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Systematic Review: Design search

Copy of published guide 'systematic review' (Paula Todd version) for Editing taken in July 2021

The search process

A scoping search is an important first step in the searching process, and can be done once you have developed a well-defined and focused question, and considered your inclusion and exclusion criteria. Normally, you would have an idea of some original research on your topic. It's helpful to use these studies to see how they've been indexed within a database (see the section below on identifying relevant keywords and subject headings).  

Allow adequate time for the scoping phase and try not to rush this process; remember the end point is to develop a reproducible and transparent search strategy to locate the available evidence for your question. Searching can be understood as an iterative process. You will need to develop a search and continually review and refine it for constant improvement.

A scoping search helps identify:

  • relevant headings and terms
  • the extent of the literature
  • further articles to use as part of your gold set.

Make sure to keep track of how your search strategy evolves - documentation is key!

Create a gold set of relevant articles

It is useful to build a 'gold set' of relevant references before you develop your search strategy.  The papers in your 'gold set' can then be used to help you identify relevant search terms and to test that your search strategy will retrieve these items and other relevant references on your topic.

Sources for this 'gold set' may include:

  • Key papers recommended by subject experts or your supervisor, or team members on your review
  • Results of preliminary 'scoping searches' from key databases such as Google Scholar, Scopus, Ovid Medline and Pubmed
  • Citation searching ('pearl growing' or 'snowballing') techniques to locate further relevant references cited in or by key papers. VOSviewer and Connected Papers are just two of the tools you could use to conduct forward and backward citation tracking.
  • References used in similar systematic reviews

Develop a search strategy - subject headings and keywords

A comprehensive search would usually entail a combination of subject headings, plus a wide range of keywords/phrases for each concept. Not all databases will have subject heading searching and for those that do, the subject heading categories may differ between databases. This is because databases classify articles using different criteria. Ensure that your keywords/phrases are necessarily robust.


Look for synonyms that may be listed in reference books or in other background reading material. .

Many systematic reviews are required to append a copy of their search strategies on publication. It can be helpful to look at the terminology that is used in reviews on related topics, even if not all concepts or elements of the paper are relevant.

You can manually ‘text mine' the papers in your gold set for useful search terms. Check if the author has provided a list of key terms that describe the content of the article, and look for relevant keywords or phrases that have been used in the title and abstract field. It is also helpful to search for the papers from your gold set in a key database such as Ovid Medline or Embase and then harvest the database reference for subject headings and useful keywords or phrases. Use a title search to locate the papers in the database Specialised software is available to help you in identifying keywords or subject headings. identifying subject headings. Key databases such as Medline allow you to do a subject heading search to locate relevant subject headings for your key concepts. It is important to remember that subject headings are often unique to a particular database, so you will need to look for appropriate subject headings in each database you intend to use.

Text mining tools you may want to try out are:

PubReMiner Enter a simple search string

Termine Upload a pdf or URL to mine text

MeSH on Demand Enter abstract or full text

Documenting the search strategy

You may create a concept map to list the subjects and key terms that you have identified.  This list can be quite extensive and it can be helpful to use an Excel spreadsheet.

  • The concept map/grid may change after you have done some test searches.  You may also discover new or different ideas as you explore the subjects and look for more clues.


In people at risk of cardiometabolic disorders whose weight is classified as obese (P), how effectively does chocolate (I) reduce cholesterol level (O)?




‘P’ (Population/patient)
People whose weight is classified as obese who may or may not be hypercholesterolemic

  • Overweight
  • Over weight
  • Obesity
  • Cholesterol
  • Hypercolesterolemia
  • Cardiovascular
  • LDL
  • HDL
  • CVD
  • Overweight
  • Obesity
  • Cholesterol
  • Cholesterol, Dietary
  • Cholesterol, LDL
  • Cholesterol, HDL
  • Hypercholesterolemia
  • Cardiovascular system
  • Cardiovascular diseases
  • Hyperlipdemias

‘I’ (Intervention)


  • Chocolate
  • Cocoa
  • Cacao
  • Flavanols

  • Cacao
  • Theobromine



  • Comparison ‘C’ is not generally used when searching but if a comparison is included, this is often done as a separate search using P and C.
  • If the outcome ‘O’, is similar (in terms of subject headings and keywords) as the Problem, then they are not repeated as search terms.

Combining and truncating search terms

Boolean operators

Boolean operators are used to combine the different concepts in your topic (or the elements in your PICO question).

The operators that are used to combine terms in a search strategy are AND, OR, NOT.

venn diagram of boolean operators

OR is used to combine synonyms.

In a systematic review, you will usually have a long list of terms that will describe a concept. Usually these terms are either synonyms or alternative descriptors for the same idea/concept.

Using OR will search for resources that discuss concept A or B.


AND is used to combine concepts.

Once you have created the lists of synonyms for a concept and grouped those terms into the one search statement, you can then use AND to combine Concept A with Concept B.

Using AND will search for resources that combine both concept A and B.

NOT is a subtraction operator.

You can use NOT to subtract a term or an idea or a topic that you do not wish to appear in your search results.

Use with caution as you may accidentally omit something that is still required.

Using NOT will search for resources that only discuss concept A.

Proximity Operators

Proximity operators are extremely useful when looking for combinations of keywords in a phrase.


Using ADJn retrieves words within ‘n’ words of each other, in any order.

For example: ADJ4 retrieves words within 4 words of each other in ANY order (up to 3 words in between).

Use DEFINED ADJACENCY for phrase searching fields such as the TITLE or ABSTRACT.

For example: (lung* ADJ2 injur*) retrieves: lung injury, lung injuries, injured lungs, injury to lungs.

Using N+ retrieves words within '+' words of each other, in any order.

For example: N4 retrieves words with 4 words of each other in ANY order.

Using W+ retrieves words within '+' words of each other, in the same order.

For example: W2 retrieves words within 2 words of each other in the SAME order.

Use proximity operators for phrase searching fields such as the TITLE or ABSTRACT.

For example: heart N2 disease* retrieves: heart diseases, diseased heart, diseases of the heart.

For example: coronary W1 disease* retrieves: coronary diseases, coronary heart disease, coronary artery disease.

NOT is a subtraction operator.

Using NEAR/number retrieves words within ‘number’ words of each other, in any order.

For example: NEAR/5 retrieves words within 5 words of each other in ANY order.

Use proximity operators for phrase searching fields such as the TITLE or ABSTRACT

For example: heart NEAR/2 disease* retrieves: heart diseases, diseased heart, diseases of the heart.

Using NEXT/number retrieves words when they appear next to each other in the order specified.

For example: lung NEXT injur* retrieves: lung injury BUT NOT injured lung

Truncation and wildcards

When using keywords, consider the need for Truncation and Wildcards to capture all relevant research.  These can be used to ensure you retrieve any keyword variations, such as synonyms, plurals and variant endings and spellings.


Truncation allows you to search for a word and all its variant endings. For example:

Infect* will retrieve:







Beware of truncating too soon!

Fall* will retrieve fall, falls, falling BUT it will also retrieve fallopian

Work out what is likely to be picked up in order to avoid irrelevant results.

Wildcards allow you to search for an undefined character within a word.

Isch?emia will retrieve:



The syntax (wildcards, proximity operators) may vary across database platforms. Use the database tips sheet below as a handy reference to ensure that you use the correct symbol/operator for the database that you are using.

    Database search tips - reference guide


The tutorial below demonstrates how to develop and perform systematic searches in Ovid Medline. The techniques demonstrated in the tutorial can be translated to other databases.

  Systematic searching for a review