# How to Search for Literature in a Scoping Review: A Step-by-Step Strategy ## Quick Answer - Use a three-step search approach: database searching, grey literature searching, and reference list checking to capture the breadth a scoping review requires. - Document every search decision, including databases, dates, Boolean operators, and filters, so another researcher can reproduce your strategy. - Scoping reviews prioritize breadth over depth, so your search must be iterative and transparent, not a single one-time database query. ## Understanding Scoping Review Search Requirements A scoping review maps the existing literature on a broad topic to identify key concepts, types of evidence, and gaps. Unlike a systematic review that answers a narrow clinical question with a highly focused search, a scoping review requires a wider net. The search strategy must balance sensitivity, finding as many relevant studies as possible, with specificity, avoiding an unmanageable volume of irrelevant results. The [National Library of Medicine](https://www.ncbi.nlm.nih.gov/books) hosts research methods resources that describe the distinctions between review types and the expectations for evidence synthesis. These resources emphasize that the search phase is foundational because every subsequent step, screening, data extraction, and synthesis, depends on what the search retrieves. Researchers often underestimate the time and skill required for a proper scoping search. A search that misses key databases or uses overly narrow terms will produce a map that does not reflect the true state of the literature. A search that is too broad will return thousands of records that overwhelm the screening team. The goal is a reproducible, transparent process that another researcher could follow and obtain comparable results. ## Core Principles of a Scoping Search Strategy ### Iterative Search Development Scoping review searches are iterative. You do not write a final search string on the first attempt. Instead, you develop a preliminary strategy, test it against known relevant articles, and refine the terms based on what you find. This process may repeat several times before the search is finalized. The iterative nature means you must track every version of your search strategy. Record the date, the database, the exact string used, and the number of results returned. This documentation becomes part of your review methods and allows readers to assess the rigor of your approach. ### Transparency and Reproducibility Transparency is a core expectation in evidence synthesis. The [EQUATOR Network](https://www.equator-network.org/) provides reporting guidelines that help researchers present their methods clearly. For scoping reviews, the PRISMA-ScR checklist is the relevant guideline, and it requires detailed reporting of the search strategy, including the full search strings for at least one database. Reproducibility means another researcher can take your documented strategy and run it in the same databases to obtain the same set of records. This requires exact search strings, not paraphrased descriptions. Include database-specific syntax, field tags, and filters in your appendix. ### Peer Review of the Search A search strategy benefits from peer review. A colleague who is not familiar with your topic can test whether your search terms are logical and whether the strategy is likely to miss relevant studies. This is a quality control step that catches errors in Boolean logic, spelling, or database syntax before you commit to the full search. ## The Three-Step Search Approach The [National Library of Medicine](https://www.ncbi.nlm.nih.gov/books) research methods resources describe a structured approach to searching for evidence syntheses. The three-step method is widely taught and applied in scoping reviews. ### Step One: Database Searching The first step is a structured search of bibliographic databases. For life sciences and biomedical topics, this typically includes PubMed or MEDLINE, Embase, and Web of Science or Scopus. For topics that cross into agriculture, veterinary science, or environmental science, additional databases such as CAB Abstracts, Agricola, or AGRIS may be relevant. Develop a search string using Boolean operators. Combine synonyms for each key concept with OR, then combine the concepts with AND. For example, if your scoping review addresses antimicrobial use in dairy cattle, your concepts might be: - Concept 1: dairy cattle OR dairy cows OR bovines - Concept 2: antimicrobial OR antibiotic OR antibacterial - Concept 3: mastitis OR udder infection OR intramammary infection The search string would combine these concepts with AND, and within each concept, synonyms are combined with OR. Use truncation symbols to capture word variants, such as antibiot* for antibiotic, antibiotics, or antibacterial. Use quotation marks for exact phrases and parentheses to group terms correctly. Database-specific syntax matters. PubMed uses [Mesh] for controlled vocabulary terms and [tiab] for title and abstract searches. Embase uses Emtree terms and different field tags. Web of Science uses TS= for topic searches. You must adapt your strategy to each database instead of copying the same string verbatim. ### Step Two: Grey Literature Searching Grey literature includes reports, theses, conference proceedings, government documents, and other materials not published in commercial academic journals. Scoping reviews often include grey literature because it captures current practice, policy documents, and research that has not undergone