Skip to Main Content

EPIB 629 Knowledge Synthesis

What is a systematic search?

Knowledge syntheses involve systematically searching the literature. For example,

Systematic reviews of interventions require a thorough, objective and reproducible search of a range of sources to identify as many relevant studies as possible (within resource limits). This is a major factor in distinguishing systematic reviews from traditional narrative reviews (...)

Lefebvre C, Glanville J, Briscoe S, Littlewood A, Marshall C, Metzendorf M-I, Noel-Storr A, Rader T, Shokraneh F, Thomas J, Wieland LS. Chapter 4: Searching for and selecting studies. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.1 (updated September 2020). Cochrane, 2020. Available from https://training.cochrane.org/handbook/current/chapter-04#section-4-2-2

Systematic searching is:

  • Thorough: It involves searching more than one database and using a combination of textwords plus subject headings (the latter when available) to identify relevant literature. It also involves using supplementary search methods in addition to database searching
  • Objective: Search terms include variations in terminology and the searcher should avoid biasing the results of the search through the selection of search terms
  • Reproducible: The searcher is careful to record all the details of the search, including the database and platform with dates of coverage (may differ based on institutional subscriptions), the search strategy as executed, and the date the searches were run

Selecting databases to search

Need help choosing which databases to search for a health sciences review? 

Chapter four of the Cochrane Handbook states that it is mandatory to search CENTRAL, MEDLINE, and Embase for all Cochrane reviews (4.3.1.1). Other databases are also suggested in the technical supplement to chapter 4, particularly if you are performing region- or subject-specific searches covered in more specialized databases (e.g., CINAHL, PsycINFO, or regional databases).

Bramer et al. (2017) recommend searching the following based on their analyses: Embase, Ovid MEDLINE ALL, Web of Science Core Collection, and the first 200 records in Google Scholar. Keep in mind that Web of Science Core Collection may include different indices depending on the institutional subscription. 

Large, multidisciplinary citation databases are certainly worth considering, particularly if studies on your topic are likely to be found across many disciplines.

Core databases

This table lists some of the differences between the core databases used in health sciences.

Database (Platform) Subject coverage Publication types included Dates covered
MEDLINE (Ovid) / PubMed Primary biomedical database for health care research; we recommend searching MEDLINE on the Ovid platform (enhanced options for searching) or via PubMed (free platform) Journal articles, editorials 1946 to present
CINAHL (EBSCOhost) Extensive coverage of nursing and allied health, including nursing and rehabilitation journals not covered by MEDLINE Journal articles, editorials, trade magazines 1937 to present
CENTRAL (Cochrane Library) CENTRAL is a sub-database (identified as Trials) in the Cochrane Library and contains records of randomized and quasi-randomized studies. The majority of the records come from MEDLINE and Embase but records from CINAHL and KoreaMed are also included, along with trial records from ClinicalTrials.gov, ICTRP, and additional records from handsearching and those flagged in the Cochrane Review Groups' Specialized Registers. Journal articles, records from clinical trial registries Earliest available to present
Embase Classic + Embase (Ovid) European coverage in biomedicine, rehabilitation, pharmacology Journal articles, editorials, conferences 1947 to present
PsycINFO (Ovid) Excellent resource for research on psychological, social, behavioural, and mental health questions Journal articles, books, book chapters, & dissertations 1806 to present
Scopus Multidisciplinary citation database; "largest database" of peer-reviewed article records covering the arts, medicine, science, social sciences, and technology Journal articles, books, conference proceedings 1823 to present
Web of Science Core Collection Multidisciplinary citation database; McGill coverage includes the Science Citation Index Expanded 1900- (SCI-EXPANDED), Social Sciences Citation Index 1956- (SSCI), Arts & Humanities Citation Index 1975- (A&HCI), Conference Proceedings Citation Index-Science 1900- (CPCI-S), Conference Proceedings Citation Index-Social Science & Humanities 1900- (CPCI-SSH), and the Emerging Sources Citation Index 2015- (ESCI) Journal articles, conference proceedings 1900 to present

