The purpose of a literature search is to obtain an overview of the current state of the field; that is, what is currently known about a specific topic and what questions remain unanswered. There are a variety of reasons why you may wish to perform a literature search over the course of your research career. Some of the most common are as follows:
- Designing a project - A major motivation for performing a literature search is to help identify a research question that is worth pursuing. Performing a thorough and complete literature search can help identify a gap in our knowledge, understanding, or capabilities that is worth investigating.
- Writing a Background section - The role of the Background section of an academic paper is to explain to readers how your study fits into the wider field. Performing a literature search for previously published articles related to your study is an important first step in gathering the information and resources you need to write this section.
- Writing the introductory chapter to your thesis - Similar to a Background section, the introduction to a thesis provides the context for a project by describing what the state of the field is, what unanswered question the project addressed, and why answering this question is important and/or useful to other researchers. Performing a literature search can help you put together a strong argument for the relevance and significance of your work to the field at large.
- Writing a review paper - When writing a review of a specific topic, performing a thorough literature search is crucial to ensuring that you have adequately covered the state of the field and have not missed any important or relevant papers.
How to perform a literature search
The first step in performing a literature search is to generate a list of keywords related to the main theme that you are interested in exploring. These keywords should ideally be specific enough to retrieve a reasonable number of responses, without being so specific that they unintentionally exclude relevant papers. For example, if the topic you are interested in is the use of different imaging modalities for detecting prostate cancer, the search terms ‘cancer’, ‘prostate cancer’, and ‘imaging’ are likely to be too broad, and will retrieve an unmanageably large number of references. On the other hand, simply searching for ‘prostate cancer AND computed tomography (CT)’ is too specific, as this search will not retrieve studies that used magnetic resonance imaging (MRI) to detect prostate cancer, for example. Ideally, you will want to come up with a list of phrases and words that can be searched for in different combinations to retrieve as much relevant material as possible, without requiring you to sift through too many irrelevant results. Using the example above, a reasonable search strategy might include the following list of terms:
- prostate cancer AND magnetic resonance imaging
- prostate cancer AND computed tomography
- prostate cancer AND bone scintigraphy
- prostate cancer AND detection AND imaging
Note the use of Boolean operators (AND, OR, and NOT), which can help you put together a master search phrase instead of searching multiple phrases, if you prefer. For example, the first three items on the list above could be condensed into a single search string as follows: ‘prostate cancer AND (magnetic resonance imaging OR computed tomography OR bone scintigraphy)’. It is also good practice to search for both full terms and abbreviations (e.g. ‘magnetic resonance imaging’ and ‘MRI’) for the sake of completeness.
The next step in performing your literature search is to identify which database or databases you wish to search. This will vary depending on your field, but the most commonly used databases in the biomedical sciences are PubMed, Scopus, Web of Science, and GoogleScholar. It can often be helpful to search more than one database, again for completeness, so consider carefully whether this might be useful in your situation. If you are unsure, you can always try performing the identical literature search in two or more databases and comparing the results to see if they retrieve similar items.
One thing to keep in mind when selecting a database to search and performing the search itself is that most database default to searching only English-language content, which includes articles published in English, as well as articles published in other languages accompanied by an English translation of the abstract. While this is acceptable for many literature searches, in some cases you may wish to specify the language(s) of the target material. For example, if you are specifically interested in the use of different imaging modalities to detect prostate cancer in rural China, then it would seem useful to search the Chinese-language literature as well as the English-language literature, to retrieve papers that may have been published in regional journals.
How to refine and shortlist the papers retrieved by your literature search
Once you have performed your initial literature search and are satisfied with the breadth and volume of the references that you have retrieved, it is time to start evaluating this list of potentially relevant papers in more detail to determine which are most pertinent to your topic of interest. To do this, we recommend beginning by reading the abstracts and asking yourself whether the paper is directly related to your research question or topic of interest, or whether the connection is only peripheral. For example, the main focus of the paper may be an unrelated topic, even though the text contains the target search terms, so reading the abstracts can help rule out papers that will not ultimately be helpful to you. Only the most relevant papers should be included in your shortlist.
Performing a preliminary evaluation of the papers that you retrieved from your initial search also provides a useful opportunity to identify other potentially relevant papers. While reading through the abstracts from your first search, we recommend keeping an eye out for any keywords or search terms that appear in these texts that you may have inadvertently overlooked in your original search. If this is the case, you can then update your search strategy accordingly and add any new papers that this additional search retrieves to the batch that you are evaluating to add to your shortlist.
Another helpful strategy when shortlisting your literature search results is to identify the most important or seminal papers related to your topic, and explore them in more detail. For example, using the sample search strategy described above, you may retrieve a study that investigated MRI imaging for prostate cancer detection in your target population. In this case, we recommend searching for any other recent papers from the same research group, as they are likely to have explored very similar questions, as well as reading through the Discussion and scanning the reference list to identify any related papers that may not have come up in your original search.
Finally, consider using online search tools or databases to identify any papers that cite the most important or interesting papers that you retrieved in your search. PubMed, for instance, makes this easy by providing a list of such papers to the right of the abstract in each search result.
Any additional papers that you identify using these follow-up methods can then be evaluated and shortlisted, if appropriate, as described above.
How to organise the shortlisted results from your literature search
Once you have curated your initial search results into a shortlist containing only the most relevant and interesting papers, it is important to organise this shortlist in a way that will be useful to you when formulating your research question or writing your Background section, thesis introduction, or review article. If you have access to a reference management tool, this is one of the best ways to organise a collection of papers. All reference management tools can be used to record references for a collection of papers, and many also offer an option to store full texts of papers in PDF format.
You can also store downloaded papers on your computer; if you choose to do so, we recommend developing a file name strategy that makes it easy for you to identify a specific paper without reading through all of the possibilities. It can also be helpful to write up brief summaries of all of the papers in your shortlist (a few sentences at most) highlighting the most important features with regards to your topic, as well as any shortcomings or limitations. For example, a paper selected using the strategy described above could be summarised in your personal archive as follows: ‘CT-based prostate cancer screening in men aged 55-70 in tertiary hospitals in France; small sample size; findings inconclusive’.
Finally, organising all of the papers on your shortlist into subcategories can provide the dual advantages of making each individual paper easier to find, as well as helping to structure your thinking when you begin to write. For example, dividing all of the shortlisted prostate cancer detection papers into three categories based on imaging modality (CT, MRI, bone scintigraphy) can help create a logical approach to the structure of a Background section or thesis introduction you are writing (e.g. one imaging modality per paragraph or subsection, respectively), or help narrow down the possibilities when generating a new research hypothesis.
In summary, performing a thorough literature search by identifying appropriate search terms, expanding the list of retrieved items by reading key papers in more detail, and creating a targeted shortlist by evaluating abstracts in more detailed can help ensure that you are well-informed and prepared to begin writing, or starting a new project.
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