Systematic Reviews

Creating a search strategy

Bibliographic databases are highly structured with sophisticated indexing and so require a more strategic search approach compared to a simple Web search. To design your search strategy you need to:

Identify key concepts

Decide on which keywords to use 

Find relevant Subject Headings

We advise that you plan out your search strategy before you start searching any databases. 

Key concepts

The search should be structured around the key concepts being examined.

For clinical questions we could use the PICO formula, discussed in a previous section, to help our thinking here as it is an easy way to work out what information we want and which search terms should be included.

Keyword searching

Keyword searching (or free-text searching) is a mode of searching where you find exact matches for your search terms within the titles and the abstracts of journal articles. Here you are looking for terms used by the author(s), in contrast to subject headings which are defined by the database.

Include likely variations in terms for your key concepts to broaden your search and avoid missing studies which are relevant to your topic. Think about synonyms, related terms and spelling variations (particularly UK/US).

Shortcuts and proximity

Use  Shortcuts (truncation or wildcards) to help with keyword searching. Check what is available in the database you are searching by consulting the Library’s Quick Guides or by checking the database’s help pages.

Many databases allow you to use the asterisk to expand on the stem of a word:
e.g. dent* would find dental, dentistry, dental

Phrase Searching: use quotation marks to specify that you want to retrieve records containing this exact phrase, e.g. “bronchial asthma”  ensures that you retrieve records containing these words next to each other in this order.

You may also be able to specify proximity – that certain words occur near each other.

Using Subject Headings

Many databases, including PubMed, Medline and Embase use subject headings to make it easier to find articles on a specific subject. Each article listed in the database is assigned a number of subject headings which represent what the article covers. The advantage of this is that all of the articles on the same subject will be given the same subject heading and this is independent of the terminology used by the individual authors. For example, papers in Medline about low blood pressure will all be assigned the subject heading hypotension regardless of whether the author has used terms such as ‘low blood pressure’, ‘hypotensive’, hypotension’ and so on.

OR, AND, NOT

Use the commands OR, AND and NOT to instruct the database how to combine your searches logically.

Use OR to combine searches about the same concept - e.g. synonyms, related concepts, variant spellings, e.g. vitamin C OR ascorbic acid

Use AND to combine searches about different concepts, e.g. caffeine AND asthma

Use NOT to exclude terms from your search. If you use NOT, an article containing the term you specify will be excluded from your results. This can be useful, but use it with caution as you risk losing articles which may have been relevant.

Combining searches

Combine subject heading and keyword searches

If you are doing a quick search, you only need to use one of these search methods. 

If you want to retrieve as much as you can on your topic, you should use both methods. This overcomes the limits of both approaches.

Subject headings make it easier to cover all the variant terminology and offer a precise method of searching. However, they can take time to catch up with new developments. Indexers occasionally make mistakes.

Keyword searching allows you to search for the exact terminology you want. However, you need to think of variant terminology and to watch out for false matches.

Refining and limiting

When you have run your search, you will probably find that you have either more or fewer results than you were hoping for.

Too many? - Look for ways to make your search more specific: can you add terms? Can you limit your results? Can you use more specific terms?

Too few? - Look for ways to make your search more general: can you remove any terms? Can you remove any limits? Can you use more general terms?

Limits - Look for options to limit your results, such as by year of publication, by age group, language and so on.

Search filters

Sometimes it is helpful to use a search filter to find types of study such as a Randomised Controlled Trial or some qualitative research. Search filters have also been developed to find results about certain age groups, geographical regions etc.

ISSG Search Filter Resource - search filters collated by the InterTASC Information Specialist Sub-Group covering study designs and other filters.   

The BMJ Study design search filters

 

Next step: Grey literature