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Systematic Review Searching

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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:

  • prepare a systematic search
  • translate a search between databases
  • document your search

Introduction to systematic reviews

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:

  • a predefined and clearly focused question
  • explicit criteria for inclusion and exclusion of relevant studies
  • documentation of search process and methods applied
  • methods for extracting data from studies
  • quality assessment (assessing risk of bias) of the studies
  • how findings from studies will be analysed and synthesised

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.

Prepare your search

Preparing the literature retrieval process is crucial, as this will be the foundation for your further work.

  • Identify the main elements of your research question
    Defining which elements from your research question to include in the search will help you choose search words effectively. Too few elements will lead to an unspecific search and many irrelevant hits. Too many elements can make it difficult to find anything at all, or lead to a biased selection of studies.
  • Make a choice on relevant and accessible databases to search
    Your library may subscribe to relevant databases that you’re not aware of. Choose as many relevant ones as necessary. Even though there may be overlap in the search results, you will probably gain additional references.Consult the manuals of the chosen databases. Here you will find information about such advanced search options as truncation symbols, Boolean operators and syntax in that specific database.
  • Unpublished material
    In addition to databases of published research there are several databases of registered clinical trials. Here you can discover unpublished results from completed trials, or you can decide to wait for the results to be published.
  • Grey literature
    ‘Grey literature’ is a common name for information or publications not appearing in regular channels for scholarly communication. These publications are typically from research institutes or government bodies, and are often not registered (or indexed) in reference databases. The publications can be located on the institutional websites, in library catalogues or in designated databases for grey literature.
  • Strategies for tracing references
    To be as exhaustive as possible, and make sure all relevant literature is included, the reference lists of relevant studies should also be traced.Read more about Tracing references in Discover your field.
  • Find search terms
    In order to capture all previous research, use an exhaustive set of search words. Include synonyms and use thesauruses and the subject headings relevant to the selected databases for your overall search.

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

  • From the top of your head. If you know the field, you know several relevant terms.
  • Journal articles on the topic. A good exercise is to text analyse articles on the topic to get an overview of the variety of words used in the field. Check for both text words and author’s keywords.
  • In addition, do some test searches on the topic. See if any unexpected but relevant words appear.
  • Search dictionaries and thesauruses. The same concepts may be described differently in different thesauruses; they might have different subheadings or include several related concepts.
MeSH EMTREE
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.

  • Look up relevant references in databases and see what subject headings they are indexed with. Check the same reference in databases with different thesauruses.
  • Take advantage of others’ published search strategies. It has become more common to publish search strategies in review articles. Search for reviews on your topic or parts of it, and see if there is anything you can use. Systematic reviews from the Cochrane Collaboration usually present their full search strategies in an appendix to their published review.
  • Text analysing programs. There are various text analysers available online which can help you discover even more search terms. Reference managers such as Endnote also generate a term list based on library references, which can be useful.
  • Be careful to not overlap search terms.
Examples Explanation
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.


Translate a search between databases

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

  • electronic cigarettes
  • obstructive lung diseases
  • second hand smoking

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.

