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Systematic Reviews: Step 3: Conduct Literature Searches

Created by Health Science Librarians

About Step 3: Conduct Literature Searches

It takes an average of 44 hours for a systematic review team to conduct literature searches

Click an item below to see how it applies to Step 3: Conduct Literature Searches.

For PRISMA, there are specific items you will want to report from your search.  For this step, review the PRISMA-S checklist.

 

For this step of the review, in Covidence you can:

  • Document searches in Covidence review settings so all team members can view
  • Add keywords from your search to be highlighted in green or red while your team screens articles in your review settings

 

When designing and conducting literature searches, a librarian can advise you on:

  • How to create a search strategy with Boolean operators, database-specific syntax, subject headings, and appropriate keywords 
  • How to apply previously published systematic review search strategies to your current search
  • How to test your search strategy's performance 
  • How to translate a search strategy from one database's preferred structure and syntax to another

 

Partner with a librarian

The goal of a systematic search is to retrieve all results that are relevant to your topic. Because systematic review searches can be quite extensive and retrieve large numbers of results, an important aspect of systematic searching is limiting the number of irrelevant results that need to be screened. Librarians are experts trained in literature searching and systematic review methodology. Ask us a question or partner with a librarian to save time and improve the quality of your review. Our comparison chart detailing two tiers of partnership provides more information on how librarians can collaborate with and contribute to systematic review teams.

Systematic Searching Process

Magnifying glass looking at city lights

Search Process


  • Identify search concepts and terms for each of them
    • Use controlled vocabulary, if applicable
    • Include synonyms/keyword terms
  • Choose databases, websites, and/or registries to search
  • Construct search strategy
    • Use nesting, Boolean operators, and field tags
    • Translate to other databases
    • Search using other methods (e.g. hand searching)
  • Validate and peer review the search
  • Document the search

Choose a few databases

Databases can be multidisciplinary or subject specific. Choose the best databases for your research question. Databases index various journals, so in order to be comprehensive, it is important to search multiple databases when conducting a systematic review. Consider searching databases with more diverse or global coverage (i.e., Global Index Medicus) when appropriate. A list of frequently used databases is provided below. You can access UNC Libraries' full listing of databases on the HSL website (arranged alphabetically or by subject).

Databases for Literature Searching
Database Scope
PubMed
  • Biomedical and life sciences literature 
  • Includes MEDLINE, manuscripts deposited in PubMed Central, and some e-books
  • Publicly available
Scopus
  • Large research database covering 240 disciplines, including medicine, science, psychology, and education
  • Includes cited references and citing references for articles
  • Includes MEDLINE, select conference proceedings
  • Subscription required (paid by UNC Libraries - access with Onyen and password)
Embase
  • Large biomedical research database 
  • Special focus on pharmaceutical and medical device research
  • Includes MEDLINE, select conference proceedings
  • Subscription required (paid by UNC Libraries - access with Onyen and password)
CINAHL
  • Focuses on nursing and (allied) health sciences literature
  • Includes nursing, biomedical sciences, health sciences librarianship, and 17 allied health disciplines
  • Subscription required (paid by UNC Libraries - access with Onyen and password)
PsycInfo
  • Focuses on psychological, behavioral, and mental health literature
  • Created and maintained by the American Psychological Association
  • Subscription required (paid by UNC Libraries - access with Onyen and password)
Global Index Medicus
  • Biomedical and public health literature
  • Search by and within low-middle income countries
  • Maintained by World Health Organization Regional Office Libraries
  • Publicly available
Global Health
  • Public health database; includes literature on subjects including international health, non-communicable diseases, nutrition, food safety, maternal and child health, and more
  • Subscription required (paid by UNC Libraries - access with Onyen and password)

Search with controlled vocabulary and keywords

Generally speaking, when literature searching, you are not searching the full-text article. Instead, you are searching certain citation data fields, like title, abstract, keyword, controlled vocabulary terms, and more. When developing a literature search, a good place to start is to identify searchable concepts of the research question, and then expand by adding other terms to describe those concepts. Read below for more information and examples on how to develop a literature search, as well as find tips and tricks for developing more comprehensive searches.


Identify search concepts and terms for each

Start by identifying the main concepts of your research question. If unsure, try using a question framework to help identify the main searchable concepts. PICO is one example of a question framework and is used specifically for clinical questions. If your research question doesn't fit into the PICO model well, view other examples of question frameworks and try another!

Question: For patients 65 years and older, does an influenza vaccine reduce the future risk of pneumonia?

