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Systematic Review: Identify key concepts

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

Identify key concepts

To search effectively on a topic you must convert your research question into a format that is searchable in a database. Start by breaking your question down into key concepts.

A useful tip to help you identify the key concepts is to try to describe articles that will answer your question. What are the elements that are essential for an article to be able to address your research question? These elements (or 'concepts') might be diseases, exposures, interventions, population characteristics, services, settings, and so on.

Then, for each identified concept, identify the words most likely to have been used in the published literature on this topic. Prior reading on your topic will help with this, and some preliminary searching of the literature is indispensable for confidence that you've identified all relevant terms and synonyms. Also consider singular and plural forms of words, variant spellings, acronyms (if well known), and relevant subject headings.

There are usually between 2 and 4 key concepts, however, these may not all be included in your search strategy. The more concepts you include, the higher the risk of missing relevant references. Again, your preliminary searching is important as it can help to assess whether there are articles that answer your research question but don't include a key concept in their title/abstract/keywords.

A handy tool that might help you to get a feel for the importance of different concepts/terms, is PubVenn, which gives you a look at the relative amount of the literature for each term entered, and the crossover of literature with each term. The visualisations are based on citations in PubMed.

There are lots of frameworks you can use to organise the key concepts of a research question. Here we’ll focus on the popular ‘PICO’ format (Richardson et al., 1995).

P, I, C and O each represent different concepts that will be part of the question, roll over the image for a description of each element:


For questions where C = comparison:

  • In (P), is (I) more effective than (C) at affecting (O)?

For questions where C = control:

  • In (P), how effectively does (Intervention) affect (Outcome)?

There are variants of PICO that prompt you for other concepts, such as time (PICOT), research type (PICOR) or study type (PICOS).

Not all elements of PICO should necessarily be used in a search strategy. For example, often, to minimise bias, you may just search on the P and I elements. (When including outcome terms it is easy to bias results towards a specific outcome).

As well as PICO, there are many other frameworks for conceptualising your question:

  • ECLIPSE - Expectation/Client group/Location/Impact/Professionals/Service (Evaluating services) (Wildridge & Bell, 2002)
  • SPIDER - Sample/Phenomenon of Interest/Design/Evaluation/Research type (Qualitative studies, especially with samples rather than populations) (Cooke et al., 2012)
  • SPICE - Setting/Perspective (or Population)/Intervention/Comparison/Evaluation (Evaluating outcomes of a specific intervention) (Booth, 2006)

As part of documenting the SR process, it is recommended that you keep a master list of search terms for each key concept. This will make it easier when it comes to translating your search strategy across multiple database platforms.