Search strategy for researchers
1.Position your question
• For who?
• Main point / side issue?
2. Select your source
• The visible or the deep web
3. Create your search strategy
• Identify the main concepts
• Search for alternative terms, technical jargon and index terms
• Combine the search terms
4. Evaluate your result
Formulate a research question in which, if possible, all aspects of your topic are named. The PICO model is a tool for setting up a clinical research question. In this model the patient category (Population/Patient), the intervention (Intervention/Indicator), the control treatment or comparison (Comparison/Comparator/Control) and the outcome (Outcome) are clearly described.
For example: ‘In men, does having a vasectomy (compared to not having one) increase the risk of getting testicular cancer in the future?’
Population/patient = adult males
Intervention/indicator = vasectomy
Comparator/control = no vasectomy
Outcome = testicular cancer
Spin-offs of PICO such as PICOT (including Time) and/or PICOS (including Setting) contain more information than the classic PICO and may therefore be a better alternative. Here is a template for formulating PICOT questions. If you need other types of PICO and spin-offs (e.g. in the context of a systematic review of case control studies), please contact us.
In addition to formulating a clear research question, it is essential to define and issue selection criteria when writing a systematic review (preferably also per aspect of your research question). Each of the aspects is described in yet more detail in the inclusion and exclusion criteria (e.g. age limit or comorbidities under the description of the Population). Other important criteria to be described are: setting (e.g. Western society), duration of administration of the intervention and/or the follow-up, study types and language of the studies that will be included/excluded.
The choice of a suitable source depends on the type of source and the aim of your question.
When you are looking for background information (subject information and/or public information), it may be useful to conduct a search in Google or consult an encyclopaedia (e.g. Wikipedia) and/or search in MedlinePlus.
Scientific information can be found in journals and databases.
For clinical evidence-based information, databases such as UpToDate are suitable.
For each type of source it is useful to know more about the information source (e.g. users in Wikipedia), the search algorithm (e.g. order of results in Google Scholar) and the search options (e.g. Advanced Search option and thesaurus in certain medical databases).
For more information, consult the information sheets about databases.
The Internet has different layers: the free (or visible) web and the deep (or invisible) web. The deep web essentially refers to databases, for which you will find at most the homepage via search engines such as Google, Bing or Yahoo. In order to search through the content, you will have to use the search engine associated with the database in question.
In the Cochrane Library you can only search for systematic reviews in the Cochrane Database of Systematic Reviews.
Via PubMed you can find systematic reviews using the Clinical Queries filter. Please note: the results obtained using this filter are not exclusively systematic reviews, but include for example guidelines or Health Technology Assessments (HTAs).
Via Embase you can add [systematic review]]/lim to your search query or select the filters Systematic Review and/or Cochrane Reviews under the EBM (Evidence-Based Medicine) filters.
In the database Scopus you will find that your search query also shows the link View # patent results in the bar with the number of results. This link is only visible if there are patents that correspond to your search query. The information is checked and retrieved from five patent organisations via LexisNexis.
Look for the most commonly occurring (subject)specific search terms and synonyms for every aspect of your question (see How do I formulate my research question?). The use of the ‘correct’ search terms is crucial for finding the desired information. Wikipedia, Oxford Reference Online and Thesaurus.com are potential sources for English search terms. Always assume that search systems simply check whether the terms you have specified in your search strategy appear in for example title and abstract. In most cases, if your term is written slightly differently or your term only appears in the full text of the article, the information will not be found.
If you are searching in a database with a thesaurus (e.g. PubMed or Embase), then it is best to use relevant keywords/index terms from that thesaurus as well. Below is a table that you may help you search and list relevant search terms.
|Population||search term including singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym, etc. *||synonym(s) including subject terminology, singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym(s), etc. *||keyword (e.g. MeSH term for searching via PubMed; Emtree term for searching via Embase)|
|Intervention||search term including singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym, etc. *||synonym(s) including subject terminology, singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym(s), etc. *||keyword (e.g. MeSH term for searching via PubMed; Emtree term for searching via Embase)|
|Comparison||search term including singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym, etc. *||synonym(s) including subject terminology, singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym(s), etc. *||keyword (e.g. MeSH term for searching via PubMed; Emtree term for searching via Embase)|
|Outcome||search term including singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym, etc. *||synonym(s) including subject terminology, singular/plural, used as noun/adjective and verb conjugations, spelling variations (British and American spelling), Latin names, acronym(s), etc. *||keyword (e.g. MeSH term for searching via PubMed; Emtree term for searching via Embase)|
*Use single or double quotation marks (depending on the database) for terms that must appear together and in exactly this order (= phrase searching).