Element Example

P

Patient(s) / Population(s)

 

patients 65 years and older

I

Intervention(s)

 

influenza vaccine

C

Comparison(s)

 

not applicable

O

Outcome(s)

 

pneumonia


Controlled Vocabulary

Controlled vocabulary is a set of terminology assigned to citations to describe the content of each reference. Searching with controlled vocabulary can improve the relevancy of search results. Many databases assign controlled vocabulary terms to citations, but their naming schema is often specific to each database. For example, the controlled vocabulary system searchable via PubMed is MeSH, or Medical Subject Headings. More information on searching MeSH can be found here.

Note: Controlled vocabulary may be outdated, and some databases allow users to submit requests to update terminology.

Question: For patients 65 years and older, does an influenza vaccine reduce the future risk of pneumonia?

As mentioned above, databases with controlled vocabulary often use their own unique system. A listing of controlled vocabulary systems by database is shown below.

Database Controlled Vocabulary Indicated By Example
PubMed (MEDLINE)

Medical Subject Headings (MeSH)

[MeSH] "Influenza Vaccines"[Mesh]
Embase EMTREE /exp 'influenza vaccine'/exp
CINAHL CINAHL Headings MH or MM (MH "Influenza Vaccine")
PsycINFO APA Thesaurus DE DE "Influenza"
Sociological Abstracts Thesaurus of Sociological Indexing Terms MAINSUBJECT.EXACT MAINSUBJECT.EXACT("Influenza")


Keyword Terms

Not all citations are indexed with controlled vocabulary terms, however, so it is important to combine controlled vocabulary searches with keyword, or text word, searches. 

Authors often write about the same topic in varied ways and it is important to add these terms to your search in order to capture most of the literature. For example, consider these elements when developing a list of keyword terms for each concept:

  • Terms with similar meaning
    • flu
    • influenza
  • Terms that have different spelling
    • American versus British spelling
    • hyphenated terms
  • Acronyms
  • Concepts described inconsistently
    • quality of life
    • satisfaction
  • Broad versus specific terms
    • vaccination
    • influenza vaccination

There are several resources to consider when searching for synonyms. Scan the results of preliminary searches to identify additional terms. Look for synonyms, word variations, and other possibilities in Wikipedia, other encyclopedias or dictionaries, and databases. For example, PubChem lists additional drug names and chemical compounds.

Question: For patients 65 years and older, does an influenza vaccine reduce the future risk of pneumonia?

PICO Element Example Controlled Vocabulary Synonyms/Keyword Terms

 

 

 

P

Patient(s) / Population(s)

 

 

 

 

patients 65 years and older

 

 

 

 

"Aged"[Mesh]

elder

elders

elderly

aged

aging

geriatric

geriatrics

gerontology

gerontological

senior citizen

senior citizens

older adult

older adults

older individuals

older patients

older people

older persons

advancing age

 

 

 

I

Intervention(s)

 

 

 

 

influenza vaccine

 

 

 

"Influenza Vaccines"[Mesh]

influenza vaccines

flu vaccine

flu vaccines

influenza virus vaccine

influenza virus vaccines

((flu OR influenza) AND (vaccine OR vaccines OR vaccination OR immunization))

C

Comparison(s)

 

not applicable

 

-

 

-

O

Outcome(s)

 

pneumonia

 

"Pneumonia"[Mesh]

pneumonias

pulmonary inflammation

 

Combining controlled vocabulary and text words in PubMed would look like this:

"Influenza Vaccines"[Mesh] OR "influenza vaccine" OR "influenza vaccines" OR "flu vaccine" OR "flu vaccines" OR "flu shot" OR "flu shots" OR "influenza virus vaccine" OR "influenza virus vaccines"

Acknowledge outdated or offensive terminology

Social and cultural norms have been rapidly changing around the world. This has led to changes in the vocabulary used, such as when describing people or populations. Library and research terminology changes more slowly, and therefore can be considered outdated, unacceptable, or overly clinical for use in conversation or writing.

For our example with people 65 years and older, APA Style Guidelines recommend that researchers use terms like “older adults” and “older persons” and forgo terms like “senior citizens” and “elderly” that connote stereotypes. While these are current recommendations, researchers will recognize that terms like “elderly” have previously been used in the literature. Therefore, removing these terms from the search strategy may result in missed relevant articles. 

Research teams need to discuss current and outdated terminology and decide which terms to include in the search to be as comprehensive as possible. The research team or a librarian can search for currently preferred terms in glossaries, dictionaries, published guidelines, and governmental or organizational websites. The University of Michigan Library provides suggested wording to use in the methods section when antiquated, non-standard, exclusionary, or potentially offensive terms are included in the search.

Helpful tip - Building your search

Lightbulb- Helpful Tip

Check the methods sections or supplementary materials of published systematic reviews for search strategies to see what terminology they used. This can help inform your search strategy by using MeSH terms or keywords you may not have thought of. However, be aware that search strategies will differ in their comprehensiveness.

