Literature Searching and Reviews

When writing an academic paper or review, you need to read and study the literature on a topic, issue or dilemma you are working on. Time must be set aside to search for, process, select and evaluate the literature.

Literature searches, for example when preparing a review, consist of a number of steps. Whether some or all steps are necessary for you depends on the purpose of the literature search. If it is for a review, it is also relevant to consider what type of review you are aiming for. Types of reviews

The literature search is an iterative or circular process and it takes time. There are different ways to search for literature, the most systematic approach is the block search. The purpose of a literature search is to collect the evidence and knowledge that will form the basis of a paper or review. A systematic literature search is basically a properly conducted and thought-out literature search.

A good literature search is a combination of controlled keywords (if available in the database) and free text words.

When conducting a more complex literature search that consists of multiple search terms and you intended to find more specific literature, you should divide your literature search into blocks. Block searching is a systematic approach to searching.

Block searches use the Boolean operators AND, OR (and NOT). 

OR: The Boolean operator OR is used to include keywords/terms that are considered to be synonyms or closely related. This means that there must be at least one of the entered keywords, e.g. crime OR violence. Entering this into a database will return search results containing either crime or violence or both crime and violence.

AND: The Boolean operator AND is used to combine keywords / subject terms that both occur. This means that both of the entered keywords must be mentioned, e.g. crime AND abuse. Entering this will return search results containing both crime and abuse.

NOT: Using the Boolean operator NOT will omit search results that contain the following keyword/phrase. This means that if you enter crime NOT women, only search results containing crime but not women will appear.
It is rarely appropriate to use NOT, as this function is unfortunately very likely to exclude relevant literature.

There are more search techniques

Citation searching provides insight into how a publication cites other publications and how the publication itself is cited and can be a good way to find relevant literature. There are many terms that describe citation searching, such as pearl growing, snowballing, citation chaining, forward and backward chaining (1). Citation search is a general term for different methods for identifying potentially relevant publications - methods that are based on citations (or co-citations, where publications cite the same references) (2). It can be a good idea to perform forward and backward citation searches, especially if you are preparing reviews / overview articles where you want as exhaustive a search as possible:

• forward citation search: you identify which publications have cited a selected publication (“seed reference”). When preparing a review article, “seed references” will typically be included studies/publications (2). Forward citation search is performed in a citation index such as Scopus, Web of Science or Google Scholar.

• backward citation search: using a Citation Index such as Scopus, Web of Science or Google Scholar, you identify the publications that your “seed reference” has cited (i.e. the reference list of your “seed reference”). If you are preparing a review article, seed references will typically be included studies/publications. Backward citation search is thus a review of the reference list via a citation index (2). “Reference list check” also refers to reviewing the reference list, but only manually (i.e. not using Scopus, for example). It can be a good idea (if you are preparing a review article) to do a backward citation search, as you can then export the reference list to Covidence, sort the results and have an overview.

By reviewing journal tables of contents, you can quickly get an overview of the latest topics and articles, making it easier to find relevant research. Electronic journals are continuously updated which means you always have access to the latest journals in your field. This ensures that your academic paper or work is based on the most up-to-date data and theories. Tables of contents can also provide inspiration for new perspectives and angles on your topic.

In principle, a systematic literature search is no different from a properly conducted literature search. It is recommended that your search has a high recall (a theoretical measure of how much of the relevant literature in a database is identified in the search (3), which can be synonymous with low precision (how much of the searched result is relevant) (4).
In practice, this means that when conducting a comprehensive, systematic literature search the aim is to gather as much evidence/literature on a topic as possible, this will often result in some irrelevant literature (low precision). 

Preperation and reporting the systematic literature search

In order to perform a systematic literature search, it is important to divide/structure your search into correct search blocks (see the point above about this), so that you can then create lists of keywords. If you have used a conceptualization model (e.g. the PICO model), this can help you identify the most important elements to search for.

Preparing the systematic literature search itself requires a good knowledge of complex literature searches, including controlled keywords, subject coding, free text searches and search techniques, as well as how each of the databases to search works. There is a lot of work involved in preparing the search before you can execute the final search and proceed with the search result. Bramer et al. describe how to plan and perform a systematic literature search step by step (5).

A critical evaluation/peer review of the specific literature search is important, and the PRESS 2015 Guideline Evidence-Based Checklist (published in McGowan et al. (6) can be used for this.

The validity and reproducibility of the systematic search is essential. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) has developed a PRISMA for Searching, which is a checklist of essential items that should be reported in connection with a systematic literature search (7). The checklist ensures that what should be included in a systematic search is reported and clear, so that the search is reproducible, valid and transparently conducted. The checklist can also be used prospectively, i.e. in the planning of the systematic search. See Reporting and PRISMA.

1. Gusenbauer M. Beyond Google Scholar, Scopus, and Web of Science: An evaluation of the backward and forward citation coverage of 59 databases' citation indices. Res Synth Methods. 2024, doi: 10.1002/jrsm.1729.

2. Hirt J., Nordhausen T, Fuerst T, Ewald H, Appenzeller-Herzog C, TARCiS study group. Guidance on terminology, application, and reporting of citation searching: the TARCiS statement. BMJ. 2024 May 9:385:e078384. doi: 10.1136/bmj-2023-078384.

3. Frandsen TF, Dyrvig AK, Christensen JB, Fasterholdt I, Ølholm AM. En guide til reproducerbare og systematiske litteratursøgninger. Ugeskr Læger (2014); 176: V02130141. 

4. Lefebvre C, Glanville J, Briscoe S, Littlewood A, Marshall C, Metzendorf M-I, Noel-Storr A, Rader T, Shokraneh F, Thomas J, Wieland LS. Chapter 4: Searching for and selecting studies. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.0 (updated July 2019). Cochrane, 2019. Link.

