Main search
Creating the main search is a critical phase of the literature searching process. The main search builds on the exploration from preliminary searching. The purpose of a main search is to develop a structured, reproducible search strategy that ensures thorough coverage of a topic while minimising bias.
In a systematic literature search, the main search is designed to comprehensively and transparently retrieve relevant studies to address a well-defined research question.
Characteristics of your main search should include:
Structured and Comprehensive
Employs a systematic approach to minimise gaps in the literature and ensure no relevant studies are overlooked.
Balanced
All potentially relevant literature is captured to reduce the risk of missing critical evidence.
Iterative
Adapts and refines search terms and strategies based on preliminary search to improve accuracy.
Advanced Techniques
Applies Boolean operators, truncation, phrase searching, and proximity operators to refine the search.
Transparent and Reproducible
Clear documentation reporting the search strategy so others can verify your methods and replicate the process.
Creating the main search
During the preliminary search phase you found seeding articles, identified key concepts, gathered search terms, and selected relevant databases. To create the main search, these components will be brought together along with other components, such as subject headings, limiters and search hedges.
Tip
Remember, searching is an iterative process. Your search can and should be refined. For further help with refining your search check out the Testing page.
Line-by-line searching
Line-by-line searching involves entering each individual term for a concept on a separate line in a search interface. This approach separates each term, allowing the database to treat each term as an independent search line, instead of combining all terms for a concept in a single line. Line-by-line searching is helpful in maximising control and transparency in your main search.
How does line-by-line searching work?
Drag the slider left and right below to discover the different parts of a line-by-line search exploring adolescent obesity in Australia using the Medline ALL database via the Ovid platform:
Subject headings
Subject headings (also known as controlled vocabulary, thesauri searching, subject searching, subject terms, descriptors or index terms) are standardised terms or phrases used in databases to describe the main topics of an article or resource. Subject headings are assigned to articles to ensure that materials about the same subject are consistently categorised, regardless of the terminology used by the authors.
Searching subject headings within databases locates articles indexed (or tagged) with that subject heading. In contrast, searching with keywords finds articles based on their title or abstract. Combining both keywords and subject headings in your search helps ensure no relevant studies are missed. Ideally, we would find subject headings appropriate for each of our concepts.
Tip
Subject headings vary between databases. They often reflect the specific discipline the database focuses on. For example, CINAHL includes more nursing-related subject headings, while PsycINFO emphasises psychological terms and concepts.
How do I find and select subject headings?
The method for finding and entering subject headings differs across databases, but it’s essential to browse and select subject headings directly from the database’s controlled vocabulary or thesaurus, rather than typing them in freehand. This ensures you are using the exact terms recognised by the database for accurate and comprehensive results.
What are the subject heading selection options?
When selecting subject headings, many databases offer additional features to refine and enhance your search:
- Explode: This option retrieves all articles indexed under the selected subject heading and any narrower, related terms within its hierarchy. It's ideal for broadening your search to ensure comprehensive coverage.
- Focus / Major: This limits the search to articles where the subject heading is a primary focus. It is not recommended for comprehensive searches.
- Subheadings: These allow you to target specific aspects of a subject, such as diagnosis, therapy, or complications. Be aware it's often best not to apply subheadings for comprehensive searches, as they may exclude relevant studies.
Always check the database's help guide for details on how these options work.
How do I combine subject headings with keywords?
Just like keywords, add your subject headings on a separate line in the database, and combine them with the other keywords for that concept using OR. This ensures your search captures articles that either have those subject headings applied or mention the keywords in the title or abstract (or a combination of both!)
Drag the slider left and right below to discover how subject headings (in bold) have been added to the line-by-line search exploring adolescent obesity in Australia using the Medline ALL database via the Ovid platform. Notice that the subject headings have been combined in the same way as the keywords in their relevant concepts, and in some cases, multiple subject headings are relevant to a single concept. The subject heading for Australia has been exploded to include all its narrower terms:
Alert
Be aware that subject headings are displayed differently across databases and platforms. For example, in Medline via Ovid, subject headings are indicated with a forward slash (/), and the prefix exp is used for the "explode" feature. In contrast, Embase uses the prefix DE for subject headings and /exp for "explode".
Limiting the search
Limiting your search is a useful strategy for refining results and focusing on the most relevant studies. Applying limits can help reduce the volume of irrelevant results by narrowing the search to specific criteria that align with your research topic.
Common database limiters that can be applied to a search strategy:
- Date - e.g. restricting to articles published in the last 10 years
- Language - e.g. English
- Population - e.g. age, human, animals
- Geography - e.g. low and middle-income countries only
- Study design - e.g. randomised controlled trials, systematic reviews
Caution
For systematic searches, limiters should be applied cautiously to ensure the search remains comprehensive and unbiased. Only language and date limiters are typically recommended. Other limiters, such as study design or population may prematurely exclude studies that could meet inclusion criteria.
Instead of relying on database limiters, add an additional concept to your search strategy. This approach is often more precise and ensures relevant studies are not accidentally excluded.
For example, when searching for studies about teenagers, rather than using database filters, you could include keywords and subject headings to clearly define the concept.
Search hedges
Search hedges, also known as search filters, are pre-tested structured search strategies that help identify studies with specific attributes, such as study design, population, or methodology. Search hedges are particularly useful for systematic searches, as they ensure precision and consistency.
Why use search hedges?
Search hedges help:
- Identify specific types of studies, as well as those focusing on particular populations (e.g., paediatric studies) or geographical regions.
- Apply consistent and validated search strategies, reducing the risk of bias and missed results.
- Save time by leveraging pre-built strategies rather than creating them from scratch.
What are the different types of search hedges?
The following are common search hedges:
- Study design hedges: To identify specific study types, such as randomised controlled trials (RCTs), qualitative research, or economic evaluations.
- Population hedges: To target specific groups, such as children, Indigenous peoples, particular patient populations (e.g., individuals with diabetes).
- Geographical hedges: Narrow results to specific locations, such as low- and middle-income countries, OECD countries, or regions like Asia.
Where to find search hedges?
Search hedges are readily available from reputable sources that specialise in creating filters for different databases and topics. Below are some key resources where you can find pre-built search hedges:
How to use search hedges?
Incorporate hedges into your main search as an additional concept. Combine the search hedge with your existing concepts using the Boolean operator (e.g. AND).
Always ensure you reference and acknowledge the use of search hedges.
Tip
While search hedges are pre-made, they may require slight adjustments to fit your research question or database platform. Always test the hedge to ensure it retrieves relevant results.
Searching scenario
Tip
Download the Example main search for further details on how line-by-line search can be applied to a research question.