You can also run a preliminary search for your topic, sort the results by Relevance or Best Match, and skim through titles and abstracts to identify terminology from relevant articles that you should include in your search strategy.

Use nesting, Boolean operators, and field tags

Nesting

Nesting is a term that describes organizing search terms inside parentheses. This is important because, just like their function in math, commands inside a set of parentheses occur first. Parentheses let the database know in which order terms should be combined. 

Always combine terms for a single concept inside a parentheses set. For example: 

("Influenza Vaccines"[Mesh] OR "influenza vaccine" OR "influenza vaccines" OR "flu vaccine" OR "flu vaccines" OR "flu shot" OR "flu shots" OR "influenza virus vaccine" OR "influenza virus vaccines")

Additionally, you may nest a subset of terms for a concept inside a larger parentheses set, as seen below. Pay careful attention to the number of parenthesis sets and ensure they are matched, meaning for every open parentheses you also have a closed one.

("Influenza Vaccines"[Mesh] OR "influenza vaccine" OR "influenza vaccines" OR "flu vaccine" OR "flu vaccines" OR "flu shot" OR "flu shots" OR "influenza virus vaccine" OR "influenza virus vaccines" OR ((flu OR influenza) AND (vaccine OR vaccines OR vaccination OR immunization)))


Boolean operators

Boolean operators are used to combine terms in literature searches. Searches are typically organized using the Boolean operators OR or AND. OR is used to combine search terms for the same concept (i.e., influenza vaccine). AND is used to combine different concepts (i.e., influenza vaccine AND older adults AND pneumonia). An example of how Boolean operators can affect search retrieval is shown below. Using AND to combine the three concepts will only retrieve results where all are present. Using OR to combine the concepts will retrieve results that use all separately or together. It is important to note that, generally speaking, when you are performing a literature search you are only searching the title, abstract, keywords and other citation data. You are not searching the full-text of the articles.

boolean venn diagram example


Field tags

The last major element to consider when building systematic literature searches are field tags. Field tags tell the database exactly where to search. For example, you can use a field tag to tell a database to search for a term in just the title, the title and abstract, and more. Just like with controlled vocabulary, field tag commands are different for every database.

If you do not manually apply field tags to your search, most databases will automatically search in a set of citation data points. Databases may also overwrite your search with algorithms if you do not apply field tags. For systematic review searching, best practice is to apply field tags to each term for reproducibility.

For example:

("Influenza Vaccines"[Mesh] OR "influenza vaccine"[tw] OR "influenza vaccines"[tw] OR "flu vaccine"[tw] OR "flu vaccines"[tw] OR "flu shot"[tw] OR "flu shots"[tw] OR "influenza virus vaccine"[tw] OR "influenza virus vaccines"[tw] OR ((flu[tw] OR influenza[tw]) AND (vaccine[tw] OR vaccines[tw] OR vaccination[tw] OR immunization[tw])))

Database Select Field Tags Example
PubMed (MEDLINE)
  • title = [ti]
  • title and abstract = [tiab]
  • title, abstract, keyword, and MeSH = [tw]
  • author = [au]
  • "influenza vaccine"[ti]
  • "influenza vaccine"[tiab]
  • "influenza vaccine"[tw]
  • Poland, GA[au]
Embase
  • title = :ti
  • title and abstract = :ti,ab
  • title, abstract, and keyword = :ti,ab,kw
  • author = :au
  • 'influenza vaccine':ti
  • 'influenza vaccine':ti,ab
  • 'influenza vaccine':ti,ab,kw
  • 'Poland G.A.':au
CINAHL, PsycInfo, & other EBSCO databases
  • title = TI
  • abstract = AB
  • author = AU
  • dissertation number = DN
  • TI "influenza vaccine" 
  • AB "influenza vaccine"
  • AU Poland GA
  • DN 28195829
Sociological Abstracts & other Proquest databases
  • title = ti()
  • abstract = ab()
  • author = au()
  • ti("influenza vaccine"
  • ab("influenza vaccine")
  • au(Poland)

For more information about how to use a variety of databases, check out our guides on searching.

Build your search

Combining search elements together

Organizational structure of literature searches is very important. Specifically, how terms are grouped (or nested) and combined with Boolean operators will drastically impact search results. These commands tell databases exactly how to combine terms together, and if done incorrectly or inefficiently, search results returned may be too broad or irrelevant.

For example, in PubMed:

(influenza OR flu) AND vaccine is a properly combined search and it produces around 50,000 results.

influenza OR flu AND vaccine is not properly combined.  Databases may read it as everything about influenza OR everything about (flu AND vaccine), which would produce more results than needed.

