Searching is a critical part of conducting the systematic review, as errors in the search process potentially result in a biased or otherwise incomplete evidence base. Searches for systematic reviews need to be constructed to maximise recall and deal effectively with a number of potentially biasing factors. (McGowan, 2005, p. 75)
You should aim to be as extensive as possible when conducting searches for systematic reviews. However, it may be necessary to strike a balance between the sensitivity and precision of your search.
Increasing the comprehensiveness of a search will reduce its precision and will retrieve more non-relevant results. However,
... at a conservatively-estimated reading rate of two abstracts per minute, the results of a database search can be ‘scanread’ at the rate of 120 per hour (or approximately 1000 over an 8-hour period), so the high yield and low precision associated with systematic review searching is not as daunting as it might at first appear in comparison with the total time to be invested in the review. (Cochrane Handbook for Systematic Reviews of Interventions, 2011, Section 6.4.4)
A useful technique is to check the search strategies used in other systematic reviews for hints on terms and combinations to use. Several groups have also developed pre-tested search filters.
A technique often used in health research for formulating a clinical question is the PICO Model.
Using PICO, a clinical question will have 4 elements - Patient, Intervention, Comparison and Outcome.
The Cochrane Handbook for Systematic Reviews of Interventions (2011,Sections 5.2-5.6) includes the following factors to consider when developing criteria for your PICO elements.
|Patient, Population or Problem||
How is the disease/condition defined?
|Interventions and Comparisons||
What are the experimental and control (comparator) interventions of interest?
Main outcomes, for inclusion in the 'Summary of findings' table, are those that are essential for decision-making, and should usually have an emphasis on patient-important outcomes.
It is important not to overlook this stage in the search process. Time spent identifying all possible synonyms and related terms for each of your PICO elements or concepts will ensure that your search retrieves as many relevant records as possible.
- Think laterally about how others may describe the same concept
- What terminology is used internationally?
- Are there spelling differences in UK English and US English words?
- Are there any colloquial terms or phrases used?
- Check the search terms used in other papers or systematic reviews - other terms may be suggested from these.
It might be useful to check relevant dictionaries, encyclopedias and key texts for alternate terms.
Build a list of each of the search terms you identify. For example, if you were searching for:
exercise-based rehabilitation for coronary heart disease
Your list of synonyms and related terms might include:
|Exercise-based rehabilitation||Coronary heart disease|
physical education and training
coronary heart bypass
The definition of ‘truncation’ is to shorten or cut-off at the end. Truncation is used in database searches to ensure the retrieval of all possible variations of a search term. All databases allow truncation, but the symbols used may vary, so it is best to check the database help for details.
Databases usually allow words to be truncated either at the end, or internally:
Be careful not to truncate terms too early, or you may retrieve a high number of irrelevant documents.
Most databases use an asterisk (*) to find alternate endings for terms. For example:
|therap* will retrieve therapy, therapies, therapists, therapeutic, therapeutical, etc|
Internal truncation is available in some databases, allowing you to search for alternate spellings of words - extremely useful when searching for American and English spellings of words.
For example, using the OVID databases (MEDLINE, EMBASE, PsycINFO, etc), a question mark included within a word can designate zero or one character in that place:
|colo?r will retrieve either colour or color|
Boolean operators allow you to link terms together, either to widen a search or to exclude terms from your search results.
Use to broaden your search, increasing the number of references retrieved. Use "OR" to search for synonyms and related terms for each concept within a research question.
For example, when searching for the concept "exercise based rehabilitation" you might use the following terms:
rehabilitation OR exercise OR exercise therapy OR sports OR exertion OR physical training OR aerobics OR kinesiotherapy
Used to narrow a search, therefore decreasing the number of references.
For example, searching for:
Would retrieve just those references covering both topics.
Used to narrow a search, therefore decreasing the number of references.
female NOT male
Would retrieve references dealing with females, but not those which discuss males. Caution should be exercised when using NOT, In the example above, research dealing with both females and males would be excluded from the search results.
Proximity (sometimes called “adjacency”) searching is similar to using Boolean operators in that you are specifying relationships between 2 or more terms. However, proximity searching allows you to specify the proximity of words to each other.
Some databases allow you to search for words within a specified number of words from each other.
physician adj3 relationshipWould retrieve physician patient relationship, patient physician relationship, or relationship of the physician to the patient.
Nest search terms to control the logic of your search. For example:
(rehabilitation OR exercise OR exercise therapy OR sports OR physical training)
(Coronary heart bypass OR myocardial ischemia OR myocardial infarction OR coronary disease OR coronary thrombosis)
You should next think about the limits you intend to apply to your search.
|Criteria||Questions to Ask||Advise from the Cochrane Handbook for Systematic Reviews of Interventions (2008, p. 134)|
|Time Period||Will your review be restricted by year of publication, or is it important that you cover all years?||"Date restrictions should be applied only if it is known that relevant studies could only have been reported during a specific time period, for example if the intervention was only available after a certain time point."|
|Language||Should you restrict to English language publications only?||"Whenever possible review authors should attempt to identify and assess for eligibility all possibly relevant reports of trials irrespective of language of publication. No language restrictions should be included in the search strategy."|
|Publication Type||Are you restricting your search by publication type?||"Format restrictions such as excluding letters are not recommended because letters may contain important additional information relating to an earlier trial report or new information about a trial not reported elsewhere."|
|Geographic Considerations||Are there any geographic considerations to include in your search strategy?||For example, if you were researching Chinese herbal medicine you would need to consult Chinese literature.
Controlled vocabularies (such as the MESH subject headings used in Medline and EMTREE subject headings used in EMBASE) provide an organised approach to the way knowledge is described.
Their use is extremely important as they bring uniformity to the indexing of publications included within a database. Using the same terminology throughout a database creates consistency and precision and helps you to find relevant information no matter what terminology the author may have used within their publication.
Indexing is usually a manual process. Databases such as MEDLINE employ specially trained indexers to read the full-text of each publication then identify all of the concepts covered within the article. These concepts are then translated to the controlled vocabulary used within the database. It is the indexer’s job to ensure that each concept included in the article are identified and assigned a term.
Each database may use different subject headings to describe the same concept. As an example, the term “complementary medicine”:
|The MESH heading (MEDLINE) is “complementary therapies”|
|The EMTREE heading (EMBASE) is “alternative medicine”|
|The CINAHL heading is “alternative therapies”|
The OVID (MEDLINE, EMBASE, AMED, PsycINFO, etc) and CINAHL databases provide a search option to “explode” terms. Exploded searches retrieve indexed records for a term, plus other terms which are a derivative (more specific, narrower terms) of the search term. Exploding search terms provides a fast way to find related concepts in a single search.
For example, if a search for "complementary therapies" in MEDLINE was exploded:
The search results would also include records indexed with the MESH headings acupuncture”, “anthroposophy”, “auriculotherapy”, and so on.
NOTE: Clicking on a MESH heading will display its tree, including the exploded terms.
The following sites include examples of pre-tested search filters. These are useful for checking for additional terms for your search strategies.