Preparing to search
Before you begin - are you doing a search for a systematic review or a literature review?
Knowing the difference between these two concepts is key. A systematic review aims to answer a specific research question through rigorous identification, appraisal and synthesis of research evidence, so it's important that the search strategy is comprehensive and reproducible.
In contrast, a literature review is a descriptive and evaluative summary of the research in your area of interest and is likely to cover a broader research topic. It is less likely to involve:
- criteria for including or excluding studies;
- appraising the quality of studies; and
- synthesising data.
If you are unsure about the differences between a systematic review and a literature review, this document provides a helpful summary: What’s in a name? The difference between a Systematic Review and a Literature Review and why it matters, by Lynn Kysh, Information Services Librarian, University of Southern California, Norris Medical Library, MLGSCA Poster 2013 – CC-BY.
Defining your research question
It is important to have a well-defined research question and to be clear about your objectives when you start work on a systematic review. You will need to spend time planning your search; thinking about sources to search, terms to use and search language. The School of Health Sciences have a useful introduction outlining why ‘Asking the right question’ can help in the search for evidence, and explaining how to construct good questions using the PICO technique discussed below.
Defining your search concepts - PICO, SPICE, SPIDER and other frameworks
When thinking about your search, you will need to break your research question down into its key concepts. For example, if your research question was ‘Which complementary therapies work for acne?’ you should start by breaking your research question down into separate concepts. You may find using a framework such as PICO (Population/Patient, Intervention, Comparison, Outcome) helpful, although often you won’t have search terms for every concept. You can find out more about PICO from this 5-minute tutorial from the Cushing/Whitney Medical Library at Yale University.
For the research question above, two of the PICO concepts are helpful in thinking about search terms:
- Population - is people with acne, and
- Intervention - is complementary therapies.
Next, try to think of as many different ways as possible to express the terms associated with each concept in your research question. Think about variant word endings, hyphenated words, different spellings and which way round terms might appear.
For the Population in the example above (people with acne), your list might include additional terms like acne vulgaris, comedones, pustules, etc.
For the Intervention, complementary therapies, you might consider including terms like alternative therapy, aromatherapy, homeopathy, holistic health, etc.
The PICO framework outlined above works well for conventional systematic reviews of effectiveness using a single comparative study design. Other frameworks work better for other types of research question, including:
- SPICE (useful for qualitative evidence synthesis) - Setting; Perspective; Intervention/Interest, of Phenomenon; [Comparison]; Evaluation.
- SPIDER (useful for mixed methods systematic review) - Sample; Phenomenon of Interest; Design; Evaluation; Research type.
There are lots of other search frameworks you can use for your particular systematic review. Andrew Booth (Reader in Evidence-Based Information Practice at the University of Sheffield) has summarised these in his document Alternative Question Structures for Different Types of Systematic Review. And, if your topic doesn't fit into a framework, that’s not a problem – you don’t have to use one. You can either:
- Use only the parts of a framework which do fit; or
- Don't use a framework! Just follow the principles of separating your topic into different search concepts.
Refining your search: phrase searching, truncation, wildcards, proximity operators, limits and filters
Remember to consider phrase searching, truncation, wildcards and proximity operators when thinking about your search terms. The use of limits and filters may also be appropriate in specific cases.
The most effective way to search for an exact phrase is to enclose it within quotation marks "...". For example "complementary therapy"
will ensure the database looks for exact matches of this phrase, which in turn means the information found should be more relevant. See this short tutorial about phrase searching
Truncation allows you to search for different endings to a word. The symbol most commonly used for truncation is an asterisk (*) but check the help screens in the databases you use for more information. See this short tutorial about truncating terms.
Using complementary therap* would find:
complementary therapy, therapies
A wildcard replaces a letter within a word. For example, wom?n would search for woman and women. Wildcards can be helpful for searching for alternative spellings, for example, hum?r will search for both humour and humor. The symbol used for wildcards varies between different databases, so check the help pages. See this short tutorial about wildcard searching.
Proximity operators allow you to search for two or more terms which occur within a specified number of words of each other, in any order. Examples include the ADJ (Adjacency) and N (Near) operators (note that the operators will vary according to the database you are using).
complemtary ADJ3 therap* would find the terms complementary and therap* within 3 words of each other, in any order, e.g.: complementary and alternative therapies or therapy as a complementary alternative. See this short tutorial about proximity operators.
A database may offer predefined limits which you can apply to your search (e.g. date, age group, study design, language). You should use limits with care and only if appropriate. For example, if you know a drug or intervention has only been available from a specific date, then you can justify the use of a date limit.
A better way to focus your search is to use a tried-and-tested search filter. For example, there are filters validated by Cochrane for finding randomized controlled trials (RCTs). You can copy and paste the filters line by line into your own search.
The King's College London website has some useful guidance on using limits and filters in systematic reviews, including links to pre-tested filters for RCTs, qualitative research, geographic regions, populations, etc.
Ready to search
The terms you identify during this initial preparation stage will form the basis of your strategy when you come to search bibliographic databases. It is worth spending time at this point:
- Clearly defining your research question;
- Identifying the major search concepts in the question;
- Developing a robust set of search terms, including truncated terms or those that need a wildcard; and
- Identifying the appropriate subject headings in databases which use a controlled vocabulary (e.g. MeSH in MEDLINE).
Using a combination of subject headings and keywords will ensure the most comprehensive set of results. For an overview of all aspects of keyword identification, watch the video on Search skills: thinking about keywords.