We recommend one or more of the following:

  • put all your synonyms together inside a set of parentheses, then put AND between the closing parenthesis of one set and the opening parenthesis of the next set
  • use a separate search box for each set of synonyms
  • run each set of synonyms as a separate search, and then combine all your searches
  • ask a librarian if your search produces too many or too few results

Question: For patients 65 years and older, does an influenza vaccine reduce the future risk of pneumonia? 

PICO Element Example Controlled Vocabulary (Database-Specific) Synonyms/Keyword Terms Sample Search Strategies (Combine Controlled Vocabulary & Keywords)

 

 

 

P

Patient(s) /  Population(s)

 

 

 

 

patients 65 years and older

 

 

 

 

"Aged"[Mesh]

elder

elders

elderly

aged

aging

geriatric

geriatrics

gerontology

gerontological

senior citizen

senior citizens

older adult

older adults

older patients

advancing age

 

 

 

(“Aged”[Mesh] OR elder[tiab] OR elders[tiab] OR elderly[tw] OR aged[tw] OR aging[tiab] OR “older adult”[tw] OR “older adults”[tw] OR “older patients”[tw] OR “advancing age”[tiab] OR geriatric[tw] OR geriatrics[tw] OR gerontology[tw] OR gerontological[tw] OR “senior citizen”[tw] OR “senior citizens”[tw])

 

 

 

I

Intervention(s)

 

 

 

 

influenza vaccine

 

 

 

"Influenza Vaccines"[Mesh]

influenza vaccines

flu vaccine

flu vaccines

influenza virus vaccine

influenza virus vaccines

(flu OR influenza) AND (vaccine OR vaccines OR vaccination OR immunization)

 

 

("Influenza Vaccines"[Mesh] OR “influenza vaccines”[tw] OR “flu vaccine”[tw] OR “flu vaccines”[tw] OR “flu shot”[tw] OR “flu shots”[tw] OR “influenza virus vaccine”[tw] OR “influenza virus vaccines”[tw] OR ((flu[tw] OR influenza[tw]) AND (vaccine[tw] OR vaccines[tw] OR vaccination[tw] OR immunization[tw])))

C

Comparison(s)

 

not applicable

 

-

 

-

 

-

O

Outcome(s)

 

pneumonia

 

"Pneumonia"[Mesh]

pneumonias

pulmonary inflammation

 

("Pneumonia"[Mesh] OR pneumonia[tw] OR pneumonias[tw] OR “pulmonary inflammation”[tw])

Translate to other databases and other searching methods

Translating search strategies to other databases

Databases often use their own set of terminology and syntax. When searching multiple databases, you need to adjust the search slightly to retrieve comparable results. Our sections on Controlled Vocabulary and Field Tags have information on how to build searches in different databases.  Resources to help with this process are listed below.


Other searching methods

Hand searching

Literature searches can be supplemented by hand searching. One of the most popular ways this is done with systematic reviews is by searching the reference list and citing articles of studies included in the review. Another method is manually browsing key journals in your field to make sure no relevant articles were missed. Other sources that may be considered for hand searching include: clinical trial registries, white papers and other reports, pharmaceutical or other corporate reports, conference proceedings, theses and dissertations, or professional association guidelines.

 

Searching grey literature

Grey literature typically refers to literature not published in a traditional manner and often not retrievable through large databases and other popular resources. Grey literature should be searched for inclusion in systematic reviews in order to reduce bias and increase thoroughness. There are several databases specific to grey literature that can be searched.

Document the search

Systematic review quality is highly dependent on the literature search(es) used to identify studies. To follow best practices for reporting search strategies, as well as increase reproducibility and transparency, document various elements of the literature search for your review. To make this process more clear, a statement and checklist for reporting literature searches has been developed and and can be found below.

At a minimum, document and report certain elements, such as databases searched, including name (i.e., Scopus) and platform (i.e. Elsevier), websites, registries, and grey literature searched. In addition, this also may include citation searching and reaching out to experts in the field. Search strategies used in each database or source should be documented, along with any filters or limits, and dates searched. If a search has been updated or was built upon previous work, that should be noted as well. It is also helpful to document which search terms have been tested and decisions made for term inclusion or exclusion by the team. Last, any peer review process should be stated as well as the total number of records identified from each source and how deduplication was handled. 

If you have a librarian on your team who is creating and running the searches, they will handle the search documentation.

You can document search strategies in word processing software you are familiar with like Microsoft Word or Excel, or Google Docs or Sheets. A template, and separate example file, is provided below for convenience. 

*Some databases like PubMed are being continually updated with new technology and algorithms. This means that searches may retrieve different results than when originally run, even with the same filters, date limits, etc.