There are instances when searching for literature is part of the research methodology. This would be the case if you intend to do a systematic review or research synthesis of empirical research.
This page focuses on how you:
Due to the ever-increasing amount of research output, we see the systematic review emerging as a scientific exploration in itself, particularly in biomedicine, health, and the life sciences. The research society needs research syntheses to help direct research efforts, and the larger society needs them as an aid in policy and decision-making.
The role of the systematic review is to examine the evidence across multiple single studies with similar research questions to arrive at some conclusion about our previous knowledge. A systematic review ideally examines all previous research concerning the research question.
A systematic review is question-led, i.e. the synthesis answers a research question rather than giving a topical overview. Using a focused research question as the starting point of a systematic review enables an exhaustive examination of what is known and not known about the question at hand.
The key is in the systematic approach, which, simply put, predefines what you are looking for. Although you are looking for all previous research, there need to be enough commonalities among the studies to make comparisons that will aid in a conclusion. You will be looking for studies that address the same research question, have similar samples and methods for sample selection, look at the same intervention, exposition, experiment, or technique, and use similar research methods.
You begin by specifying all the elements of the review work ahead, especially:
The search will usually try to capture as much literature as possible, while inclusion and exclusion criteria will be used to reduce the amount of it. Documentation of the search will make the review work replicable for other researchers. The quality assessment will help to determine the strength of the evidence.
In order to scope your field for relevant research, prevent replication of already published work and to discover research gaps, it is important to identify already published reviews. This can be done by searching extensively in several reference databases, utilizing the review filter, and searching in databases designated for reviews.
In medicine and health, there are international registers of protocols for systematic reviews, e.g. Cochrane Library and Prospero. These also register planned systematic reviews in order to prevent duplication of work effort.
Preparing the literature retrieval process is crucial, as this will be the foundation for your further work.
In searching, we distinguish between text words and subject headings, which are in different searchable fields of a reference. Text words (also called free text) are an author’s own written words that appear in the title and abstract. Subject headings describe the content of an article and are added by the database providers; these are often standardized.
Strategies of finding search terms
|Second hand smoking
||Tobacco Smoke Pollution||Passive smoking|
|Vaporizers||Nebulizers and Vaporizers||Vaporization|
This example shows subject headings in Medline (which uses MeSH) and Embase (which uses Emtree), using different terms describing the same concepts.
|women AND pregnancy||No need to search for women as only
women get pregnant.
|pregnancy AND preeclampsia
||No need to search for pregnancy as
preeclampsia only occurs in pregnancy.
Language, publication year, demography and methodology filters
If your research question allows limitations, these may be applied in the literature search as well. Most databases allow limitations by year, language, demography, research design, etc. Consider the use of these.
Use a gold standard
If you have a set of key articles on your topic, use them to test your search. First, check whether they are indexed in the database, then check whether they come up in your search. If they are not included in the search, try to find out why and revise your strategy. Missed search terms? Too narrow limitations?
Test and revise your search
A part of the process of searching is to revise your search several times. By including or excluding terms and testing your search, you will end up with what best suits your research question.
Peer review of search strategy
When critically appraising a systematic review it is important to determine whether all key, relevant studies are included. This can be assessed by evaluating the literature search, that is, the sources and search terms used. Similarly, you can have a colleague or information specialist peer-review your search strategy during the review process to help raise the quality of the review. Others might identify missed terminology, syntax errors and other mistakes which are easily overlooked.
The PRESS checklist (Peer Review of Electronic Search Strategies) is developed to identify elements of accuracy of literature searches.
How exactly you build up your search strategy will differ between persons and databases. Here we present an example of how the process of developing a search strategy can look and how the literature searches will look in different databases.
In the following, our research question is: Does exposure to smoke from e-cigarettes increase the risk of obstructive lung disease?
In this example, we will search PubMed, Embase and Web of Science. In a real-life systematic review on the topic, we would search more databases.
