Structured literature reviews – A guide for students

There are several different kinds of literature reviews, but any literature review typically includes an extensive literature search. Whenever a systematic approach is used, the literature search features a methodical step-by-step procedure. However, as a Master's student, it might not be possible to fulfill all the criteria of a systematic review when writing a literature review-based thesis; you should rather do a structured literature review, which will include only certain aspects of the systematic review methodology.

In this guide we will go through the different steps of a structured literature review and provide tips on how to make your search strategy more structured and extensive. Additionally, make sure to follow any programme and course specific requirements.

Step 1: Formulate and delimit your research question

The formulation of your research question is partly connected to what kind of literature review you are doing. This article by Maria J. Grant and Andrew Booth usefully compares different kinds of reviews. While a systematic literature review is usually grounded in a clearly delimited and structured question, a scoping review may, for instance, feature a wider problem formulation. The wider a research question is, the larger number of search hits it tends to generate.

To be able to perform a literature review, you need to consider a subject area in which there seems to be a sufficiently large number of original research studies. Therefore, it may be a good idea to test search a database for previous research on the subject while you are trying to formulate and delimit your research question.

One way to structure your research question is to break it down into different parts. A well-delimited question often consists of three to four different parts. PICO and PEO are two examples of frameworks that can help you identify and define your research question.

• PICO (Population, Intervention, Comparison, Outcome) is primarily used for quantitative research questions.
• PEO (Population, Exposure, Outcome) is primarily used for qualitative research questions.

Structuring your research question in accordance with a framework, such as PICO or PEO, will also help you decide on the inclusion and exclusion criteria of your literature review.

After you have delimited your research question, you also need to identify the key concepts that make up your question. Based on these key concepts, you will create "search blocks" that you will use to organise your search terms.

Step 2: Find search terms and create search blocks

While working on a literature review-based thesis, you will need to search for articles on several occasions. In the beginning of your project, it is often good to do a couple of unstructured and simple search queries, so-called test searches, in academic databases. This way you are off to a good start, as test searching helps you investigate the terminology of your subject area and find relevant search terms. While the final search strategy is typically reported in full, you don't need to present your test queries in your thesis.

Try to find a couple of key articles, that is, articles that correspond to the type of studies that you are planning to include in your review. Use key articles to gather additional search terms for your final search strategy. Analyse the terminology of your key articles by examining what subject headings (MeSH terms, etc.) that the articles have been tagged with and what words that occur in the titles and abstracts.

To retrieve as many relevant studies as possible, you will need to include free-text search terms as well as subject headings in your final search strategy. Free-text search terms are words that occur in the article's title and abstract – words used by the authors themselves. Subject headings are subject-related words that an article is tagged with when the article is added to the database.

• In PubMed, articles are tagged with MeSH terms (Medical Subject Headings). You can look up and browse MeSH terms in the US National Library of Medicine's MeSH database.
• Databases such as CINAHL, PsycInfo, ERIC, and Sociological Abstracts have their own subject heading lists; look up subject headings in each database's subject heading list.
• There are also so-called free-text databases, such as Web of Science. These databases lack subject heading lists. Hence, when searching a free-text database, you can only use free-text search terms.

An effective way to increase the structure of your final search strategy is to arrange your search terms in so-called search blocks. Create your search blocks based on the key concepts of your research question.

This search strategy worksheet might help you document and organise your search terms.

Step 3: Search in a structured way

When doing a literature review-based thesis it is often wise to use at least two different databases. Many databases overlap, but may also contain unique content. At KI it is common for Master's students to use PubMed and Web of Science when doing a structured literature review as part of their Master's thesis. Depending on your research question, other databases may also be appropriate and useful. Read more about the most frequently used databases at KI.

Your search strategy should be as uniform as possible in every database. However, as mentioned in Step 2, databases may use different subject headings, and some databases only let you use free-text search terms. This means that you need to adapt your use of subject headings depending on the database.