traditional peer review. Search grey literature through targeted sources. These may include: - Government agency websites, such as the USDA or FDA for agricultural topics - Institutional repositories for theses and dissertations - Conference proceedings and abstracts - Professional organization websites - Preprint servers such as bioRxiv or medRxiv Grey literature searching is less standardized than database searching. You cannot apply the same Boolean syntax to a government website search box. Instead, use a structured approach: identify the key sources, search each with a simplified set of terms, and document what you find and what you do not find. The [National Institutes of Health](https://grants.nih.gov/) provides guidance on grant-funded research expectations, and many NIH-funded projects produce reports and data that appear in grey literature sources. Searching these sources can capture findings that never appear in peer-reviewed journals. ### Step Three: Reference List Checking The third step involves checking the reference lists of included studies and relevant reviews. This is sometimes called backward citation searching. When you read an included study, examine its reference list for additional studies that your database search may have missed. Forward citation searching is the complementary approach. Use a citation index such as Web of Science or Scopus to find articles that have cited a key included study. This identifies newer research that builds on the original work. Reference list checking is labor intensive but valuable for scoping reviews because it catches studies that use unusual terminology or appear in journals not indexed in your chosen databases. This step also helps you identify seminal works in the field that may have been published before the databases began comprehensive indexing. ## At a Glance: Search Strategy Components | Component | Purpose | Key Decisions | |-----------|---------|----------------| | Database search | Retrieve peer-reviewed literature from indexed sources | Which databases, which Boolean operators, which field tags | | Grey literature search | Capture reports, theses, and policy documents | Which websites, which simplified terms, which date range | | Reference list checking | Identify studies missed by database searches | Which included studies to check, whether to do forward citation searching | | Search documentation | Ensure transparency and reproducibility | What to record, where to store the records, who reviews the strategy | ## Developing Your Search Terms ### Identifying Key Concepts Start by breaking your research question into its main concepts. A scoping review question often follows the PCC framework: Population, Concept, and Context. For each element, list all the terms that authors might use to describe it. For example, if your question is about biosecurity practices on small-scale poultry farms, your concepts might be: - Population: poultry OR chickens OR layers OR broilers - Concept: biosecurity OR disease prevention OR hygiene protocols - Context: small-scale OR backyard OR smallholder OR family farms Do not limit yourself to the terms you would use. Consider how different authors, disciplines, and countries might describe the same concept. A term used in the United States may differ from the term used in the United Kingdom or Australia. ### Using Controlled Vocabulary Most databases have a controlled vocabulary. PubMed uses Medical Subject Headings (MeSH), Embase uses Emtree, and CAB Abstracts uses CAB Thesaurus terms. Controlled vocabulary terms capture articles that use different wording for the same concept. Combine controlled vocabulary terms with free-text terms. A search that uses only MeSH terms will miss recent articles that have not yet been indexed or articles that use novel terminology. A search that uses only free-text terms will miss articles that use different synonyms than you anticipated. The [National Library of Medicine](https://www.ncbi.nlm.nih.gov/books) resources explain how to build searches that combine both approaches. The goal is a search that captures the breadth of the literature while remaining manageable in size. ### Testing and Refining Terms Test your search strategy against a set of known relevant articles. These are articles that you already know should be included in your review. Run your search and check whether it retrieves these articles. If it misses any, examine why and adjust your terms. This testing process is iterative. You may need to add synonyms, adjust truncation, or change field tags. Document each version of your search so you can describe the development process in your methods. ## Boolean Operators and Search Syntax ### AND, OR, NOT Boolean operators are the building blocks of database searches. Use OR to combine synonyms within a concept, which broadens the search. Use AND to combine different concepts, which narrows the search to records that address all concepts. The NOT operator is rarely used in scoping reviews because it can exclude relevant records unintentionally. For example, using NOT to exclude a specific animal species might also exclude records that discuss multiple species, including your target species. Avoid NOT unless you have a clear justification. ### Truncation and Wildcards Truncation uses a symbol, usually an asterisk, to capture word variants. For example, therap* captures therapy, therapies, therapeutic, and therapist. Wildcards use a symbol, usually a question mark, to replace a single character. For example, wom?n captures woman and