Searching PubMed: Literature Reviews

The content in the Literature Review section defines the literature review purpose and process, explains using the PICO format to ask a clear research question, and demonstrates how to evaluate and modify search results to improve the accuracy of the retrieval.

A literature review seeks to identify, analyze and summarize the published research literature about a specific topic.  Literature reviews are assigned as course projects; included as the introductory part of master's and PhD theses; and are conducted before undertaking any new scientific research project.

The purpose of a literature review is to establish what is currently known about a specific topic and to evaluate the strength of the evidence upon which that knowledge is based. A review of a clinical topic may identify implications for clinical practice. Literature reviews also identify areas of a topic that need further research.

A systematic review is a literature review that follows a rigorous process to find all of the research conducted on a topic and then critically appraises the research methods of the highest quality reports. These reviews track and report their search and appraisal methods in addition to providing a summary of the knowledge established by the appraised research.

The UNC Writing Center provides a nice summary of what to consider when writing a literature review for a class assignment. The online book, Doing a literature review in health and social care : a practical guide (2010), is a good resource for more information on this topic.

Obviously, the quality of the search process will determine the quality of all literature reviews. Anyone undertaking a literature review on a new topic would benefit from meeting with a librarian to discuss search strategies. A consultaiton with a librarian is strongly recommended for anyone undertaking a systematic review.

Use the email form on our Ask a Librarian page to arrange a meeting with a librarian.

The first step to a successful literature review search is to state your research question as clearly as possible.

It is important to:

• be as specific as possible
• include all aspects of your question

Clinical and social science questions often have these aspects (PICO):

• People/population/problem  (What are the characteristics of the population?  What is the condition or disease?)
• Intervention (What do you want to do with this patient?  i.e. treat, diagnose)
• Comparisons [not always included]  (What is the alternative to this intervention?  i.e. placebo, different drug, surgery)
• Outcomes  (What are the relevant outcomes?  i.e. morbidity, death, complications)

If the PICO model does not fit your question, try to use other ways to help be sure to articulate all parts of your question. Perhaps asking yourself Who, What, Why, How will help.  

Example Question:  Is acupuncture as effective of a therapy as triptans in the treament of adult migraine?

Note that this question fits the PICO model.

• Population: Adults with migraines
• Intervention: Acupuncture
• Comparison: Triptans/tryptamines
• Outcome: Fewer Headache days, Fewer migraines

A literature review search is an iterative process. Your goal is to find all of the articles that are pertinent to your subject. Successful searching requires you to think about the complexity of language. You need to match the words you use in your search to the words used by article authors and database indexers. A thorough PubMed search must identify the author words likely to be in the title and abstract or the indexer's selected MeSH (Medical Subject Heading) Terms.

Start by doing a preliminary search using the words from the key parts of your research question.

Step #1: Initial Search

Enter the key concepts from your research question combined with the Boolean operator AND. PubMed does automatically combine your terms with AND. However, it can be easier to modify your search if you start by including the Boolean operators.

migraine AND acupuncture AND tryptamines

The search retrieves a number of relevant article records, but probably not everything on the topic.

Step #2: Evaluate Results

Use the Display Settings drop down in the upper left hand corner of the results page to change to Abstract display.

Review the results and move articles that are directly related to your topic to the Clipboard.

Go to the Clipboard to examine the language in the articles that are directly related to your topic.

• look for words in the titles and abstracts of these pertinent articles that differ from the words you used
• look for relevant MeSH terms in the list linked at the bottom of each article

The following two articles were selected from the search results and placed on the Clipboard.

Here are word differences to consider:

• Initial search used acupuncture. MeSH Terms use Acupuncture therapy.
• Initial search used migraine.  Related word from MeSH Terms is Migraine without Aura and Migraine Disorders.
• Initial search used tryptamines. Article title uses sumatriptan. Related word from MeSH is Sumatriptan or Tryptamines.

With this knowledge you can reformulate your search to expand your retrieval, adding synonyms for all concepts except for manual and plaque.

#3 Revise Search

Use the Boolean OR operator to group synonyms together and use parentheses around the OR groups so they will be searched properly. See the image below to review the difference between Boolean OR / Boolean AND.

Here is what the new search looks like:

(migraine OR migraine disorders) AND (acupuncture OR acupuncture therapy) AND (tryptamines OR sumatriptan)