The main elements of this question are
To search as thoroughly as possible we need all the terms describing these elements. To systemize this work, all the words are organized in the table below.
|Electronic cigarettes||-Electronic Cigarettes||-electronic cigarette||-electronic cigaret*
|Vaporizers and tobacco smoking||-Nebulizers and Vaporizers
-Tobacco Use Cessation Products
– smoking cessation
– smoking cessation program
|Passive smoking||-Inhalation Exposure
-Tobacco Smoke Pollution
|– passive smoking
|– passive smoking
– tobacco Smoke pollution
– second hand smoking
|Lung disease||– Asthma
-Pulmonary Disease, Chronic Obstructive
– Lung Diseases, Obstructive
|– obstructive airway disease
– chronic obstructive lung disease
– obstructive pulmonary disease*
– obstructive airway disease*
– obstructive lung disease*
Searches in PubMed, Embase Ovid and Web of Science can look like this
Date for searches: 6th October 2015.
Example PubMed search strategy
|#1||electronic cigarettes[MeSH Terms]||351|
|#2||electronic cigaret*[Title/Abstract] OR e-cigaret*[Title/Abstract] OR electronic nicotine[Title/Abstract] OR e-nicotine[Title/Abstract]||2299|
|#3||#1 OR #2||2332|
|#4||Nebulizers and Vaporizers[MeSH Terms]||8887|
|#5||vapor*[Title/Abstract] OR vapour*[Title/Abstract] OR vaper*[Title/Abstract] OR vaping[Title/Abstract] OR vaporizer*[Title/Abstract]||41836|
|#6||#4 OR #5||50309|
|#7||Tobacco products[MeSH Terms] OR Smoking[MeSH Terms] OR Tobacco Use Cessation Products[MeSH Terms] OR Tobacco use[MeSH Terms] OR Smoking cessation[MeSH Terms] OR Nicotine[MeSH Terms]||149671|
|#8||smoking[Title/Abstract] OR nicotine[Title/Abstract] OR tobacco[Title/Abstract]||2327822|
|#9||#7 OR #8||238856|
|#10||#6 AND #9||946|
|#11||#3 OR #10||3059|
|#12||Inhalation Exposure[MeSH Term] OR Tobacco Smoke Pollution[MeSH Term]||17475|
|#13||passive[Title/Abstract] OR exposure[Title/Abstract] OR pollution[Title/Abstract] OR second hand[Title/Abstract]||735510|
|#14||#12 OR #13||741069|
|#15||Asthma[MeSH Terms] OR Bronchitis[MeSH Terms] OR Lung Diseases, Obstructive[MeSH Terms] OR Pulmonary Disease, Chronic Obstructive[MeSH Terms]||178505|
|#16||Asthma[Title/Abstract] OR Bronchitis[Title/Abstract] OR COPD[Title/Abstract] OR obstructive pulmonary disease*[Title/Abstract] OR obstructive airway disease*[Title/Abstract] OR obstructive lung disease*[Title/Abstract]||174515|
|#17||#15 OR #16||230092|
|#18||#11 AND #14 AND #17||77|
Example Embase Ovid search strategy
(Electronic cigaret* or E-cigarette* or electronic nicotine or e-nicotine).tw.
1 or 2
(vapor* or vapour* or vaper* or vaping or vaporizer*).tw.
4 OR 5
Smoking/ or smoking cessation/ or smoking cessation program/ or nicotine/
(Tobacco or Nicotine or Smoke or Smoking).tw.
|9||7 OR 8||407264|
|10||6 AND 9||1264|
|11||3 OR 10||2517|
passive smoking/ or exposure/
(passive or pollution or second hand or exposure).tw.
|14||12 OR 13||964912|
obstructive airway disease/ or asthma/ or bronchitis/ or chronic obstructive lung disease/
(asthma or COPD or obstructive pulmonary disease* or obstructive airway disease* or obstructive lung disease*).tw.
|17||15 OR 16||342694|
|18||11 AND 14 AND 17||123|
Note: The / indicates a subject heading from Emtree and .tw. indicates text words.
