the pyramid: this is where ideas and lab
    research takes place. Ideas turn into therapies and diagnostic tools, which then are tested with lab models and
    animals.

Sources:
Greenhalgh, Trisha.  How to Read a Paper: the Basics of Evidence Based Medicine.  London: BMJ, 2000.
Glover, Jan; Izzo, David; Odato, Karen & Lei Wang. EBM Pyramid.  Dartmouth University/Yale University. 2006.

 

Types of Medical Literature

Different types of publications have different characteristics.

Primary Literature
Primary sources are original materials.  It is authored by researchers, contains original research data, and is usually published in a peer-reviewed journal. Primary literature may also include conference papers, pre-prints, or preliminary reports.

Secondary Literature
Secondary literature consists of interpretations and evaluations that are derived from or refer to the primary source literature. Examples include review articles (specifically meta-analysis and systematic reviews) and reference works. Professionals within each discipline take the primary literature and synthesize, generalize, and integrate new research.

Tertiary Literature
Tertiary literature consists of a distillation and collection of primary and secondary sources such as textbooks, encyclopedia articles, and guidebooks or handbooks. The purpose of tertiary literature is to provide an overview of key research findings and an introduction to principles and practices within the discipline.

 

Primary 
Literature

 

 

Secondary 
Literature

 

 

Tertiary 
Literature

 

 

Original research results in journals, 
dissertations, conference proceedings, correspondence

 

Abstracting and indexing services, review articles, systematic reviews, meta-analysis, practice guidelines Textbooks,encyclopedias, handbooks, newspapers

 

Sources: NEJM, JAMA

 

Sources: PubMed, CINAHL, Cochrane Library, Web of Science Sources:  Goodman & Gilman's, Williams Obstetrics

Adapted from the Information Services Department of the Library of the Health Sciences-Chicago, University of Illinois at Chicago.

Formats

Different kinds of information sources are useful depending on your topic and the type of information you need.

Books

Books are good for general background and in-depth coverage of a topic. They are often not as current as journal articles because they take a long time to research, write, and publish.

Books may be useful when:

  • You need a broad overview. There are times when you want someone to explain everything to you - beginning to end. Books are very appropriate for this.
  • Your research topic is historical. Books lend themselves to topics in which the facts don't change much over time.
  • You want several opinions from one place. Some books collect essays that give you several points of view in one source.

Books may not be useful when:

  • The topic is very recent. Books take years to get researched, written, published, purchased, and put on library shelves. If the issue you are researching is constantly changing, a book may be outdated by the time it gets to the library.
  • You have a fairly narrow topic. Sometimes books are too broad-based to address specific or narrow points.

Articles

Articles tend to be narrow in scope and are good for focused treatment of a topic. Scholarly journals contain high-quality articles usually written by experts and use data and statistics to back arguments. Popular magazines and newspapers (such as Newsweek, People, or the New York Times) are good for current treatment of a topic and are good resources for editorials and opinions. Note that popular magazines are not peer-reviewed.

Articles may be useful when:

  • Your topic is very recent. Articles are intended to keep people up-to-date on the latest developments in various issues.
  • Your research topic is very narrow in scope. Some topics are so specific, whole books will not be written on them.

Articles may not be useful when:

  • You need background or overview information. Articles tend to focus on a specific aspect of a topic.
  • Your topic covers a long time span. When an issue has a long history, you may only find one aspect discussed in an article.

Web Sites

Web sites can be very good for finding quality information including primary sources, statistical information, educational sites on many levels, policy, opinion of all kinds, and much more. However, you have to take the responsibility to rigorously evaluate each site for quality; anyone can post a Web page, regardless of their expertise or intentions.

 

Adapted from the University of Connecticut Libraries