There are many different ways of searching databases. Most databases have one simple, basic Google-like search box and one advanced search form. One advantage of the latter is that combining search terms with AND and OR is usually easier in an advanced search form, especially if you will be using both AND and OR within the same search query. However, you can often combine search terms with AND and OR in a basic search box too, and in that case, you often isolate your different search blocks from each other by enclosing each block in parentheses.

By choosing the advanced search form you will also be able to exert more control over your search process. The advanced search form lets you specify more closely and decide exactly how you want the database to interpret your search terms; this way you can make your search query more precise.

You should always document your search strategy in order to remember what search terms you have used, how these search terms have been combined, and whether you have applied any limits to your search. The easiest way to do this is to copy and paste your search history from the database into a text document. Also, academic databases often let you create a personal account, so that you may save your searches online.

Step 4: Narrowing or broadening your search

Prepare yourself for having to modify and redo your search query several times, before deciding on your final search strategy. After you have combined all your search terms and made your very first database search, you should examine the search results and analyse whether your search query is able to generate the type of search hits that you are looking for.

It is important to remember that there is nothing wrong, per se, if your search query generates irrelevant hits. This is quite normal when performing a structured literature search. What's important is that your search strategy is able to retrieve the type of articles that you are looking for, and that you are not overwhelmed by the total number of hits (given the time frame of your thesis project).

We recommend that you use the advanced search form when improving your search strategy. By using the advanced search form, you will for example be able to specify which search fields your search terms must be present in.

Step 5: Select and review articles

Selecting articles

When you have completed and feel satisfied with your search, it is time to go through all the search hits and select which studies to include in your review. All relevant studies, that is, those studies that correspond to your research question and your previously set inclusion criteria, should be included. You decide on the relevancy of a study primarily by reading through the title and abstract. If you feel unsure, go through the whole article. You can describe your selection process with the help of a flow chart, such as the frequently used PRISMA flow diagram.

One of the challenges of systematic literature searches is that the search strategy should be exhaustive, but at the same time the number of search hits also needs to be kept within reasonable boundaries. A search query needs to be broad enough to retrieve all relevant studies, but on the other hand, this also means that a large portion of the search results will be irrelevant. Hence, even though your search strategy may have generated hundreds of hits, it is fine to only include ten to twenty articles in your review in the end.

Saving articles

If you create a personal account in a database it will be easier for you to save any references that you may find there. Another way of saving and organising article references is to use reference management software. There are several different reference management software, for example Endnote Online and Zotero.

Read more about reference management and see software guides.

Reviewing articles

When you have made your selection, you should critically examine the quality of all articles included in your review. The assessment is typically performed with the help of a critical appraisal guide or checklist. The purpose is to assess the reliability of the study results and whether there are any methodological flaws that may have impacted the results. Qualitative research articles are often reviewed with a focus on authenticity, credibility, and validity.

There are many different critical appraisal worksheets and checklists. Some examples are the SBU checklists for assessing the quality of randomized studies, observational studies, and qualitative research. In the course book How to do a systematic review in nursing there is a review guide that can be used for assessing different kinds of studies (both qualitative and quantitative); the original source is Caldwell, Henshaw & Taylor, 2011.

Review worksheets and checklists contain criteria and questions that may help you identify flaws, errors, or bias. Sometimes different aspects of the study are scored separately. Later, all scores make up a final score that indicates whether the study is of high, medium, or low quality.

Many programmes and courses provide instructions on which checklists to use when reviewing articles, so check your course guidelines.

Step 6: Report your search strategy

An important aspect of doing a structured literature review is transparency. It has to be easy for your readers to follow what you did when you searched and selected the articles that you have included in your review. In the method section of your literature review you should describe how you searched different databases. This is also where you describe any manual searches that you did. Search strategies are commonly reported in the form of tables. Present one table for each database.

You can examine your search terms and any limits you have applied when searching a database by visiting its search history.

Read more about how to report your search strategy and view examples.

Checklist for search strategies

Here is a checklist to help you review your own or someone else's search strategy.