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Systematic Reviews: Introduction & Review Types

An introductory guide to Systematic Reviews

Definition of a Systematic Review

“A systematic review attempts to identify, appraise and synthesize all the empirical evidence that meets pre-specified eligibility criteria to answer a given research question. Researchers conducting systematic reviews use explicit methods aimed at minimizing bias…”  - Cochrane Library

A systematic review uses robust methods to reduce bias in the gathering, summarizing, presenting, interpreting, and reporting of the research evidence. The key characteristics of a systematic review are:

  • clearly stated objectives;
  • pre-defined eligibility criteria;
  • explicit, reproducible methodology;
  • systematic search of the literature;
  • assessment of validity of included studies;
  • systematic synthesis and presentation of findings.

Cochrane Library. About Cochrane Reviews | Cochrane Library:  https://www.cochranelibrary.com/about/about-cochrane-reviews

As systematic reviews summarise the results of all original studies within a given field, it is commonly regarded as high quality evidence. Referring to the hierarchy of evidence shown below, we can see that as the rigour of scientific method increases, we can be more confident of the reliability and robustness of the methodology used.

 

 

 

 

 

 

 

 

 

Karolinska Institutet University Library (2022). Systematic reviews [Evidence Based Pyramid] : https://kib.ki.se/en/search-evaluate/systematic-reviews

Key Differences: A Literature Review or a Systematic Review?

There are four essential criteria for a systematic review:

  1. It should be exhaustive: all relevant literature in a research field should be included.
  2. rigorous methodology must be followed throughout – from defining the research question, writing a protocol and searching the literature, to gathering, screening and analysing. The entire process should also be thoroughly documented.
  3. At least two people should be involved, particularly for screening articles and extracting data.
  4. Plenty of time resources are needed, but also in terms of availing yourself of others' expertise – for instance in database searching – and tools and software.

For a condensed overview, see the comparison below (from Jesson, Matheson & Lacey, 2011, p. 105).

  Traditional (Scoping Review) Systematic Review
Aim To gain a broad understanding, and description of the field Tightly specified aim and objectives with a specific review question
Scope Big picture Narrow focus
Planning the review No defined path, allows for creativity and exploration Transparent process and documented audit trail
Identifying Studies Searching is probing, moving from one study to another, following up leads Rigorous and comprehensive search for ALL studies
Selection of studies Purposive selection made by the reviewer Predetermined criteria for including and excluding studies
Quality assessment Based on the reviewer's opinion Checklists to assess the methodological quality of studies
Analysis and synthesis Discursive In tabular format and short summary answers
Methodological report Not necessarily given Must be presented for transparency

Karolinska Institutet University Library (2022). Systematic reviews : https://kib.ki.se/en/search-evaluate/systematic-reviews

Types of Reviews

In an article from 2009, Grant & Booth described 14 review types, for example scoping reviews, and their associated methodologies.

    Methods Used (SALSA)      
Label Description Search Appraisal Synthesis Analysis
Critical review

Aims to demonstrate writer has extensively researched literature and critically evaluated its quality. Goes beyond mere description to include degree of analysis and conceptual innovation. Typically results in hypothesis or model.

Seeks to identify most significant items in the field.

No formal quality assessment. Attempts to evaluate according to contribution.

Typically narrative, perhaps conceptual or chronological.

Significant component: seeks to identify conceptual contribution to embody existing or derive new theory.

Literature review

Generic term: published materials that provide examination of recent or current literature. Can cover wide range of subjects at various levels of completeness and comprehensiveness. May include research findings.

May or may not include comprehensive searching.

May or may not include quality assessment.

Typically narrative

Analysis may be chronological, conceptual, thematic, etc.

Mapping review / systematic map

Map out and categorize existing literature from which to commission further reviews and/or primary research by identifying gaps in research literature.

Completeness of searching determined by time/scope constraints.

No formal quality assessment.

May be graphical and tabular.

Characterizes quantity and quality of literature, perhaps by study design and other key features. May identify need for primary or secondary research.

Meta-analysis

Technique that statistically combines the results of quantitative studies to provide a more precise effect of the results.

Aims for exhaustive, comprehensive searching. May use funnel plot to assess completeness.

Quality assessment may determine inclusion/exclusion and/or sensitivity analyses.

Graphical and tabular with narrative commentary.

Numerical analysis of measures of effect assuming absence of heterogeneity.

Mixed studies review / mixed methods review

Refers to any combination of methods where one significant component is a literature review (usually systematic). Within a review context it refers to a combination of review approaches for example combining quantitative with qualitative research or outcome with process studies.