  • The most obvious search words are those in our research question and they will be the starting point for the search. These words will be used as text words both in the singular and plural, truncated with an asterisk * in the table.
  • Other natural language words and abbreviations should also be included as text words.
  • We then search the MeSH-database for relevant MeSH-terms and Embase for Emtree-terms. These subject headings are also used as text words, as subject headings and text words are placed in different fields of a reference.
  • By searching and surfing the databases you can identify additional search terms. In this example, by searching for electronic cigarettes we stumbled upon an article on electronic nicotine delivery systems, which was a new term for us. As it is the ‘electronic nicotine’ that is the core of our question and not the ‘delivery system’, we chose to only use ‘electronic nicotine’.
  • ‘Vaporizers’ is also a common term used for the smoking equipment. However, as vaporizers are also used in other contexts (e.g. inhaling medications), it is important to relate the term to smoking or nicotine to prevent retrieval of irrelevant references. Therefore the latter must be combined within brackets/parentheses with AND, or their respective search lines can be combined with AND.
  • ‘Airway disease’, ‘lung disease’ and ‘pulmonary disease’ are synonyms to ‘obstructive lung disease’ and are therefore included as alternative text words.
    MeSH-terms Emtree-terms Text words
    Electronic cigarettes -Electronic Cigarettes -electronic cigarette -electronic cigaret*
    -e-cigarette*
    -electronic nicotine
    -e-nicotine
    Vaporizers and tobacco smoking -Nebulizers and Vaporizers
    -Tobacco products
    -Smoking
    -Tobacco Use Cessation Products
    -Tobacco use
    -Smoking cessation
    -Nicotine
    vaporization
    – smoking
    – smoking cessation
    – smoking cessation program
    – nicotine
    – vapor*
    – vapour*
    – vaper*
    – vaping
    – vaporizer*
    – tobacco
    – nicotine
    – smoke
    – smoking
    Passive smoking -Inhalation Exposure
    -Tobacco Smoke Pollution
    – passive smoking
    – exposure
    – passive smoking
    – tobacco Smoke pollution
    – second hand smoking
    Lung disease – Asthma
    -Bronchitis
    -Pulmonary Disease, Chronic Obstructive
    – Lung Diseases, Obstructive
    – obstructive airway disease
    – asthma
    – bronchitis
    – chronic obstructive lung disease
    – asthma
    chronic obstructive pulmonary disease
    – chronic obstructive airway disease*
    – COPD
    – obstructive pulmonary disease*
    – obstructive airway disease*
    – obstructive lung disease*
  • By combining text words and subject terms on the same concept with OR you will get either of the variations and probably retrieve all literature concerning that concept.
  • Then combine all search terms across concepts with AND.
  • Our search terms are then applied in the chosen databases. The database syntax is considered and we adapt the search strategy for each specific database.
  • As electronic cigarettes are quite a new phenomenon, we have not applied any publication limitations. The topic itself will limit by date.
  • To retrieve all literature on the topic we have not applied any language limitations.

Searches in PubMed, Embase Ovid and Web of Science can look like this

Date for searches: 6th October 2015.

Example PubMed search strategy

# Searches Results
#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

#
Searches Results
1
electronic cigarette/
1183
2
(Electronic cigaret* or E-cigarette* or electronic nicotine or e-nicotine).tw.
1262
3
1 or 2
1467
4 Vaporization/ 3729
5
(vapor* or vapour* or vaper* or vaping or vaporizer*).tw.
45443
6
4 OR 5
46229
7
Smoking/ or smoking cessation/ or smoking cessation program/ or nicotine/
271967
8
(Tobacco or Nicotine or Smoke or Smoking).tw.
323191
9 7 OR 8 407264
10 6 AND 9 1264
11 3 OR 10 2517
12
passive smoking/ or exposure/
159848
13
(passive or pollution or second hand or exposure).tw.
951121
14 12 OR 13 964912
15
obstructive airway disease/ or asthma/ or bronchitis/ or chronic obstructive lung disease/
294695
16
(asthma or COPD or obstructive pulmonary disease* or obstructive airway disease* or obstructive lung disease*).tw.
238289
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

Searches Result
1 TOPIC: (e-cigarette* OR Electronic Cigarette* electronic nicotine OR e-nicotine) 973
2

 

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.

  • In total, from the three databases there are 235 references (respectively 123, 77, 35).
  • After automatic deduplication in Endnote there are 166 references left.
  • After manual deduplication there are 158 references left.
  • The total result is 158 references.
  • A high number of references will be excluded by employing the inclusion and exclusion criteria when screening the title and abstract, and by reading the full text of articles/publications.
  • The final list of studies are those included in your review.

Document your search

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.

Include

  • Names of databases
  • Which vendor/host of databases
    (did you use Medline through PubMed or Ovid? Embase on Ovid or Embase.com?)
  • Last date for the search or alert service
  • Which search words were used; include both subject headings and text words
  • How the search words were combined
  • Which limitations were used: publication dates, study designs, languages, etc.
  • If reference lists were reviewed
  • Number of retrieved records
  • Number of internal duplicates/number of duplicates removed
  • Number of articles after removal of duplicates

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

  • Describing the search strategy in plain text in the methodology section of your manuscript.
    This is where you inform your readers how you made your choices and proceeded.
  • Documenting precisely how your literature search was performed, by saving or copying the search strategy from each database. This will fit in an appendix to your article and can be presented in line with the examples in ‘Translate a search between databases’.
  • A flow chart can illustrate the workflow from the initial search results to the final studies included. The PRISMA flow chart is a well-known chart for documenting the search process. The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines is a set of minimum criteria for reporting systematic reviews and meta-analyses. It includes a chapter on reporting the literature search..

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.

References
– 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.

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