Additional databases

  • If your question spans multiple disciplines and you would like more information on databases outside of this list, we suggest consulting the subject guides (also known as LibGuides) produced by McGill librarians to get a sense of resources available at McGill.
  • Other high-quality knowledge syntheses published in the area of your research question can also be used for guidance. Cross-reference the methods sections of published syntheses performed in the field of your synthesis topic.
  • Consider consulting topic experts for further guidance.
  • Preliminary/exploratory searches in databases you are considering including can also give you a sense of how useful the database will be.
  • The Cochrane group also has a living list of resources you can consult.

Developing and running the database searches

We recommend developing the search strategy in a primary database before translating the search strategy to the other selected databases: This will make it easier to keep track of things. If you subsequently find terms in the other selected databases, you can then go back and add them to the search(es) that has (have) already been developed as well as integrate them into the remaining searches.

We also recommend running all the searches on the same day to make it easier to document the date in your manuscript. 

Once you have your searches developed and you are ready to run them, you can the export the records from each database to an EndNote library, which you will keep for your files.

Example of a worksheet

Worksheet for search terms

Keep track of the terms you will be using in your search strategy in whatever way works best for you. Below, we provide an example of a worksheet that can be used for this purpose. Depending on your research question, the list of terms can get quite long and can be difficult to manage if you are not recording them as you are finding them.

Subject headings - definition

Subject headings are assigned descriptors, similar to hashtags but from a controlled vocabulary, used in some databases to uniformly capture a concept. Searching using these standardized words or phrases, instead of text words, means you do not need to worry as much about synonyms and spelling variations, and also allows you to retrieve more precise results. In MEDLINE, the subject headings are MeSH terms, in Embase, they are EMTREE terms: It is important to keep in mind that the subject headings will in most cases by database-dependent.

Keep in mind that there may be a time delay between the addition of records to databases like MEDLINE and their indexing with subject headings like MeSH terms -- and in some databases, e.g., MEDLINE, some records will never be indexed, even when subject headings are available.

Example: The subject heading for cancer in MEDLINE (via PubMed) is the MeSH term Neoplasms. This means that all articles selected for indexing in MEDLINE that are about cancer at a general level will be tagged or indexed with this subject heading, or if the article is about a specific cancer like breast cancer, with a narrower term.

How you actually use subject headings in a database search (if they're even available) depends on the platform you're searching, e.g., to use the subject heading for 'Neoplasms' in PubMed or Ovid MEDLINE, which can both essentially be used to search MEDLINE:

  • PubMed: The search syntax is "Neoplasms"[Mesh] (You can also enter "Neoplasms"[mh] for the same effect; the safest way to use subject headings is to find them first in the MeSH Database, then to add them to the PubMed Search Builder, and then to Search PubMed). They will then be available in PubMed from Advanced > History and Search Details, and can then be combined with other search lines.
  • Ovid MEDLINE: The search syntax is exp neoplasms/ (The Advanced Search in Ovid MEDLINE at McGill is set up by default to "Map term to subject heading", so if you type in 'cancer', you will get a list of suggested subject headings, with 'Neoplasms' at the top; PubMed does not work that way). 

For thorough searches, you would generally include subject headings and their text word equivalents, plus any alternative terms (related terms, broader terms if needed, specific terms, synonyms, alternative spellings or variants, abbreviations).

Tutorials on using subject headings

These tutorials will help you explore subject headings, subheadings, keywords, and search strategy refinement in more detail.