women. Different databases use different truncation symbols. PubMed uses an asterisk, while some databases use a dollar sign or other symbols. Check the database help documentation before running your search. ### Phrase Searching Use quotation marks to search for exact phrases. For example, "antimicrobial resistance" searches for that exact phrase instead of records that contain the individual words. Phrase searching can improve precision but may miss records that use slightly different phrasing. ### Field Tags Field tags tell the database which part of the record to search. Common fields include title, abstract, author keywords, and subject headings. Searching only the title and abstract is common for scoping reviews because it balances sensitivity and specificity. The [National Library of Medicine](https://www.ncbi.nlm.nih.gov/books) resources provide examples of field-tagged searches in PubMed. These examples show how to construct a search that targets specific fields while maintaining the Boolean logic. ## Grey Literature Search Strategy ### Identifying Grey Literature Sources Grey literature sources vary by discipline. For agricultural and veterinary topics, relevant sources include: - USDA publications and reports - FDA and EPA documents - State agricultural extension publications - University theses and dissertations - Conference proceedings from veterinary and animal science meetings - Industry association reports For biomedical topics, relevant sources include: - WHO and CDC publications - Clinical trial registries - Preprint servers - Government health agency reports - Professional society guidelines The [National Institutes of Health](https://grants.nih.gov/) website provides access to NIH-funded research outputs, including reports and data that may not appear in commercial journals. Searching these sources can identify relevant grey literature. ### Searching Grey Literature Sources Grey literature searching requires a different approach than database searching. Government websites and institutional repositories have their own search interfaces that do not support complex Boolean syntax. Use simplified search terms and document the exact terms you used. For each grey literature source, record: - The name of the source - The date you searched - The search terms you used - The number of results - Which results you included This documentation is essential for transparency. A reader should be able to see exactly what you searched and what you found. ### Managing Grey Literature Results Grey literature results may be less structured than database results. Reports may not have abstracts, and theses may be lengthy documents. Decide in advance how you will screen grey literature and how you will extract data from these sources. Some scoping reviews exclude grey literature because of the additional effort required. However, for topics where current practice or policy is important, grey literature can provide valuable context that peer-reviewed articles lack. ## Reference List Checking ### Backward Citation Searching Backward citation searching involves examining the reference lists of included studies. When you identify a study that meets your inclusion criteria, review its reference list for additional studies that may also be relevant. This step is particularly useful for finding older studies that may not be well indexed in databases. It also helps you identify the foundational literature in your field. ### Forward Citation Searching Forward citation searching uses a citation index to find articles that have cited a key included study. This identifies newer research that builds on the original work. Web of Science and Scopus both support forward citation searching. Forward citation searching is valuable for scoping reviews because it captures recent research that may not yet be indexed with the same subject headings as older work. ### Documenting Citation Searches Record which included studies you used for citation searching and the date you performed the search. This documentation allows readers to understand the scope of your citation checking and to reproduce your approach. ## Practical Implementation Steps ### Step 1: Draft Your Research Question Write your scoping review question using the PCC framework. Identify the population, concept, and context. This framework guides your search term development and your inclusion criteria. ### Step 2: Identify Relevant Databases Select databases based on your topic. For life sciences, include PubMed or MEDLINE. For agricultural topics, add CAB Abstracts or Agricola. For interdisciplinary topics, add Web of Science or Scopus. ### Step 3: Develop Preliminary Search Terms List synonyms for each PCC element. Consult a librarian or information specialist if available. Test your terms against known relevant articles. ### Step 4: Build and Test Your Search String Construct your search string using Boolean operators, truncation, and field tags. Run the search in one database and review the results. Refine the string based on what you find. ### Step 5: Run the Search in All Databases Adapt your search string to each database's syntax. Run the search in each database and record the number of results. Export all results to your reference management software. ### Step 6: Search Grey Literature Identify relevant grey literature sources. Search each source with simplified terms. Record your search process and results. ### Step 7: Check Reference Lists Select key included studies for citation searching. Examine their reference lists and use citation indexes