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Systematic Literature Review

Overview of Systematic Literature Review (SLR)

Developing a search strategy

Researchers conducting a systematic literature review need to perform comprehensive searches to ensure they have retrieved all of the relevant information. Below is an overview of the steps involved in conducting a search for literature. For further information on conducting a comprehensive search, please see the Cochrane handbook.

Scope the topic. This will help to contextualise your topic within the broader context of your subject and also indicate the volume of literature available on your subject.  It also gives an opportunity to refine your search strategy to ensure an effective and accurate search.

Inclusion/Exclusion criteria is used to identify the specific attributes of material that you want to include in the review.  For example, the type of study or population, etc.

Identify the concepts in your research question.  You can use a framework to help with this, such as PICO, SPIDER or another suitable method.

Identify keywords, synonyms and alternative keywords.  You can develop your synonyms and alternative keywords using relevant articles or papers that you are already aware of.  You will also need to use subject headings to ensure you do a comprehensive search (see subject headings tab). 

Combine searches using Boolean operators, OR and AND. Use OR to combine the keywords and subject headings, and use AND to retrieve material that includes all the concepts in your research question. See the getting the best out of your searching tab for further details on truncation, phrased searching, etc. The idea of a search strategy is that it can be used across multiple sources, however, you may need to re-map your keywords to the subject headings of different databases.

Apply search filters. Many databases allow researchers to limit search results through the use of filters or hedges. For example results can be limited by publication type, study type, etc.  However, these in-built filters may exclude relevant studies.  For this reason, and to maintain consistency across databases, researchers conducting systematic literature reviews use standard filters that can be added to searches using the AND operator. The filters are re-useable and shared widely.

Sensitivity versus precision involves balancing your strategy between retrieving a large number of documents that might include some irrelevant material, against are a more precise strategy that might miss relevant material.

Review your strategy.  Make sure that your results are relevant, identify any keyword/concepts from your strategy that don't appear in your results.  Check retrieved material for any relevant keywords or subject headings that you may have omitted.

Document your search.  You need to record the details of your searches to ensure transparency and consistency. You can use a table or spreadsheet to do this. Include;

  • search terms
  • database/catalogue searched
  • refinements or limitations (if used)
  • data range
  • date searched
  • basic/advanced search
  • number of records (hits)
  • articles retained
  • articles you can't access​

Manage your results. Use bibliographic management software (Endnote, Mendeley or Zotero) to remove duplicate records. 

Select material. Use the agreed criteria to select material.  This is done in stages, initially by screening the title, then the abstract and finally by full text.  This should be done by more than one person, to help reduce bias in the process.  Having more than one person involved also means that it can be easier to resolve queries regarding selection of individual papers, should a query arise.

Move on to the data extraction phase.  For further information see chapter 5 of the Cochrane handbook.

For further information on conducting searches for SLR, please see the following useful links:​

 

There are many different types of information resources.  Students and researchers can use a combination of resources when completing assignments or research. Some of the most widely used sources are described below.

Textbooks that you find in the library are usually written by experts.  Textbooks are structured for ease of use; they begin with simpler aspects of a topic before progressing towards more complicated aspects. Books often have useful glossaries and an index of subjects and authors.  They also include references and bibliographies, which can help to expand your search.

Conference Proceedings can be very important for researchers because often, the first time research is published, is at a conference. Proceedings provide access to specialist and focused information.

Journals are published at regular intervals, such as monthly, bi-monthly or even annually.  This means that journals can be a good source of up to date information.  They can be written by either academics, specialist researchers or professional practitioners.  Some journals are peer-reviewed, this means any material published in the journal has undergone an evaluation process to ensure the quality and validity of the information.  Journals can be highly focused on a specific subject and also include references.

Official Publications are published by governments and government departments all over the world.  They can be a useful source of information on areas such as legal, education, finance, science, health and social policies.

Reference Material includes subject directories, almanacs and encyclopedia.  They can provide useful definitions and can be a good sources of primary data (statistics, speeches, diaries). There is no evaluation or interpretation of the material provided, which allows the student or researcher to use the data for their own purpose and to draw their conclusions.

Standards are an agreed set of procedures, processes or technical specifications that provide guidance across multiple disciplines and industries.

Social media, such as blogs and twitter feeds can highlight key topics and discussions that are current and fast moving. Social media has the potential to help researchers to stay up to date with developments in different disciplines.

Theses are final year research projects submitted by students completing degree and doctorate level qualifications. Theses are a valuable information source for researchers as they are highly focused on a particular topic, They will also contain a detailed literature review section, and have references and a bibliography.

Grey literature describes material and research that is not published by academic or commercial publishers.  It is often produced by professional bodies or organisations.  For example, theses, annual reports, technical guidelines, conference posters or government papers.