Ovid MEDLINE

Ovid MEDLINE part 1 - Starting your search (using subject headings)

Health Sciences Library, McMaster University. (2020, January 7). OVID Medline - Part 1 - Starting Your Search [Video file]. Retrieved from https://hslmcmaster.libguides.com/tutorials/ovid 

OVID Medline - Part 2 - Refining your search (using keywords, operators, and limits)

Health Sciences Library, McMaster University. (2014, October 16). OVID Medline - Part 2 - Refining Your Search [Video file]. Retrieved from https://hslmcmaster.libguides.com/tutorials/ovid

PubMed

PubMed: Building a search

Welch Medical Library, Johns Hopkins University. (2020 June 30). PubMed: Building a Search [Video File]. Retrieved from https://www.youtube.com/watch?v=xGYFDrORpzA

CINAHL

CINAHL Part 1: Starting your search

Health Sciences Library, McMaster University. (2020, January 7). CINAHL Part 1 - Starting Your Search [Video file]. Retrieved from https://www.youtube.com/watch?v=mLL76nwhYUY

CINAHL Part 2 - Refining your search

Health Sciences Library, McMaster University. (2014, May 15). CINAHL Part 2 - Refining Your Search [Video file]. Retrieved from https://www.youtube.com/watch?v=0kszOLxyI6g&feature=youtu.be

Keywords - definition

Keyword (or textword, natural language, or free-text) searching is when we, for example, search for words which we expect to find in the title, abstract, or author-assigned keywords of relevant articles; it is how we typically interrogate web search engines like Google. Draw up a list of words or phrases related to each key concept in your research question. When using this technique, you will need to be aware of synonyms and spelling variations.

Example: Keywords (or textwords) for cancer can include cancer / cancers / cancerous / malignancies / malignancy / malignant / metastasis / metastases / metastatic / neoplasia / neoplasm / neoplasms / neoplastic / tumor / tumors / tumour / tumours etc.

  • Note that cancer is a very broad concept that refers to many specific types of cancer as well, such as astrocytomas / carcinoma / chordoma / craniopharyngioma / ependymoma / esthesioneuroblastoma /  glioma / leukemia / leukaemia / lymphoma / medulloblastoma / melanoma / mesothelioma / myeloma / myelodysplastic syndromes / osteosarcoma / retinoblastoma / rhabdomyosarcoma / sarcoma / thymoma etc.
  • When searching for articles about cancer, subject headings come in very handy for this reason, but they cannot be relied upon for thorough searching.

Search operators and parentheses

Boolean operators

OR

  • retrieves records that contain at least one of your search terms
  • e.g., (dog OR canine)

AND

  • retrieves records that include all of your search terms
  • e.g., ultrasound AND tuberculosis

NOT

  • retrieves records that contain your first term but exclude your second term
  • e.g., dementia NOT alzheimer's
  • we do not usually recommend that you use NOT in your searches, as you may exclude relevant results

Note: Capitalize your operators as a matter of practice. In some platforms or search systems, it does not matter whether you enter them in uppercase or lowercase, but others (like PubMed or Google Scholar) require them to be in uppercase to work properly.

Proximity operators

  • Many bibliographic databases also allow use of proximity operators
  • The operator, if available, is dependent on the database and platform being searched
  • e.g., on the Ovid platform, adjn is used: primary adj3 care
  • Please see the database-specific operators and search fields

Parentheses/brackets

Use parentheses to set the order of execution of the Boolean logic. Parentheses work in most but not all systems (e.g., they work on the Ovid, PubMed, EBSCOhost, ProQuest, Scopus, and Web of Science platforms, but they are ignored in Google and Google Scholar)

e.g., (chest OR thorax OR thoracic) AND (imaging OR radiographs OR radiography)

Boolean operators - General tutorial

Boolean operators can be placed between your search terms to narrow or broaden a search, or to exclude search terms.

You will find below a brief video on how to use the Boolean operators:

Liaison Librarian

Profile Photo
Genevieve Gore
Liaison Librarian, Schulich Library of Physical Sciences, Life Sciences, and Engineering
Contact: Website

Need help? Ask us!

McGill LibraryQuestions? Ask us!
Privacy notice