for forward searching. Add any relevant studies to your results. ### Step 8: Document Everything Record your complete search process, including all search strings, dates, databases, and results. This documentation becomes part of your review methods. ## Records and Measurements ### What to Record Maintain a search log that records: - The date of each search - The database or source searched - The exact search string used - The number of results retrieved - Any filters or limits applied - Notes on search refinements This log is the foundation of your methods section. It also helps you troubleshoot if a search returns unexpected results. ### Managing Search Results Export all results to a reference management tool such as EndNote, Zotero, or Mendeley. Remove duplicate records and document how many duplicates you removed. The [National Institutes of Health](https://sharing.nih.gov/data-management-and-sharing-policy) data management expectations emphasize the importance of documenting your data handling processes, and this applies to search results as well. ### Measuring Search Performance Track the number of records retrieved at each stage. This includes: - Total records retrieved from all sources - Records after duplicate removal - Records screened by title and abstract - Full-text articles assessed - Studies included in the review These numbers appear in your PRISMA flow diagram and demonstrate the transparency of your search process. ## Common Failure Patterns ### Using Only One Database A search limited to PubMed will miss relevant literature indexed only in other databases. Each database has unique coverage, and scoping reviews require multiple databases to achieve breadth. ### Overly Narrow Search Terms Using only the exact terms from your research question will miss studies that use different terminology. Always include synonyms and consider how different disciplines describe the same concept. ### Ignoring Grey Literature Scoping reviews that exclude grey literature may miss policy documents, reports, and current practice information. This is particularly problematic for topics where practice outpaces peer-reviewed publication. ### Poor Documentation Failing to record search strings, dates, and results makes your search unreproducible. This undermines the credibility of your review and prevents readers from assessing your methods. ### Not Testing the Search Running a search without testing it against known relevant articles risks missing key studies. Always test your strategy before committing to the full search. ### Skipping Reference List Checking Reference list checking catches studies that database searches miss. Skipping this step can result in an incomplete map of the literature. ## Limitations of the Three-Step Approach The three-step approach is widely used but has limitations. Database searching depends on the quality of indexing, and articles may be missed if they use unusual terminology or appear in journals with incomplete indexing. Grey literature searching is less standardized and may miss relevant sources that are not easily discoverable. Reference list checking depends on the quality of the included studies' reference lists and may miss studies that are not cited by other work. The [Committee on Publication Ethics](https://publicationethics.org/core-practices) core practices emphasize the importance of transparent and ethical research conduct. This includes accurately reporting your search methods and acknowledging the limitations of your approach. Scoping reviews are not intended to provide a detailed explanation to a clinical or policy question. They map the literature and identify gaps. The search strategy should be designed to achieve this mapping function, not to answer a narrow question with high precision. ## Quality Control and Peer Review ### Peer Review of the Search Strategy Have a colleague or information specialist review your search strategy before you run the full search. This review can identify errors in Boolean logic, missing synonyms, or inappropriate database selection. ### Checking Against Known Literature Test your search against a set of known relevant articles. If your search misses any of these articles, refine your strategy until it retrieves them. ### Documenting Search Refinements Record every version of your search strategy. This documentation shows the development process and allows readers to understand how your final strategy was derived. ## Professional Escalation Criteria ### When to Consult an Information Specialist If you are unfamiliar with database searching or if your search is returning unexpected results, consult a librarian or information specialist. These professionals have expertise in search syntax and database coverage. ### When to Revise Your Research Question If your search returns an unmanageable number of results, your research question may be too broad. Consider narrowing your scope or adding additional inclusion criteria. If your search returns very few results, your question may be too narrow or your search terms may be inadequate. ### When to Seek Ethics Guidance If your scoping review involves sensitive topics or if you have questions about authorship or data handling, consult the [Committee on Publication Ethics](https://publicationethics.org/core-practices) core practices for guidance on ethical research conduct. ## Researcher Identity and Documentation ### Registering Your ORCID An [ORCID](https://info.orcid.org/researchers) identifier provides a persistent digital identity for researchers. Registering your