Requires either very sensitive search to retrieve all studies or separately conceived quantitative and qualitative strategies.

Requires either a generic appraisal instrument or separate appraisal processes with corresponding checklist.

Typically both components will be presented as narrative and in tables. May also employ graphical means of integrating quantitative and qualitative studies.

Analysis may characterise both literatures and look for correlations between characteristics or use gap analysis to identify aspects absent in one literature but missing in the other.

Overview

Generic term: summary of the [medical] literature that attempts to survey the literature and describe its characteristics.

May or may not include comprehensive searching (depends whether systematic overview or not)

May or may not include quality assessment (depends whether systematic overview or not)

Synthesis depends on whether systematic or not. Typically narrative but may include tabular features.

Analysis may be chronological, conceptual, thematic, etc.

Qualitative systematic review / qualitative evidence synthesis

Method for integrating or comparing the findings from qualitative studies. It looks for ‘themes’ or ‘constructs’ that lie in or across individual qualitative studies.

May employ selective or purposive sampling.

Quality assessment typically used to mediate messages not for inclusion/exclusion.

Qualitative, narrative synthesis.

Thematic analysis, may include conceptual models.

Rapid review

Assessment of what is already known about a policy or practice issue, by using systematic review methods to search and critically appraise existing research.

Completeness of searching determined by time constraints.

Time-limited formal quality assessment.

Typically narrative and tabular.

Quantities of literature and overall quality/direction of effect of literature.

Scoping review

Preliminary assessment of potential size and scope of available research literature. Aims to identify nature and extent of research evidence (usually including ongoing research).

Completeness of searching determined by time/scope constraints. May include research in progress.

No formal quality assessment.

Typically tabular with some narrative commentary.

Characterizes quantity and quality of literature, perhaps by study design and other key features. Attempts to specify a viable review.

State-of-the-art review

Tend to address more current matters in contrast to other combined retrospective and current approaches. May offer new perspectives on issue or point out area for further research.

Aims for comprehensive searching of current literature.

No formal quality assessment.

Typically narrative, may have tabular accompaniment.

Current state of knowledge and priorities for future investigation and research.

Systematic review

Seeks to systematically search for, appraise and synthesis research evidence, often adhering to guidelines on the conduct of a review.

Aims for exhaustive, comprehensive searching.

Quality assessment may determine inclusion/exclusion.

Typically narrative with tabular accompaniment.

What is known; recommendations for practice. What remains unknown; uncertainty around findings, recommendations for future research.

Systematic search and review

Combines strengths of critical review with a comprehensive search process. Typically addresses broad questions to produce ‘best evidence synthesis’.

Aims for exhaustive, comprehensive searching.

May or may not include quality assessment.

Minimal narrative, tabular summary of studies.

What is known; recommendations for practice. Limitations.

Systematized review

Attempt to include elements of systematic review process while stopping short of systematic review. Typically conducted as postgraduate student assignment.

May or may not include comprehensive searching.

May or may not include quality assessment.

Typically narrative with tabular accompaniment.

What is known; uncertainty around findings; limitations of methodology.

Umbrella review

Specifically refers to review compiling evidence from multiple reviews into one accessible and usable document. Focuses on broad condition or problem for which there are competing interventions and highlights reviews that address these interventions and their results.

Identification of component reviews, but no search for primary studies.

Quality assessment of studies within component reviews and/or of reviews themselves.

Graphical and tabular with narrative commentary.

What is known; recommendations for practice. What remains unknown; recommendations for future research.

Grant MJ, Booth A. [Table 1 - Main review types characterized by methods used]. A typology of reviews: an analysis of 14 review types and associated methodologies. Health Info Libr J. 2009 Jun;26(2):91-108. doi: 10.1111/j.1471-1842.2009.00848.x. PMID: 19490148. .: https://pubmed.ncbi.nlm.nih.gov/19490148/

Further Reading

Munn, Z., Stern, C., Aromataris, E., Lockwood, C., & Jordan, Z. (2018). What kind of systematic review should I conduct? A proposed typology and guidance for systematic reviewers in the medical and health sciences. BMC Medical Research Methodology, 18(1)5. (Open Access) https://bmcmedresmethodol.biomedcentral.com/articles/10.1186/s12874-017-0468-4

Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.3 (updated February 2022). Cochrane, 2022. Available from www.training.cochrane.org/handbook.

The Campbell Collaboration. Campbell systematic reviews: Policies and guidelines https://doi.org/10.4073/cpg.2016.1.