Example Web of Science search strategy
|1||TOPIC: (e-cigarette* OR Electronic Cigarette* electronic nicotine OR e-nicotine)||973|
|TOPIC: ((vapour* OR vaper* OR vaping OR vaporizer*) and (tobacco product* OR smoking OR Tobacco Use Cessation Product*s OR tobacco use OR smoking cessation OR nicotine))||357|
|3||TOPIC: (Tobacco Smoke Pollution OR second hand OR exposure OR passive)||995 886|
|4||TOPIC: (Asthma OR bronchitis OR COPD OR (obstructive pulmonary disease*) OR (obstructive airway disease*) OR (obstructive lung disease*))||199 043|
|5||1 OR 2||1258|
|6||3 AND 4 AND 5||35|
Note: Web of Science does not have a subject heading index, therefore all search terms are text words.
Processing the search result
In total there are 228 retrieved references in these searches. As this is a relatively small number, we chose not to limit our search to observational studies that investigate risk and causation. Had the number of hits been high, a relevant step would be to add a methodological filter to narrow our search to observational studies (via the Clinical Queries option available in PubMed and Embase). What constitutes a high number of hits is a subjective judgement: how many references would you review vs how specific should the search be?
While searching multiple databases, importing the result to a reference management program can help you create an efficient workflow. The reference manager can remove duplicate references, help you sort relevant references and organize your further work.
Documentation of the search makes your research reproducible and indicate that your methodology is sound. You would want to describe the search strategy that led to the included studies.
Check the journals’ instructions for authors for any specific requirements on how to report the search strategy. It can also be a good idea to look at published reviews in the journal you want to submit your systematic review to see what is customary. For instance, some journals encourage the full search strategy in an appendix.
Documenting the search strategies can be done by
Example on how documentation can be done in a section of an article
Studies included in this review are located from searching PubMed, Embase Ovid and Web of Science, last search 6th October 2015. The literature search included MeSH-terms and text words, in the following combination: electronic cigarettes or tobacco vaporizers and tobacco exposure and obstructive lung diseases. The full search strategy in PubMed included the following MeSH-terms and text words; (electronic cigarettes[MeSH Terms] OR electronic cigaret*OR e-cigaret* OR electronic nicotine) OR ((Nebulizers and Vaporizers[MeSH Terms] OR vapor* OR vapour* OR vaper* OR vaping OR vaporizer*) AND (Tobacco products[MeSH Terms] OR Smoking[MeSH Terms] OR Tobacco Use Cessation Products[MeSH Terms] OR Tobacco use[MeSH Terms] OR Smoking cessation[MeSH Terms] OR Nicotine[MeSH Terms] OR smoking OR nicotine ))) AND (Inhalation Exposure[MeSH Term] OR Tobacco Smoke Pollution[MeSH Term] OR passive OR exposure OR pollution OR second hand) AND (Asthma[MeSH Terms] OR Bronchitis[MeSH Terms] OR Pulmonary Disease, Chronic Obstructive[MeSH Terms] or Asthma OR Bronchitis OR Chronic Obstructive Pulmonary Disease* OR COPD OR Obstructive pulmonary disease* OR Chronic Obstructive airway Disease* OR Obstructive airway disease* ).
The search was adapted to Embase with the following Emtree terms; electronic cigarette, vaporization, smoking, exposure, obstructive airway disease, asthma, bronchitis and chronic obstructive lung disease, and to Web of Science searching by topic. There are no restrictions on language or publication dates. The reference lists of included articles were screened for additional references.
For further input and advice concerning systematic searching, consult your library, help pages in databases and literature on the topic. Some libraries offer courses in systematic searching, or they do such searches on request.
– Haraldstad, A. M., & Christophersen, E. (2015). Literature searches and reference management. In P. Laake, H. B. Benestad, & B. R. Olsen (Eds.), Research in medical and biological sciences from planning and preparation to grant application and publication (pp. 125–166). Amsterdam: Elsevier.
– Higgins, J. P. T., Green, S., & Cochrane, C. (2008). Cochrane handbook for systematic reviews of interventions. Chichester: Wiley-Blackwell.