ORCID and linking it to your publications and review outputs ensures that your work is correctly attributed to you. ### Maintaining Your Research Record Keep your ORCID record current with your publications, datasets, and review activities. This record supports transparency and helps other researchers find your work. ## Data Management Considerations ### Planning for Data Sharing The [National Institutes of Health](https://sharing.nih.gov/data-management-and-sharing-policy) data management and sharing policy expects researchers to plan for data sharing. For scoping reviews, this includes sharing your search strategies and, where appropriate, your screening decisions. ### Documenting Data Handling Document how you handled search results, including how you removed duplicates and how you managed records throughout the screening process. This documentation supports the transparency of your review. ## Frequently Asked Questions ### What is the difference between a scoping review search and a systematic review search? A systematic review search aims for high precision to answer a narrow clinical question. A scoping review search prioritizes breadth to map the literature on a broader topic. Scoping searches typically use more synonyms, more databases, and include grey literature to capture the full range of evidence. ### How many databases should I search for a scoping review? The number depends on your topic. Most scoping reviews search at least three databases, and many search five or more. For life sciences topics, include PubMed or MEDLINE, Embase, and Web of Science or Scopus. For agricultural topics, add CAB Abstracts or Agricola. ### Should I include grey literature in my scoping review? Grey literature is often included in scoping reviews because it captures reports, theses, and policy documents that do not appear in commercial journals. The decision depends on your topic and research question. If current practice or policy is relevant, grey literature is likely important. ### How do I document my search strategy? Record the date, database, exact search string, and number of results for every search. Keep this information in a search log and include the full search strings in your review methods or appendix. This documentation supports transparency and reproducibility. ### What is the PCC framework? PCC stands for Population, Concept, and Context. It is a framework for developing scoping review questions and search strategies. Each element guides the development of search terms and inclusion criteria. ### How do I know if my search is too broad or too narrow? If your search returns thousands of records that are mostly irrelevant, it is too broad. If it returns very few records or misses known relevant articles, it is too narrow. Test your search against known relevant articles and refine your terms accordingly. ### Do I need to use controlled vocabulary terms like MeSH? Controlled vocabulary terms improve search sensitivity by capturing articles that use different wording for the same concept. Combine controlled vocabulary with free-text terms for the best results. A search using only free-text terms may miss relevant articles. ### What should I do if I cannot access a database? If you do not have access to a subscription database, use freely available databases such as PubMed and Google Scholar. Document your database access limitations in your methods section so readers understand the scope of your search. ## Using the Evidence | Source | Best use in this topic | Important limitation | |---|---|---| | [Research Methods Resources](https://www.ncbi.nlm.nih.gov/books) | official guidance | Check the linked page for current local requirements | | [EQUATOR Network](https://www.equator-network.org/) | official guidance | Check the linked page for current local requirements | | [Core Practices](https://publicationethics.org/core-practices) | official guidance | Check the linked page for current local requirements | ## Related Articles - [Scientific Literature Review: A Reproducible Search and Synthesis Workflow](/blog/careers/scientific-literature-review-a-reproducible-search-and-synthesis-workflow) - [Scientific Literature Review Format: A Reproducible Structure for Researchers](/blog/careers/scientific-literature-review-format-a-reproducible-structure-for-researchers) - [Protein Synthesis Step By Step](/blog/guides/protein-synthesis-step-by-step) - [Protein Synthesis: A Step-by-Step Guide to Transcription and Translation](/blog/guides/protein-synthesis-a-step-by-step-guide-to-transcription-and-translation) - [The Role of the Research Ethics Committee: What Researchers Need to Know](/blog/guides/the-role-of-the-research-ethics-committee-what-researchers-need-to-know) ## References and Further Reading - [Research Methods Resources](https://www.ncbi.nlm.nih.gov/books). National Library of Medicine. - [EQUATOR Network](https://www.equator-network.org/). EQUATOR Network. - [Core Practices](https://publicationethics.org/core-practices). Committee on Publication Ethics. - [NIH Grants and Funding](https://grants.nih.gov/). National Institutes of Health. - [ORCID for Researchers](https://info.orcid.org/researchers). ORCID. - [Data Management and Sharing Policy](https://sharing.nih.gov/data-management-and-sharing-policy). National Institutes of Health. - [NCBI Literature Resources](https://www.ncbi.nlm.nih.gov/). National Center for Biotechnology Information. - [PubMed](https://pubmed.ncbi.nlm.nih.gov/). National Library of Medicine. > This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.