Other resources

  • Patents

  • Statistics

  • Datasets

​It is important to be aware of the many different sources of information that exist, so you can select the most suitable ones for your project or research. Below is a list of some of the most popular sources.

Library Catalogues provide access to a combination of print and digital material and often cover multiple disciplines. Most library catalogues can be searched by anyone, although access to material in other institutions is subject to certain permissions.

Institutional repositories provide access to the research outputs of individual academic institutions.  This includes PhD and other research theses.  OpenDOAR allows users to find repositories. RIAN allows user to search all Irish repositories, including Arrow@TuDublin.

Databases​​​ contain many different types of information resources, such as journal articles, books, conference proceedings, newspapers and reports.  The databases that are available through the library provide access to scholarly, and frequently, peer-reviewed material.  Academic databases are designed to help researchers, so many have advanced search features to facilitate accurate and focused searches.

Search Engines​​ are useful resources discovery tools.  They contain links to and information on a wide variety of subjects in various formats.  However, there is very little, if any, oversight or review of the material.  So any information taken from search engines must be thoroughly evaluated.  Although results may be largely relevance-based, some search engines manipulate how results are displayed for commercial purpose, so there is a chance that you are not seeing the most relevant information at the top of the results page. N.B. If you are doing a systematic literature review using Google, be sure to turn off personalisation by deleting your browser history, logging out of your account and removing all Google services through the "My Activity" page.  Otherwise, your results may be influenced by the search history and this may impact the relevance of your results. Consider using other specialist search engines, such as BASE.

Subject Directories provide access to curated, and sometimes annotated, subject resources.  Sites included in the directory are usually selected by an administrator according to set criteria.  The directory is navigated by clicking through options rather than a search box.  Directories contain less information than a search engine, but they can be useful for specific subject areas.  See the subject guide in your area for further information.

OpenGrey is the system for information on grey literature in Europe and provides access to 700,000 bibliographical references of grey literature produced in Europe.

LENUS provides access to the research output of many healthcare organisations in the Republic of Ireland.

Other Sources available to researchers include government & EU websites, NGOs and other voluntary and professional organisations.

 

 
  • Phrased searching

The default Boolean term for a database or search engine (unless otherwise stated) is AND.  For example, a search for -information technology- will be interpreted as information AND technology.  However, if you want to search for information on the topic - information technology, put inverted commas around both words, "information technology".  Phrased searching should decrease the number of results you retrieve but increase the relevance of those results.

  • Wildcard & Truncation

Wildcards (?/#) allow you to search for the American and British spellings of words.  For example, a search for the word behavio?r, will include both behaviour and behavior. You can also use # to include plurals of words like man or men by using m#n.

Truncation (*) allows you to include variations of a word.  For example, teach*, will include teachers, teacher, teaching and teach.

  • Subject descriptors/headings
Headings are added by the database administrator and will help you to find more resources on your topic.  Subject headings should be used to in addition to your keyword strategy. Subject headings vary from database to database, so you will need to check the thesaurus or list of subject categories in each database.
  • Snowballing
Find a key document on your topic. Examine the references and bibliography and use them to find further resources on your topic. Continue this process, working through subsequent papers, until you have the information you require.  This is a fast and effective way of finding relevant information on your topic.  However, you are searching retrospectively so you won't find new or up to date information.
  • Citation searching
Use an author or article to find relevant, subject-specific information within a field. It involves tracing references within an article, and all the articles that have referenced that article.​ This allows you to follow a discussion, to see how an idea or theory has been developed, improved or disproved.  The process can help you to add to your search terms, find relevant information and develop a bibliography.
  • Hand-searching

​Involves searching for relevant material that may have been missed by databases.  For further guidelines on hand-searching see the Cochrane handbook.

 

Subject headings are assigned by an administrator when a document is added to the database.  The headings provide a consistent description of the subject content of the document. They are also known as descriptors or categories.  They are similar to a hashtag but are added by the publisher, not the user.  Subject headings are selected from a controlled vocabulary (a list of agreed or standard terms), which is maintained and updated by an administrator.

There are different ways of locating subject headings.  Some databases allow access through a list of subject headings, descriptors, categories or provide a searchable thesaurus.  The thesaurus will allow you to enter your keyword into the search box.  The list of results should provide the subject heading closest to your keyword, and in some databases, you will be shown broader and narrower terms associated with your topic.  Alternatively, you could do a standard keyword search and identify the subject headings or descriptors assigned to one or two relevant documents.

One of the most well-known lists of subject headings is the Medical Subject Headings or MeSH, used to search MEDLINE/PubMed or the National Library of Medicine.