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:
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 (126.96.36.199). 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.
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|
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.
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 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:
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).
These tutorials will help you explore subject headings, subheadings, keywords, and search strategy refinement in more detail.
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
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
Welch Medical Library, Johns Hopkins University. (2020 June 30). PubMed: Building a Search [Video File]. Retrieved from https://www.youtube.com/watch?v=xGYFDrORpzA
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
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
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: 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.
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 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: