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Durham University

University Library

Systematic Reviews

Systematic Reviews are most frequently conducted in the fields of education, health & medicine, social policy or psychology.

Durham University Library does not currently offer a dedicated systematic review support service. However, the resources collected here offer practical guidance and support, and you can contact the Library for assistance in constructing effective and comprehensive search strategies and identifying appropriate resources.


Definition

"A Systematic Review is a literature review that is designed to locate, appraise and synthesise the best available evidence relating to a specific research question and provide informative and evidence-based answers." (Boland et al, 2014)

It aims to review the available research evidence with the same level of rigour that should be used in producing the research evidence in the first place (Hemingway & Brereton, 2009), taking steps to "reduce/make transparent hidden bias and 'error'" (Newman & Dickson, 2012) and allowing "potentially unmanageable amounts of literature to be managed in a scientifically credible and reliable way (Torgerson, Hall & Light, 2012).

To achieve this, a Systematic Review seeks to:

  • answer a pre-defined and clearly formulated research question.
  • use a transparent, comprehensive and pre-defined (to reduce the potential bias in the search process) search strategy to locate the best available evidence, and to allow others to replicate that search process.
  • set and apply pre-defined inclusion and exclusion criteria (to reduce potential bias in the selection process) for research evidence identified.
  • critically assess the selected studies in terms of quality and relevance, and to identify and account for potential bias.
  • provide a balance and impartial summary and interpretation of the findings of the studies, drawing any relevant conclusions or identifying any identified gaps or flaws in the available research evidence.

The end goal is to produce more reliable findings that can be used to inform future decision-making in that field (Higgins & Green, 2011).


Systematic Review vs Literature Review

A Systematic Review is not just a 'big' literature review. Whilst both seek to provide a summary of the available literature on a topic, a Systematic Review is expected to be more rigorous, and ultimately to be transparent and replicable.

Systematic Review Literature Review
Objectives Focussed on a single, pre-determined research question, with clear objectives identified and stated prior to conducting the primary search of the literature. Not necessarily focussed on a single research question. May describe an overview around a theme or topic of research. Clear objectives for the literature search may not be identified in advanced, but may be shaped during the review of the literature itself.
Preparation Methods of identification, selection, evaluation and synthesis are pre-defined to minimise the risk of bias. Methods of identification, selection, evaluation and synthesis may change subject to authors changing awareness and knowledge of evidence presented, building in the authors subjective knowledge and bias into the review.
Design & Methodology Based on clear identifiable steps with the purpose of being replicable. No clear rationale; based on expert substantive knowledge of author(s).
Selection of Evidence

Explicit over how reviews included were identified and selected.
Intended to search exhaustively, and account for, all relevant studied.

Selection is not made explicit. It may be unclear whether selected evidence is representative, comprehensive or a (biased) sample.
Evaluation of Evidence All assumptions and judgments are made explicit against identified criteria and open to scrutiny and replication. Subject to author(s) knowledge and opinion.
Outcomes and data synthesis Clear summary of studies, including an objective assessment of the quality of the evidence presented and potential for bias in the studies selected. Summary based on studies may not address in detail quality of data or presence of potential bias.

The above table was based upon comparisons found in Torgerson, Hall and Light (2012) (in Arthur et al. (2012)) and Bettany-Saltikov (2012).


Stages of a Systematic Review

Any Systematic Review should be approached in a methodical way, with several key steps outlined below. There are various ways this process is outlined in the literature, although all roughly following a similar process. Examples form the literature are included in the References section below.

  1. Define a formulate the research question.
  2. Define inclusion/exclusion criteria.
  3. Define a transparent and comprehensive search strategy.
  4. Search for studies based on pre-defined search strategy.
  5. Selection of studies based on pre-defined inclusion/exclusion criteria.
  6. Coding of data collected and selected.
  7. Critical assessment of studies in terms of quality, relevance, and addressing potential bias in each study.
  8. Synthesis of findings.
  9. Interpret the results with an eye on impartiality and reducing potential bias of the authors, and draw any conclusions.

References

(Boland et al, 2014)

  1. Definition of question / problem
  2. Identification of available evidence
  3. Critical assessment of available evidence
  4. Synthesis of findings and drawing of conclusion(s)

(Newman & Dickson, 2012)

  1. Formulate a review question
  2. Search and selection of studies
  3. Coding to describe studies
  4. Critical assessment of studies (quality and relevance)
  5. Synthesis of results
  6. Deriving conclusions and implications

(Torgerson, Hall and Light, 2012)

  1. Transparent, comprehensive search strategy
  2. Clear pre-specified inclusion/exclusion criteria
  3. Explicit methods for coding, quality appraising and synthesising included studies

(Cochrane Collaboration, eight stages of doing a systematic review)

  1. Define review question and develop criteria for inclusion of studies
  2. Search for studies
  3. Select studies and collect data
  4. Assess risk of bias in selected studies
  5. Analyse data and undertake meta-analyses
  6. Address reporting biases
  7. Present results and summary of findings
  8. Interpret results and draw conclusions

Useful Examples of Systematic Reviews

Below we have tried to provide some useful examples of systematic reviews where you can access and review the search strategy's employed. This may be of interest when approaching your own Systematic Review, but also even if you are not engaged in a systematic review, as visible examples of how other researchers have employed various search tools and techniques to construct a well-focussed but comprehensive search strategy.


Examples


Online Guides and Resources


Finding Published Systematic Reviews

The resources below are available to identify if a systematic review of your topic of research already exists, beyond subject databases you may already be familiar with where published in academic journal articles.


De-duplication

One key problem you may face is that the same study may be found in more than one database searched. A single study published in the journal Nature, for example, might be indexed in Web of Science, Scopus, Medline, Embase and PsycINFO. You will need to de-duplicate your list of studies so that duplicate studies are removed from your list of results, both to save time and to correctly record in your protocol or PRISMA flow diagram the actual number of unique studies identified.

  • One common approach to managing this process is using reference management software (Endnote, Zotero, Mendeley) to both collect and store your references, and identify and remove or merge duplicates within your library of collected references. See our Reference Management Software pages for further guidance.
  • If you are using any Systematic Review specific software, this may include options for de-duplication. One example includes the de-duplication tools available within the IEBH Systematic Review Accelerator.

Google Scholar and Systematic Reviews

When approaching Systematic Reviews, it is important to be aware of the limitations of using Google Scholar for this purpose - and clearly identify if and how you have used it as a tool to support your search strategy.


Coverage and transparency

If a systematic review aims to locate all of the best available evidence relating to a specific research question, then a key aim of the researcher should be to search multiple appropriate sources to ensure a comprehensive search of the available literature.

Google Scholar might seem a logical tool to use, given its breadth of coverage. However, another criteria of a systematic review is transparency in how available research is identified and selected. Here Google Scholar creates problems for the researcher in the context of a Systematic Review:

    • There is no content coverage list for Google Scholar: Google Scholar indexes vast amounts of content, from various sources. Unlike many academic databases (e.g. Scopus, Web of Science, ERIC, British Education Index), there is no "content coverage list" to clearly indicate what is included (and what is not included) in any search made.
    • Content can be added, but is also removed, without any notice: It is possible that a seach conducted as part of a systematic review may identify results one day, which are no longer discoverable at a later date because that content is removed or no longer indexed. It is also not possible, if trying to replicate the search, to identify if a 'new' result identified was just not indexed previously, or was missed by accident or excluded by the researcher for other reasons not specified in the review.
    • Not all content is indexed to the same standard: Unlike Google, many publishers will allow web crawlers from Google Scholar to index the full-text of content, to aid discoverability. But not all sources do, which can mean some publications will appear, or be ranked, differently, dependent upon how they are indexed by Google Scholar; academic database would usually indicate differences in coverage and indexing, but Google Scholar offers no transparency for researchers to identify potential factors affecting what results are returned.
    • You can't actually view all of your results: It is not uncommon to find a Google Scholar search returning results in their thousands, ten's of thousands or hundred's of thousands. However, no matter how many results you return, Google Scholar will never let you see or download more than the first 1,001 results.

    One advantage Google Scholar does offer is its coverage of grey literature, which may not be as well indexed or covered by traditional academic databases. So Google Scholar is often used to plug these gaps in a search protocol. A useful tool for making the most of this is Publish or Perish, which more easily allows you to collect, re-order, review, filter and export your results identified from a Google Scholar search.


    Search logic

    Google Scholar cannot accurately process any search which uses more than one “concept”, each described by multiple synonyms/alternative terms.

    For example:

    • (teen OR adolescent OR juvenile) AND (achievement OR attainment OR grade)

    ... will be interpreted by Google Scholar as...

    • teen OR adolescent OR juvenile OR achievement OR attainment OR grade
    Not convinced? Try running the first search by pasting it into the Google Scholar search box (Note: you can remove the word "AND" as Google Scholar interprets a space between two terms as an "AND" connector). Then, from your list of results, go to Advanced Search and review what search Google Scholar has actually conducted:

    This can have a significant effect on the ability to identify and filter the most appropriate results. This deficiency is exagerrated further where you cannot view any results past the first 1,001 results identified, when they are ranked by Google Scholar's proprietary (and unknown) search and ranking algorithm.

    Some studies (Gehanno et al, 2013) highlight that publications found through searching other academic and professional sources can also be found via Google Scholar, and have argued that Google Scholar is therefore a useful source for Systematic Reviews as it's coverage is much broader than other tools. However, other researchers have pointed out that searching for known articles is very different to constructing a comprehensive search protocol using boolean operators to identify unknown research studies.


    Reproducibility

    Search results will be ranked by Google Scholar's proprietary (and unknown) search and ranking algorithm. Whilst unknown, it is apparent that the ranking takes into account various factors, including:

    • Keywords
    • "Venue" (e.g. journal) of publication
    • Citation count

    ... but also

    • If a result has been recently cited
    • Search history of the search operator
    • Search history of others searching for similar keywords
    • Location of the search operator

    Given this, it is possible that a search protocol will offer different results, and ranking of results, to different users. This, alongside the problems with the search logic and the lack of a clear (and changing) coverage list, make it difficult to reproduce a search on Google Scholar with the level of certainty a Systematic Review is expected to provide.


    Further reading


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