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Systematic Reviews: Search strategy

A guide to systematic review searching for University of Newcastle researchers

Searching systematically

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.

  • Sensitivity – the number of relevant results identified divided by the total number of relevant results in existence
  • Precision - the number of relevant results identified divided by the total number of results identified.

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. 

1. Developing answerable questions

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?
What are the most important characteristics that describe the people?
Are there any relevant demographic factors (eg. age, sex, ethnicity)?
What is the setting (eg. hospital, community, etc)?
Who should make the diagnosis?
Are there any other types of people who should be excluded from the review (because they are likely to react to the intervention in a different way)?
How will studies involving only a subset of relevant participants be handled?

Interventions and Comparisons

What are the experimental and control (comparator) interventions of interest?
Does the intervention have variations (eg. dosage/intensity, mode of delivery, personnel who deliver it, frequency of delivery, duration of delivery, timing of delivery)?
Are all variations to be included (for example is there a critical dose below which the intervention may not be clinically appropriate)?
How will trials including only part of the intervention be handled?
How will trials including the intervention of interest combined with another intervention (co-intervention) be handled?

Outcomes

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.
Primary outcomes are the two or three outcomes from among the main outcomes that the review would be likely to be able to address if sufficient studies are identified, in order to reach a conclusion about the effects (beneficial and adverse) of the intervention(s).
Secondary outcomes include the remaining main outcomes (other than primary outcomes) plus additional outcomes useful for explaining effects.
Ensure that outcomes cover potential as well as actual adverse effects.
Consider outcomes relevant to all potential decision makers, including economic data.
Consider the type and timing of outcome measurements.

 

2. Identify synonyms and related terms

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

      rehabilitation

      exercise

      exercise therapy

      sports

      physical education and training

      exertion

      physical training

      aerobics

      kinesiotherapy

           

      coronary heart bypass

      myocardial ischemia

      myocardial infarction

      coronary disease

      coronary thrombosis

3. Use Truncation and Wildcards

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:

  • Truncating a word at the end ensures that all variations of the word, beginning with a specific root, will be retrieved. This is particularly useful for retrieving singular and plural versions of words.

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 therapytherapiestherapiststherapeutictherapeutical, etc
  • Truncating a word internally ensures that any variations of spelling of a word can be retrieved. For example, pediatrics or paedetrics

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

4. Combining terms

Boolean Operators

Boolean operators allow you to link terms together, either to widen a search or to exclude terms from your search results.

OR   

 

 

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

AND  

Used to narrow a search, therefore decreasing the number of references.

For example, searching for:
coronary heart disease AND Australian Aboriginals

Would retrieve just those references covering both topics.

 NOT     

Used to narrow a search, therefore decreasing the number of references.
For example:

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 Operators

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.

ADJACENCY   

 

 

Some databases allow you to search for words within a specified number of words from each other.

For example:

physician adj3 relationship

Would retrieve physician patient relationship, patient physician relationship, or relationship of the physician to the patient.

 

Nesting

Nest search terms to control the logic of your search. For example:

(rehabilitation OR exercise OR exercise therapy OR sports OR physical training) 

AND

(Coronary heart bypass OR myocardial ischemia OR myocardial infarction OR coronary disease OR coronary thrombosis)

5. Identify Search Limits/Exclusion Criteria

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.
 

6. Keyword vs subject searching

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.

Keyword searches are extremely important when conducting systematic reviews, and should be used in combination with the relevant subject headings within each of your database searches:

  • Authors may not describe their methods or objectives well and indexers are not always experts in the subject areas
  • The available indexing terms might not correspond to the terms the searcher wishes to use. Cochrane Handbook for Systematic Reviews of Interventions (2011, Section 6.4.5)

Checking your Search Strategy

Harvey Cushing Videos

Search filters

The following sites include examples of pre-tested search filters. These are useful for checking for additional terms for your search strategies.

PRESS: Peer Review of Electronic Search Strategies