Key articles or systematic reviews related to your research question can be useful sources of inspiration for search terms, synonyms and keywords. You can also use tools such as PubReMiner and GoPubMed (After Search > Statistics), which list numerous commonly found terms and/or MeSH terms on the basis of your search query. On the basis of your search query or a piece of text, you can also obtain advice about possible relevant MeSH terms and key articles via MeSH On Demand.
Search filters or blocks can provide inspiration for search terms. See What are search filters and where can I find them?
Search blocks and filters can be subdivided into methodological filters (e.g. RCT filter, filter for Health Economics, filter for systematic reviews) and content filters (e.g. filter to look for literature about children, or about AIDS, etc.).
Search filters or blocks can provide inspiration for search terms. If the filter is validated, it can even be taken in its entirety and used in your search strategy – providing the source is cited. Always check that the search filter is compiled in the correct syntax. For example, a search filter in Embase may be compiled for the OVID interface or for the Embase.com interface.
The table below gives an overview of various search filters or blocks.
|Organisation||Various examples of methodological filters or blocks||Various examples of content filters or blocks|
|The Cochrane Collaboration||Randomized Controlled Trials (RCTs)|
|The InterTASC Information Specialists’ Sub-Group (ISSG)||Observational studies
Mixed method studies
Public Views & Patient Issues
|Canadian Agency for Drugs and Technologies in Health (CADTH)||Therapie
|PubMed Search Strategy blog
Via the search box
|A specific pathology, e.g. COPD
A specific target group, e.g. healthcare workers
A specific type of study, e.g. animal studies
Medication, e.g. NSAIDs (non-steroidal anti-inflammatory drugs)
|BMI van de Koninklijke Nederlandse Vereniging voor Informatieprofessionals||A specific pathology, e.g. Alzheimer’s disease
A specific target group, e.g. patients
Medication, e.g. Acyclovir or Zovirax
A specific treatment, e.g. bone marrow transplant
A specific laboratory test, e.g. blood cell count
|Caresearch Palliative Care Knowledge Network||A specific pathology, e.g. anorexia
A specific target group, e.g. the elderly
A specific theme related to palliative care, e.g. advance care planning
Combine the search terms, synonyms and keywords per aspect (e.g. Population) with the OR operator. The use of the OR operator indicates that at least one of the terms must appear in your search results. Then combine the search actions of all aspects with an AND operator (which indicates that both terms must appear in your search results). Use the advanced search function offered by many databases for this purpose.
For extensive search queries (e.g. in the context of a systematic review), it is recommended to use the syntax (≈ code) of the database. An overview of the syntax in PubMed, Embase, CENTRAL, CINAHL, Web of Science and Scopus can be found here. By using the syntax, you indicate where a term can appear, e.g. in the title and abstract when using [TIAB] in PubMed. The syntax enables you to stay in control rather than leaving it to the search engine (e.g. the Automatic Term Mapping functionality in PubMed).
It is also recommended in a systematic review to describe the search strategy in all its detail (preferably by including the syntax), listing the various sources, search terms and search method (e.g. look at the cited and citing articles of the studies included).
The PRESS (Peer Review of Electronic Search Strategies) checklist is an evidence-based guideline for assessing search strategies for systematic reviews, among other things. You can also contact us for more information in this area.
Are there any available tools to facilitate the process of selecting articles when compiling a systematic review?
Covidence is a user-friendly online tool mainly suitable for facilitating the process of selecting articles in a systematic review. Online screening is free of charge for 1 review (with a maximum of 2 reviewers).
You can probably already deduce something about the reliability from the URL and the digital information certificate of a source/website.
|URL extension||Type of organisation||Example||Objectivity, reliability|
|.be / .nl / …||Country code||https://www.kcgg.be||?|
The use of language (e.g. correct, objective language), the proprietor (e.g. Universitair Ziekenhuis) and the up-to-dateness will also give you some insight into the reliability. In any case, check whether you can verify the information provided in other reliable sources. The search engine Health On the Net delivers sources that conform to the HON label for reliable and qualitative healthcare information for patients and professionals on the Internet.
In addition to assessing the quality of the source, it is also important to check the relevance and validity of your information. Is the study recent? Was the appropriate methodology used when conducting the study? (See also Processing > Evaluating)
The aim of a systematic review is to give an objective overview of all literature relating to a certain research question. The (possibly low) quality of a study is therefore not a reason for excluding it. In this type of literature research it is common practice to allocate a quality level to each study. There are various tools for this purpose that are often divided by study type, e.g. the Cochrane tool (ROB) for Randomised Controlled Trials (RCTs) and the ROBINS for non-Randomised Controlled Trials (non-RCTs). (See also Processing > Evaluating and Writing)
If you would like more information or if you have any questions, you can contact our information specialist Nele Pauwels.
Do you have a question related to your literature review or do you need help with searching, processing or publishing of your literature/data? Then please fill in our form to give us a clear view of your question and so that we can advise you efficiently. Alternatively, you could email us at firstname.lastname@example.org.