5. Bramer, W.M., et al., A systematic approach to searching: an efficient and complete method to develop literature searches. J Med Libr Assoc, 2018. 106(4): p. 531-541.).

6. McGowan J, Sampson M, Salzwedel DM, Cogo E, Foerster V, Lefevbre C. PRESS Peer Review of Electronic Search Strategies: 2015 Guideline Statement. J Clin Epidemiol (2016); 75: 40 – 46.

7. Rethlefsen ML, Kirtley S, Waffenschmidt S, Ayala AP, Moher D, Page MJ, Koffel JB; PRISMA-S Group. PRISMA-S: an extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews. Syst Rev. 2021 Jan 26;10(1):39. doi: 10.1186/s13643-020-01542-z.).

A hermeneutic literature search differs from the systematic approach, being interpretive, iterative and context dependent. In contrast to the reproducible and linear systematic search, the hermeneutic approach is characterized by a circular movement between understanding and preunderstanding, where search and analysis develop mutually. This means that the search process is not just a technical tool, but an integral part of the epistemological process.

Planning and conducting the hermeneutic search strategy
A hermeneutic search strategy requires that the literature search is included as a reflective and theoretically grounded part of the research design. The search should be based on central texts or key concepts, which are gradually expanded through citation searches, chain searches and thematic filtering. Key text could be a review article or encyclopedia entry—which introduces core concepts and vocabulary. Keywords and databases are not selected based on fixed criteria but are continuously adjusted as the understanding of the problem area develops.

It is recommended to document the search process in a protocol or search log where the choice of sources, databases and search techniques are justified based on the research question and the theoretical framework. The hermeneutic circle involves moving between the whole (the broader literature) and the parts (individual texts), with each iteration enhancing understanding. Techniques such as snowballing, citation pearl growing, and identifying core journals or authors are used to expand the literature base. Unlike systematic searches, the hermeneutic approach allows for flexibility, enabling the research question to evolve alongside the literature review.

Validity and academic transparency
In a hermeneutic search, validity is not linked to reproducibility, but to the researcher's ability to reflect on their own choices and preconceptions. It is therefore crucial that the search process is transparent and theoretically anchored so that the reader can follow the analytical movement from search to interpretation. This approach is particularly suited to research in the humanities and social sciences, and qualitative research, as questions are often open-ended, and understanding is developed through interpretation.

Based on Boell, S. K., & Cecez-Kecmanovic, D. (2010). Literature Reviews and the Hermeneutic Circle. Australian Academic and Research Libraries, 41(2), 129–144. https://doi.org/10.1080/00048623.2010.10721450

A literature search based on grounded theory differs from both systematic and hermeneutic searches by being inductive and iterative. It is recommended that you work with an open and flexible approach in your search, where literature is not used to confirm a predefined hypothesis, but to develop concepts and theory based on data. In practice, this means that the literature search often takes place parallel with empirical data collection and analysis, it is continuously adjusted as new insights emerge.

Developing and applying the grounded theory search strategy
To conduct a literature search in accordance with grounded theory, it is important to consider the search as part of the theoretical coding and concept development. The search process can take different forms, e.g. literature can be included before, during or after empirical data collection - depending on the research design. The essential point is that literature is used analytically and reflexively, and that the search supports the development of situation-specific theories. The back-and-forth movement between data and literature searching mirrors the iterative nature of Grounded Theory. The goal is not to confirm existing theories but to build new ones that are grounded in empirical observations. Literature is used to enrich and contextualize the developing theory, not to constrain it.

The validity of this approach lies not in reproducibility, but in the researcher's ability to document and justify their analytical choices. It is therefore recommended to keep a protocol or search log where you continuously note how search terms, databases and findings relate to the concepts and patterns that emerge in the analysis. Over time, the researcher develops a deeper understanding of the topic, refining search terms and identifying key authors, journals, and concepts. The search can be considered complete when new sources no longer add significant insights—this is known as reaching “saturation.” At this point, the researcher typically has a strong grasp of the main ideas and debates in the field, providing a solid foundation for the final theory. The grounded theory search strategy is particularly suitable for humanities and social sciences and qualitative studies, where theory is developed in close correlation with empirical data and context.
 

Based on Corbin, J. M., & Strauss, A. (1990). Grounded theory research: Procedures, canons, and evaluative criteria. Qualitative Sociology, 13(1), 3–21. https://doi.org/10.1007/BF00988593                          

In addition to the literature referenced under References, the following articles and books describe systematic literature searches:
Harter, S. P. (1986). Online information retrieval : concepts, principles and techniques. Orlando

For Health science:

• Frandsen, T. F., Gildberg, F. A., & Tingleff, E. B. (2019). Searching for qualitative health research required several databases and alternative search strategies: a study of coverage in bibliographic databases. Journal of Clinical Epidemiology, 114, 118-124. https://doi.org/10.1016/j.jclinepi.2019.06.013
• Frandsen TF, Eriksen MB. Planlægning af systematisk litteratursøgning.[Planning the searches for a systematic review]. Ugeskrift for Læger 2020;182:V10200722 [danish].
• Frandsen TF, Eriksen MB. Anvendelse af søgefiltre i systematiske søgninger. [The use of search filters in systematic searches]. Ugeskrift for Læger 2022;184: V12210901 [danish].
• Frandsen TF, Eriksen MB. Afrapportering af systematiske søgninger. [Reporting of systematic searches]. Ugeskrift for Læger 2023;185:V06230420 [danish].
• Lefebvre C, Glanville J, Briscoe S, Featherstone R, Littlewood A, Metzendorf M-I, Noel-Storr A, Paynter R, Rader T, Thomas J, Wieland LS. Chapter 4: Searching for and selecting studies. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.4 (updated October 2023).