Preparing to search
Before you begin - are you doing a search for a systematic review or a scoping review?
Systematic reviews and scoping reviews are both part of the evidence synthesis family (a group of methods that also includes mixed method reviews, rapid reviews, umbrella reviews, realist reviews, evidence maps and more) and confusion can arise about which type of review should be undertaken.
If you are unsure about the differences between a systematic review and a scoping review (or any other type of evidence synthesis), there are tools which can help you distinguish the differences and decide on the right type of review for your research question. For example:
There are also numerous articles which cover this topic, including:
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. If your research question is too broad, you will be overwhelmed with a large number of irrelevant results; equally, if it's too specific you may miss relevant evidence. 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.
Frameworks to help define your research question
You will first need to define your research question. Using a research question framework can help you structure your question and enable you to find the information you need most effectively. The PICO (Population/Patient, Intervention, Comparison, Outcome) framework is commonly used in systematic reviews and works well for reviews of effectiveness using a single comparative study design. You can find out more about PICO from this 5-minute tutorial from the Cushing/Whitney Medical Library at Yale University.
There are a number of different frameworks available, and the one you use will depend on whether your review plans to explore quantitative or qualitative evidence and the specific purpose of your review:
- PCC (useful for scoping reviews) - Population; Concept; Context.
- SPICE (useful for qualitative evidence synthesis) - Setting; Perspective; Intervention/Interest, of Phenomenon; [Comparison]; Evaluation.
- SPIDER (useful for mixed methods reviews) - Sample; Phenomenon of Interest; Design; Evaluation; Research type.
If your topic doesn't fit into a framework, that’s not a problem – just use the parts that do. Many clearly defined questions do not have a comparison or control to consider, so don't worry if this category isn't applicable to your topic.
Identifying your search concepts, keywords and synonyms
When thinking about your search, you will need to break down your research question into its key concepts. For example, if your research question is ‘Which complementary therapies work for acne?’, you should start by breaking your research question down into the separate concepts.
For this question there are 2 concepts: ‘people with acne’; and ‘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.
To help break the question down into concepts and brainstorm synonyms, you might find this concept grid template and an example of a Medline (OVID) search strategy useful.
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.
Phrase searching
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
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.
Example:
Using complementary therap* would find:
complementary therapy, therapies
Wildcards
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).
Example:
complementary 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.
You can combine proximity operators and Boolean operators in the same line to make searches even more effective. This involves using parentheses (brackets) to create 'nested' search terms.
Example:
(complementary ADJ3 therap*) OR acupuncture would find records containing the terms complementary and therap* within 3 words of each other and then combine these records with any that contain references to acupuncture. See this short tutorial on using brackets and Boolean 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.